2017 Fiesta ST - 100% stock throwing CEL: P0420

[PolloVagabundo]

Hello everyone. Our fiesta (purchased new and never modded) has 45K miles and last month decided to throw a check engine light.
After reading the code: P0420 and clearing it... it came back the next day and has remained on.

I'm a bit confused since the car is stock and relatively low miles. Is it possible the oem cat went bad??? Or just a bad o2 sensor?
The stealership said the cat would be covered until 80K but the sensor is not something that would be covered. And if I brought the car in and it was anything other than the cat, they would charge me a $100 diagnostic fee.

After searching prior threads, all I was able to come across for this P0420 code being thrown is that it usually happens when people mod their exhausts... Which is clearly not the case here.

Seeing if anyone has had this happen or read about someone else having this issue with a stock fiesta ST???
We were really trying to keep the car stock until 60K miles, but maybe it is time for a catless downpipe and tune???
Any suggestions, much appreciated. Thank you!

 

[jtuffy96]

Hello everyone. Our fiesta (purchased new and never modded) has 45K miles and last month decided to throw a check engine light.
After reading the code: P0420 and clearing it... it came back the next day and has remained on.

I'm a bit confused since the car is stock and relatively low miles. Is it possible the oem cat went bad??? Or just a bad o2 sensor?
The stealership said the cat would be covered until 80K but the sensor is not something that would be covered. And if I brought the car in and it was anything other than the cat, they would charge me a $100 diagnostic fee.

After searching prior threads, all I was able to come across for this P0420 code being thrown is that it usually happens when people mod their exhausts... Which is clearly not the case here.

Seeing if anyone has had this happen or read about someone else having this issue with a stock fiesta ST???
We were really trying to keep the car stock until 60K miles, but maybe it is time for a catless downpipe and tune???
Any suggestions, much appreciated. Thank you!

While my car isn’t stock and I have an exhaust I didn’t get this code until like a year after I had it on. It went away by itself within 2 days so hopefully that is the case with yours.


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[PolloVagabundo]

While my car isn’t stock and I have an exhaust I didn’t get this code until like a year after I had it on. It went away by itself within 2 days so hopefully that is the case with yours.


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The light in our car stays on, but it does seem fine to drive otherwise.

Anyone else have any thoughts?

Should we just get a catless downpipe and accessport and call it a day?

 

[Ford ST]

I would absolutely have the dealership deal with this. OEM converters are expensive. Very expressive. You have the opportunity to get this replaced for free. A potential hundred dollar fee is absolutely worth it.
Of course I'm also against catless downpipes I actually care about the air I breathe.

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[Snrub]

P0420 is a failed cat more often than not. It doesn't normally take a long time for it to come on. I've had cats fail on two different cars in the past. Looking at Rock Auto, aftermarket an cat and o2 sensor are reasonably priced.

 

[PolloVagabundo]

P0420 is a failed cat more often than not. It doesn't normally take a long time for it to come on. I've had cats fail on two different cars in the past. Looking at Rock Auto, aftermarket an cat and o2 sensor are reasonably priced.

When you had your cats fail, did you notice a difference in driveability? slowly or right away?

 

[Erick_V]

When you had your cats fail, did you notice a difference in driveability? slowly or right away?

For me the car got really loud then quiet then loud then quiet then permanently louder as the cat blew out the back of my car. Happened over the course of a week. I’m pretty sure I’m the only person that has had this happen. I had a catback and a crackle tune. Don’t think the tune was related. Drivability felt the same. I still street drive the car so I put a catted down pipe back into the car. Was tired of smelling like gas every time I parked in the garage and walked inside. I would have the dealer look at the car before going further. Worst case the sensor is less than $100

 

[Snrub]

In the RX-8 I had to replace two cats, one under warranty, the other one not. Those cars are notorious for killing cats. The first time I noticed some hesitation under WOT acceleration right before the CEL started coming on. The second time I didn't notice any difference.
In the 6th gen Camaro, I don't *think* there was a difference, but it was POS with other issues, so it's a little difficult to say.

 

[SteveS]

Your catalytic convertor is still covered under the federally required emissions warranty. A failed catalyst is the most common reason for that obd code. Take it to the dealer. Under the federal warranty they cannot charge you to diagnose or replace a federal emission warranty part.

The other common causes are an exhaust leak ahead of the catalyst (which is also covered under the emissions warranty and/or powertrain warranty) or the O2 sensor. The O2 sensor is not covered, but they cost about $45. You can change those yourself.

 

[PolloVagabundo]

At almost 50K miles, our fiST finally needs new brake pads and going to replace the rotors.
So I decided to also order the o2 sensor finally.
Really hope this solves the problem!
Will keep anyone interested posted with the results.

 

[PolloVagabundo]

Ok Friends. so this is very odd....

I received the rotors,pads and o2 sensor from rockauto. Changed the rotors and pads this weekend but didn't have the time to do the sensor.

The next morning when we started the car, the CEL was off. Keep in mind(again) I did not replace the sensor... just rotors and pads.

And the CEL has remained off for the last couple days!

this makes absolutely no sense to me. Any thoughts???

 

[Snrub]

I'm assuming you didn't disconnect the battery with the brake job? If you did, the CEL will clear.

A CEL turns on when a monitor test fails in the computer. The tests have thresholds for measured parameters to be within, as well as time periods, etc. It could be that you only barely failed the conditions before and you drove the car a differently (eg. say a lot more highway, or more city), or something like weather conditions were different and the thresholds were no longer met for the period required by the test. Some less critical CELs will turn off under these conditions. Or it could be that the O2 sensor is functioning intermittently and it happened to work well enough for long enough for the CEL to clear. I believe these are all reasonable guesses.

 

[scooter123]

First, don't do a catless downpipe, you will lose horsepower unless you are running a BIG Turbo. The factory downpipe is actually a rather sophisticated bit of the Exhaust System and is tuned to create a Standing Wave partial vacuum at the exit point of the Turbocharger. That necked down point within the downpipe acutally isn't a restriction, it's a tuning point that acts to make the Turbocharger more efficient. Point is that you want to keep that downpipe and if you cat has a cracked element you want to replace it with an identical Ford Downpipe, which is covered under the Powertrain Warranty for 100,000 miles or 7 years. BTW, the Ford Powertrain Warranty is that generous, I actually looked it up.

 

[PolloVagabundo]

First, don't do a catless downpipe, you will lose horsepower unless you are running a BIG Turbo. The factory downpipe is actually a rather sophisticated bit of the Exhaust System and is tuned to create a Standing Wave partial vacuum at the exit point of the Turbocharger. That necked down point within the downpipe acutally isn't a restriction, it's a tuning point that acts to make the Turbocharger more efficient. Point is that you want to keep that downpipe and if you cat has a cracked element you want to replace it with an identical Ford Downpipe, which is covered under the Powertrain Warranty for 100,000 miles or 7 years. BTW, the Ford Powertrain Warranty is that generous, I actually looked it up.

Definitely not going catless anymore after reading exactly what you have mentioned also said by many other people. And that is great to know regarding Fords warranty. Thank you!

 

[PolloVagabundo]

I'm assuming you didn't disconnect the battery with the brake job? If you did, the CEL will clear.

A CEL turns on when a monitor test fails in the computer. The tests have thresholds for measured parameters to be within, as well as time periods, etc. It could be that you only barely failed the conditions before and you drove the car a differently (eg. say a lot more highway, or more city), or something like weather conditions were different and the thresholds were no longer met for the period required by the test. Some less critical CELs will turn off under these conditions. Or it could be that the O2 sensor is functioning intermittently and it happened to work well enough for long enough for the CEL to clear. I believe these are all reasonable guesses.

That is correct... The battery was NOT disconnected.

What you said makes sense.
Guess we will just have to keep an eye out for the CEL to pop back up and if it does come back on... I will finally swap the sensor.

Otherwise, the car drives great and no symptoms of anything wrong!

 

[PolloVagabundo]

Alright so while my wife has been driving the car the last few days, there has been no CEL.

But last night, I took the car to get new tires... And the CEL light turned back on!!!
The only difference I see is that my wife drives the car very economically/ casually...
While I enjoy my boost and spirited driving

Looks like I will try replacing the sensor tonight and report back if that has solved the issue.

 

[PolloVagabundo]

New motorcraft o2 sensor installed last night.

This was the first sensor before the downpipe (black wire)

I disconnected the negative off the battery to clear the code overnight and surprisingly still shows the CEL as of this morning.
Going to see if a friend or auto store can clear the code and we shall see if it is fixed?

Am I safe to say if we clear the code and it pops back up after installing this new sensor... time to bring it in to FORD for the cat inspection?

 

[Snrub]

If you clear the code and it comes back, then the problem isn't resolved. Up to you whether you want Ford to run the cat diagnostic test. It looks like cats go for ~$500 and the diagnostic is probably ~$120. I'd personally just replace the cat at that point to save the diagnostic costs, but I'd be doing it myself.

 

[PolloVagabundo]

Ford said $99.95 for a diagnostic fee if its not the cat. If not a sensor or cat, it could be potential bad wiring?
They did confirm the cat is under warranty for 8y/80K

 

[CarGuy]

I don't think disconnecting the battery will always clear the code. You need a code reader. You can pick up a $20 bluetooth one from Amazon and be able to read and clear codes on your own. There is plenty of free software to use with it. I have Torque that I use from my phone to read and clear codes.

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