Last reported case: 2 months ago
Based on 7 owner reports (2 from Reddit, 5 from forums)
What is P0420?
P0420 indicates your catalytic converter is not cleaning exhaust gases effectively, typically triggered by a failing converter or upstream engine issues. This powertrain code is moderately serious and can affect emissions and performance, with Honda vehicles like the Accord and CR-V appearing most frequently in reports. Based on 975 repair cases, the average repair cost is $3847, ranging from $0 to $4700.
Common Causes:
- Failing catalytic converter (most common cause based on code definition)
- Upstream oxygen sensor malfunction
- Exhaust leaks before the catalytic converter
- Engine misfires or rich/lean fuel mixture affecting converter efficiency
Symptoms:
- Check Engine Light illuminated (primary symptom)
- Reduced fuel economy reported by some owners
- Possible lack of power during acceleration
- No noticeable symptoms in some cases beyond the warning light
🚗P0420 on 2020 FORD Mustang
This page focuses specifically on P0420 issues reported by 2020 FORD Mustang owners. The experiences and fixes below are from real owners of this exact vehicle.
Symptoms Reported by 2020 FORD Mustang Owners:
Parts Often Involved:
What Owners Say
Real experiences from 2020 FORD Mustang owners
“I keep getting a code: P0420 and there’s a lot of things it can be, O2 sensors are good for a fact. Was getting codes for them, fixed those but the cat code persists.”View original
“Thank you so much, so if I have a vacuum leak it would be somewhere else on that line? I keep getting a code: P0420 and there’s a lot of things it can be, O2 sensors are good for a fact.”View original
“So I installed stainless power longtube headers with a catted x pipe two weeks ago and two days later after installing them cel came on. I've cleared the code multiple times but it will not go away.”View original
“I've cleared the code multiple times but it will not go away. I need to fix the issue because I am due for emissions testing and I need to pass and I know long tubes with a catted mid pipe will pass because I've watched a procharged gt with long tubes pass on the exact same emissions machine I'm testing on.”View original
“I really can't afford to replace the cats could this be due to the O2 sensor failing or another reason ? The car is basically stock besides the Flowmaster Axleback Outlaw mufflers, I am not sure if the cat is stock, but I have never received this code before in the 6000 miles I owned the car any help would be appreciated!”View original
“P0420 Reoccurring, Super Annoying Hey guys, my name is Christopher I have followed the forums for a quite a while but haven't posted much, I hate to post only when I have a problem haha.”View original
“Lack of cats is not the cause of the limp mode, there is another issue. Removing the cats should only throw p0420/p0430 codes if all else is in good working order.”View original
“The blue ones are downstream, in or after the cats. It makes a difference which connectors you plug into which side, too; most important is to get the green ones plugged into the proper connectors as you engine will run like crap if connected wrong.”View original
“It makes a difference which connectors you plug into which side, too; most important is to get the green ones plugged into the proper connectors as you engine will run like crap if connected wrong.”View original
“I have a 2020 mustang gt premium convertible with a stage 2 whipple. I inherited it from my dad and it's very sentimental to me.”View original
These are real experiences shared by 2020 FORD Mustang owners on automotive forums and communities. Results may vary based on vehicle condition and repair quality.
About This Data
This page aggregates 7 discussions about P0420 from 2020 FORD Mustang owners across automotive communities.
Success rate (14%) is calculated from posts where owners confirmed whether their fix resolved the issue.
Last updated: 12/8/2025