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2017 Fiesta ST - 100% stock throwing CEL: P0420

Messages
39
Likes
3
Location
Fort Wayne
#1
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!
 


Messages
88
Likes
23
Location
Morley
#2
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.


Sent from my iPhone using Tapatalk
 


OP
PolloVagabundo
Messages
39
Likes
3
Location
Fort Wayne
Thread Starter #3
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.


Sent from my iPhone using Tapatalk

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

1000 Post Club
Messages
1,529
Likes
1,131
Location
Pleasant Garden
#4
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.

Sent from my LG-LS997 using Tapatalk
 


Messages
26
Likes
11
Location
London, ON, Canada
#5
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.
 


Last edited:
OP
PolloVagabundo
Messages
39
Likes
3
Location
Fort Wayne
Thread Starter #6
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?
 


Messages
229
Likes
151
Location
San Antonio
#7
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
 


Messages
26
Likes
11
Location
London, ON, Canada
#8
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.
 


Messages
36
Likes
33
Location
Osage Beach, MO, USA
#9
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.
 


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