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Ford Fiesta 2012 1.25L Petrol. Code: P0420 Advice pls.

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#1
Hello!

This is my first post so sorry if I made any errors :)

Just to start off with, I saw another post regarding this matter on his forum however would like some advice regarding my situation.

Car info:
-Regularly serviced.
-Mileage: 50k
-Driven mostly as a city car low but constant daily miles.

Symptoms:
-Petrol smell from cars exhaust.
-Quite high MPG than compared to couple months back.
-High idle RPMs till warmed up around 1.1/1.2.
-Very minor performance hit (barely noticeable, no stuttering or anything either).

As most people probably on this forum I drive a 2012 1.25L petrol Ford fiesta.

On my last MOT (Yes I know, I'm a brit) I got told that my Fiesta just barely passed the emissions test on the last try/second. Got told that it could be a variety of problems ranging from a MAF sensor, oxygen sensor to a bad catalytic converter. I kind of ignored it because I'm stupid and didn't do any investigating or anything (Yeah I know I regret it now). Around 23 days later during a long drive (Around 80 miles) my check engine light popped up. Carried on driving till I got to my destination and called up my local mechanic to investigate this. Got the code P0420, he kind of just told me what the guys at my MOT told me and just changed my spark plugs to some NGK ones and deleted the code. Mentioned that most of the work that would need to be done would cost quite a lot and I'd need OEM parts which cost quite a lot as well. Mentioned a couple smallish potential additives that could help which I've done as of writing this post. I Added "Wynn's super charge oil treatment" for my engine, and "Wynn's catalytic converter & Lambda cleaner" to my tank used both as per the instructions and noticed a minor change if any, however I didn't get the engine light pop up again but I presume its a matter of time (which is perfect timing considering I'm broke atm). I'm going to get myself an OBD2 reader so I can personally diagnose and not pay these massive diagnostic fees the mechanics charge.

My questions:
-Do I really need OEM parts? specifically the oxygen sensor? As the OEM ones are like £200 whereas some other branded ones are between £30-60 which seems more reasonable. If so which brands would you recommend on a budget?
-Is changing the O2 sensor something I can do or would I need a mechanic? Like how likely am I to fuck it up?
-How likely is it to be my catalytic converter considering my car has only done 50k miles and been serviced regularly?
-If its not the O2 sensor, what order of parts to go down next?



Thanks in advance, any input would be greatly appreciated :)
 

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#2
Hello. Welcome to the forum. I would first replace the upstream O2 sensor and the downstream O2 sensor. Given the age of the vehicle and the amount of time you have continued to drive it with the present condition. Many mechanic's consider oxygen sensors to be part of a major tuneup routine. They do "age out" or as i refer to their condition "become tired" and slow down in their reaction time to adust the fuel trims as engine demands change quickly.
You don't have to purchase Ford O.E. replacement sensors. Bosch makes direct fit O2 sensors that work just fine. You will need a special design socket to remove the sensors from their bungs. On the front of the head pipe and the pipe just in front of the resonator.
The catalyst, if it proves to be worn out, would need to have new O2 sensors anyways. So , you don't have to worry that you are needlessly spending money to find out that the catalyst is bad.
Bad or worn out O2 sensors are what kills or accelerate the death of a catalyst 99 percent of the time. It's very late here. In the morning I will post some images of the tools and parts to do a service on the O2 sensors. Then you can determine whether to do that yourself or hand it to a mechanic.

This special socket has a relief cut into the side to allow the wire pigtale to rotate with the sensor. You must first unplug the sensor from the electrical harness. Or new problems of the wire harness repair type will develop! 17120649415971367764084770009368.jpg
 
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Thread Starter #3
Hello. Welcome to the forum. I would first replace the upstream O2 sensor and the downstream O2 sensor. Given the age of the vehicle and the amount of time you have continued to drive it with the present condition. Many mechanic's consider oxygen sensors to be part of a major tuneup routine. They do "age out" or as i refer to their condition "become tired" and slow down in their reaction time to adust the fuel trims as engine demands change quickly.
You don't have to purchase Ford O.E. replacement sensors. Bosch makes direct fit O2 sensors that work just fine. You will need a special design socket to remove the sensors from their bungs. On the front of the head pipe and the pipe just in front of the resonator.
The catalyst, if it proves to be worn out, would need to have new O2 sensors anyways. So , you don't have to worry that you are needlessly spending money to find out that the catalyst is bad.
Bad or worn out O2 sensors are what kills or accelerate the death of a catalyst 99 percent of the time. It's very late here. In the morning I will post some images of the tools and parts to do a service on the O2 sensors. Then you can determine whether to do that yourself or hand it to a mechanic.

This special socket has a relief cut into the side to allow the wire pigtale to rotate with the sensor. You must first unplug the sensor from the electrical harness. Or new problems of the wire harness repair type will develop! View attachment 9092
Hello!

Thank you for the reply, its given me a bit of comfort and insight knowing that I might not be completely fucked. I'll have a browse at Bosch O2 sensors, are there any other brands you'd recommend or just to stick mostly to Bosch? I've gotten a couple results for Haas and Starline, which would be better financially for me but don't mind the input. I'll have a gander at some YouTube videos to see how easy o2 sensors are to replace and the price my mechanic would do it for.

Would love any other input on this as well :)

Thanks! :)
 

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#4
I would only choose a known quality of sensor for the upstream O2. A pair of new sensors are not anywhere near as costly as the catalyst! You really want that upstream O2 to be a quality product! It has to be the most reliable one in order to be certain that the catalyst is not being force fed a steady diet of hydrocarbons. The downstream O2 sensor is a confirmatory sensor that sniffs the A/F ratio at approximately half the rate of the upstream O2 sensor.
You also mentioned that the idle speed is rather high. I would be checking the tightness of the worm gear clamps which hold the intake tube at each end! The intake system and the PCV system are sealed systems and only function properly when connections are tight.
When you get the new sensors in place and you are certain that the intake tube is tightly connected. It would be best to take your fiesta for a drive on a motorway where you can achieve steady speeds that will allow enough exhaust flow to purge the catalyst of HC. About 16km should be enough to get it back to a normal state, if its going to function efficiently enough to not trip a MIL code again. Low speed, urban style driving is not going to clear out the catalyst brick thoroughly enough to prevent a repeat of the P0420 code.
Good luck
 
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Thread Starter #5
I would only choose a known quality of sensor for the upstream O2. A pair of new sensors are not anywhere near as costly as the catalyst! You really want that upstream O2 to be a quality product! It has to be the most reliable one in order to be certain that the catalyst is not being force fed a steady diet of hydrocarbons. The downstream O2 sensor is a confirmatory sensor that sniffs the A/F ratio at approximately half the rate of the upstream O2 sensor.
You also mentioned that the idle speed is rather high. I would be checking the tightness of the worm gear clamps which hold the intake tube at each end! The intake system and the PCV system are sealed systems and only function properly when connections are tight.
When you get the new sensors in place and you are certain that the intake tube is tightly connected. It would be best to take your fiesta for a drive on a motorway where you can achieve steady speeds that will allow enough exhaust flow to purge the catalyst of HC. About 16km should be enough to get it back to a normal state, if its going to function efficiently enough to not trip a MIL code again. Low speed, urban style driving is not going to clear out the catalyst brick thoroughly enough to prevent a repeat of the P0420 code.
Good luck

Hello!

No worries, I'll stick to Bosch and check the tightness of the worm gear clamps when I'm in there as I've decided to try and do it myself (Gonna get the tools/parts first). I'm just looking at different sites trying to find the best deals, got recommended "Autodoc" by a couple friends and found several compatible Bosch O2 sensors but don't know which one to choose from. I believe the first one has the right connector but does it matter? Would love your input :)

Thanks!
 

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#6
You don't want a "universal" O2 sensor unless you are up to speed on properly doing required pin position of each of the four wires that feed into the sensor! "Direct fit" is what you want. It makes the task truly plug and play. Most parts stores have a vin database that narrows down the choices that you could make to get the right fitting part for your particular car.
 
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Thread Starter #7
You don't want a "universal" O2 sensor unless you are up to speed on properly doing required pin position of each of the four wires that feed into the sensor! "Direct fit" is what you want. It makes the task truly plug and play. Most parts stores have a vin database that narrows down the choices that you could make to get the right fitting part for your particular car.
Oh sorry, yeah I should of specified. These were the Bosch choices when ran through the compatibility search via number plate/vin. They all appear to be compatible with my fiesta but I think I will stick to plug and play as you mentioned. The first pic is plug and play and that's what I think I'll go with, looked up the OEM number and will buy the sensor through Amazon's Bosch seller. In addition, I checked through multiple websites and pic 1s sensor seems to pop up in most of them but with different prices :). Just wanted to know if any of those numbers in the pics ring any bells in case you or anyone else used those specific ones just so I can be 100% guaranteed it'll fit but I take it that its just a matter of connectors :)

Thanks!
 

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#8
Welcome!

IF it wasn't for you Brit's - we'd be still driving a Pinto and Yugo for "Economy" class cars.

So thanks for kind of being the "test subjects" while Ford figured out what the world wanted and built something we could use.

Unfortunately - the good things in life have to come to an end but I digress.

@scotman - has got you going on the right track, if you are having trouble with Idle speed RPM, might want to inspect around the intake manifold, several things are going on in there.

I'm presuming your engine looks like this...
1712109068594.png
Only with not so much dirt in it...
The reason I'm adding in to this thread is to ask you to look down in-between that "gap" in the intake manifold so you can see the PCV valve and it's hose that routes to the intake - this are may be what is causing your higher idle - or at least you should look into and make sure the integrity of the PCV ventilation system is good and no collapsed hoses or broken lines.

  • - one main being the PCV valve - that hose is subject to deterioration
1712106165180.png
Whether you motor is a Ztec or Duratorq or any variety in-between - the PCV valve uses a housing with an oil-splash guard and has been kept in a particular spot during the production run of this motor style. Its mounted to the engine block just above the oil-level line that when you're filled it for the oil change - keeps the PCV valve in the foul air, but keeps out vthe oil using a baffle. The vacuum from the intake routes to this thru, using a spring loaded port valve and the valve itself is nothing special - it looks like everything else out on the market for PCV valves - but the location of the valve and how it's routed to ventilate the system and purge out the foul air of combustion by-products makes this engine style unique.

You probably know your MAF sensor - it's on the snorkel by the air box and it's on the neck of the air filter cap.

But there is also a hose and valve setup that is part of the systems air-idle quality budget that if broken or cracked - can spill air past the throttle, forcing the engine idle up and can even set a check engine light if it's not corrected.

1712110083136.png
This valve serves to regulate air going into the intake and also provide the Brake Booster vacuum from the intake manifold for braking assist. It connects the Snorkel, Brake Booster and Intake manifold together to form a specific set of conditions to help the throttle maintain control of idle air mixture, using and applying air to regulate vacuum and also provide boost for braking.

So I'd look over the motor and make sure you don't have any cracked lines or loose hoses that can affect the idle speed like you're post describes.
 
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Thread Starter #9
Welcome!

IF it wasn't for you Brit's - we'd be still driving a Pinto and Yugo for "Economy" class cars.

So thanks for kind of being the "test subjects" while Ford figured out what the world wanted and built something we could use.

Unfortunately - the good things in life have to come to an end but I digress.

@scotman - has got you going on the right track, if you are having trouble with Idle speed RPM, might want to inspect around the intake manifold, several things are going on in there.

I'm presuming your engine looks like this...
View attachment 9098
Only with not so much dirt in it...
The reason I'm adding in to this thread is to ask you to look down in-between that "gap" in the intake manifold so you can see the PCV valve and it's hose that routes to the intake - this are may be what is causing your higher idle - or at least you should look into and make sure the integrity of the PCV ventilation system is good and no collapsed hoses or broken lines.

  • - one main being the PCV valve - that hose is subject to deterioration
Whether you motor is a Ztec or Duratorq or any variety in-between - the PCV valve uses a housing with an oil-splash guard and has been kept in a particular spot during the production run of this motor style. Its mounted to the engine block just above the oil-level line that when you're filled it for the oil change - keeps the PCV valve in the foul air, but keeps out vthe oil using a baffle. The vacuum from the intake routes to this thru, using a spring loaded port valve and the valve itself is nothing special - it looks like everything else out on the market for PCV valves - but the location of the valve and how it's routed to ventilate the system and purge out the foul air of combustion by-products makes this engine style unique.

You probably know your MAF sensor - it's on the snorkel by the air box and it's on the neck of the air filter cap.

But there is also a hose and valve setup that is part of the systems air-idle quality budget that if broken or cracked - can spill air past the throttle, forcing the engine idle up and can even set a check engine light if it's not corrected.
This valve serves to regulate air going into the intake and also provide the Brake Booster vacuum from the intake manifold for braking assist. It connects the Snorkel, Brake Booster and Intake manifold together to form a specific set of conditions to help the throttle maintain control of idle air mixture, using and applying air to regulate vacuum and also provide boost for braking.

So I'd look over the motor and make sure you don't have any cracked lines or loose hoses that can affect the idle speed like you're post describes.

Hello!

Thanks for replying! :)

I had a quick look around my engine and the different cables and wires to see if any of them have been cracked or deteriorated but it seems that all of them are fine just very dirty. I'll attach some pictures so you can have a look, I think that some of the parts on my engine are different, as my MAF sensor is in a different location compared to the picture you've put in. The RPMS are only between 1.1/1.2 only when the engine is cold when it reaches 4 temp bars the RPMs lower back down to 1 on the dot. I'll attach some pictures to this post so you can have a look. The only thing that seemed out of place was my engine filter was not properly fitted into the cable which I pushed in and tightened! But I don't think that changed anything.

NOTE: The oil near/around my spark plugs was from when I added oil additives just need to clean it up a bit better but had no time recently.
 

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#10
This is really cool!

It's like visiting the Smithsonian, without having to pay all those fees to get inside and "Look around"

Ok, the 1.25 (sigh) I wish I had your motor - far simpler - you were/are seeing a 2012 version of the Fiesta 1.6L as a USA variant and it's emissions (ugh).

The MOT - did they give any other info to you - like where you were at on their scale?

I mean, the O2 sensors - in many ways are "calibrated" to a known reference but for a given rate of flow.

So the 1.25L has a much lower air flow rate than a 1.6L. This changes things...or would be different and possibly now outdated standard which is why I asked about the MOT scale of your car on their emissions test.

Sadly I see you're kinda getting pushed into buying something more modern because the sensor used for OEM - REQUIRES OEM so it's readings match OEM calibration data for Ford's own.

Are you ok with talking about this so far? I mean, I feel bad that MOT is taking all these economy cars and basically throwing them into a heap and forcing the public to buy new whether it needs to be or not.

All the sensors I'm seeing so far are based upon the 1.6L version of this motor which makes the Sensor selection more specific because you have a smaller displacement so therefore the sensor calibrated for that type of size and the flow rate it has and it's exhaust parameters is different.

Adding this to help...

Kind of ties back to where were you at - on their scale for MOT emissions?

Because if you just need to run a little leaner - might want to look into replacing that PCV valve - as the engine ages so does that valve - you may need a different larger flow rate one to handle more combustion by products - it's spring loaded and it deals with oil and gunk - it's a part of the "foul air" cleaning and purge the system does to re-burn old oil - gunk and any other hydrocarbons that get trapped - it circulates fresh air in - so if the PCV valve is weak or clogged, the lesser fresh air can change the emission parameters the MOT sees. Might be more rich than lean - which would have helped to know so you can make adjustments and clean the system where air metered in (throttle body, air valves EGR and PCV)- is flowing in and working like it should.
 
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Thread Starter #11
This is really cool!

It's like visiting the Smithsonian, without having to pay all those fees to get inside and "Look around"

Ok, the 1.25 (sigh) I wish I had your motor - far simpler - you were/are seeing a 2012 version of the Fiesta 1.6L as a USA variant and it's emissions (ugh).

The MOT - did they give any other info to you - like where you were at on their scale?

I mean, the O2 sensors - in many ways are "calibrated" to a known reference but for a given rate of flow.

So the 1.25L has a much lower air flow rate than a 1.6L. This changes things...or would be different and possibly now outdated standard which is why I asked about the MOT scale of your car on their emissions test.

Sadly I see you're kinda getting pushed into buying something more modern because the sensor used for OEM - REQUIRES OEM so it's readings match OEM calibration data for Ford's own.

Are you ok with talking about this so far? I mean, I feel bad that MOT is taking all these economy cars and basically throwing them into a heap and forcing the public to buy new whether it needs to be or not.

All the sensors I'm seeing so far are based upon the 1.6L version of this motor which makes the Sensor selection more specific because you have a smaller displacement so therefore the sensor calibrated for that type of size and the flow rate it has and it's exhaust parameters is different.

Adding this to help...

Kind of ties back to where were you at - on their scale for MOT emissions?

Because if you just need to run a little leaner - might want to look into replacing that PCV valve - as the engine ages so does that valve - you may need a different larger flow rate one to handle more combustion by products - it's spring loaded and it deals with oil and gunk - it's a part of the "foul air" cleaning and purge the system does to re-burn old oil - gunk and any other hydrocarbons that get trapped - it circulates fresh air in - so if the PCV valve is weak or clogged, the lesser fresh air can change the emission parameters the MOT sees. Might be more rich than lean - which would have helped to know so you can make adjustments and clean the system where air metered in (throttle body, air valves EGR and PCV)- is flowing in and working like it should.


Hello!

Thanks for the reply, I will attach an image of my emissions test as we get a print out to have a look. Honestly, I am having to google some of the things you mention to keep up as its all mechanical jargon but I kind of get the gist. As for the MOT I believe the government is becoming more and more stricter with emissions trying to become more green which forces us to either switch cars or look for alternatives. According to the OEM part numbers the first Bosch o2 sensor should fit into my car and match the emission standard, the connector seems to match up too! So, I'll purchase it from a reputable seller when I get my tools to fit the part in, as I've had a look on how to replace it.

Edit: I've also added 4 photos to show my RPMs when the cars engine warms up between 1-4 temp bars. Let me know if that's anything useful.



Thanks!
 

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Thread Starter #12
Hello!

I just remembered a piece of information that might be useful in my problem. Ages ago, around a year ago. I took my car into a garage to get my core plugs replaced. However, the garage also recommended I change my coil pack and leads, considering my budget at the time they didn't go for factory parts but rather lower branded ones which I accepted. They did give me the others back so, my question is. Could these parts also explain part of my problem?

I'm going to put the factory parts back on tomorrow and report to see if anything has changed as I remember the coil pack and leads did work.

Thanks.
 

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#13
Man! They are touchy on things!

0.02% difference! Wow!

Ok, with what I see the Throttle body might need to have some cleaning and TLC, like clearing out the bore, for if the plate to bore clearance is not right - the system will have to "seek" to find a throttle plate position to keep the system "in check".

As for now you passed - but 2nd try - which may have been other issues with their equipment too - we can't just "assume" it's all on your end for all the other parameters unchanged passes again with little to no change on those other readings.

So if this is getting out of hand I realize it may seem overwhelming, but I tend to take vehicles like these cars personally. They provide a level of freedom to anyone whom is diligent and accommodating - and patient with other drivers for this is a privilege to be able to move about - in any form and with some comfort - as these vehicles provide - they become endearing to their owners even with all their quirks.

I did up some threads about throttle body and their needs. One being that they often just wear out. The teeth in the gearbox, the variable position locator (a curved long piece of carbon trail that a wiper works like a variable resistor in a stereo volume control potentionmeter) along with the plate and the rod that the plate is affixed to - that turns, so it lets in more air as it opens.

All this to help the system develop a positional sense of where these throttle plates are at - so it can correlate it to the accelerator position sensor (your throttle pedal) and make sure the angle and the mixtures are correct that the stepper motor works against - develop wear and slop which if small indications like you're experiencing with that MOT test, the simple freeplay or worn bearing surface can affect the vacuum, the air to fuel mixture and position of the throttle plate in such a way that tolerances we once thought of as acceptable are now endangering your vehicle from ever being allowed to be driven again.

Ok, to keep this simple do your best to keep the system clean - start with some simple cleaning using a throttle body intake cleaner and also inspect and if needed - regap those plugs - they can wear but that also depends on the mileage and the type of metal in the tip used.

The PCV valve? IF you can take a set of long nose or even a spark plug boot puller, try to pull out the PCV valve itself and see if the valve needs cleaning - it may need quick squirt or two to remove some build up - but also inspect that valve - have your mechanc or someone you trust with your vehicle help you understand the differences in knowing the differences in a good PCV valve or a bad PCV valve due to loose spring-loading used in the valve. It can change the way vacuum and air-to fuel ratios are senses and worked on by the O2 sensor you're thinking about swapping. PCV valves can last a long time and usually don't fail but your freedom is important. So PCV valves use springs to tension a vane closing a port - against the vacuum and open more when vacuum is low but also are open in high vacuum and meter in proportion to that level of vacuum - so if the springs wear - that affects how the port opens and closes in vacuum conditions. - and as they get used their lifetime is limited - it's not infinite. They do have springs and ports that can get plugged and operate sluggishly - keeping fumes and or air from properly entering in and disposing of the foul crankcase air - again affecting the mixture - if it stays open too long the system sees a richer mixture and can affect the exhaust which is what they look at to determine the heath of the motor.
 

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#14
Other things to look for...
1712368770002.png

When you're cleaning out the throttle body, there may be some things you'll need to VERIFY they work and are positioned like they should.

The above may look intimidating, but remember too, you're trying to save your car and pass an MOT - so if the throttle body is not perfect symmetrical round throttle plate centered in the bore - you might want to fix that too.

IT uses some small star bit - so a simple star bit tool can easily loosen (don't remove) the setscrews so you can; using your fingers - position the plate so a strong light shined thru the bore from behind can leave a ring of light that can be seen in equal distance around the bore inside clearance edge like an annular eclipse - the ring of light is symmetrical in shape.
 


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