Last reported case: 2 months ago
Based on 27 owner reports (3 from Reddit, 24 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
đźš—P0420 on 2020 HONDA Accord
This page focuses specifically on P0420 issues reported by 2020 HONDA Accord owners. The experiences and fixes below are from real owners of this exact vehicle.
Symptoms Reported by 2020 HONDA Accord Owners:
Parts Often Involved:
What Owners Say
Real experiences from 2020 HONDA Accord owners
“If it still starts, accelerates, and shifts ok, you can probably ignore some of the stuff they suggested for a *little while*. But honestly, the KBB value of your car is about $850-1000.”View original
“At 250k miles, be ready for it to go soon. If it still starts, accelerates, and shifts ok, you can probably ignore some of the stuff they suggested for a *little while*.”View original
“P0420 is for sure the cat. The transmission needs to be cleaned off, filled to the right amount and add dye to the transmission and drive it couple hundred miles IF it can be driven.”View original
“That's difficult unless you're in a state that suspends emissions/ suspends emissions on older vehicles. Knock sensor and oil pressure switch are easy swaps.”View original
“The transmission needs to be cleaned off, filled to the right amount and add dye to the transmission and drive it couple hundred miles IF it can be driven.”View original
“There is a way to properly diagnose what the problem is but I don't know what it is, have the tools to do it, nor do I have the patience to learn it. The first time I got the P0420 code I read up on what it meant, concluded that the problem could actually be the sensor itself, and decided to throw a new one at it and it worked.”View original
“The first time I got the P0420 code I read up on what it meant, concluded that the problem could actually be the sensor itself, and decided to throw a new one at it and it worked.”View original
“It takes 2 failures to turn on the P0420 code, the first time sets a pending code. In all cases you should troubleshoot any other codes set with the P0420 code first.”View original
“I just pulled this out from “a friend’s car” (yeah, that’s the ticket). So this is what I found: Apparently the previous owner(s) 6.5+ years ago, when I bought this, cheated around emissions problems as my best guess.”View original
“Is this not the sensor 2 located after the catalytic converter. My understanding is that it shouldn’t be fluctuating so much if the cat is doing its job.”View original
These are real experiences shared by 2020 HONDA Accord owners on automotive forums and communities. Results may vary based on vehicle condition and repair quality.
About This Data
This page aggregates 27 discussions about P0420 from 2020 HONDA Accord owners across automotive communities.
Success rate (7%) is calculated from posts where owners confirmed whether their fix resolved the issue.
Last updated: 1/6/2026