P0420 on a 2018 Honda Accord

P0420 means the downstream oxygen sensor pattern suggests the bank 1 catalytic converter is not storing oxygen properly.

Not your vehicle? See general P0420 info.

Most common causes on the 2018 Honda Accord

Ranked from 1,812 NHTSA owner complaints filed for this vehicle.

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1
Failing fuel injectors causing rich/incomplete combustion
Medium severity
The fuel injectors reported failing alongside the head gasket on this engine can dump excess unburned fuel into the exhaust, overloading and overheating the catalyst.
Reported in 7 NHTSA complaints
2
Catalytic converter contamination from coolant intrusion
High severity
Several complaints confirm via borescope or cooling-system pressure test that coolant is physically entering the combustion chamber on this engine; combusted coolant residue passing through the exhaust coats and poisons the catalyst, reducing its efficiency below threshold.
Reported in 4 NHTSA complaints
3
Worn or aged catalytic converter
Medium severity
Normal high-mileage degradation of the catalyst substrate reduces conversion efficiency; compare pre/post-cat O2 sensor switching activity.
Common cause
Parts:catalytic converter
4
Failing downstream (rear) oxygen sensor
Medium severity
A lazy or biased rear O2 sensor can falsely mimic poor catalyst efficiency; compare its switching pattern to the upstream sensor.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leak between the catalyst and the rear sensor lets in outside air, skewing its reading; inspect flange gaskets and welds.
Common cause
6
Catalyst substrate damage from prolonged untreated cylinder misfire
High severity
Unburned fuel dumped into the exhaust during the sustained P0300-range misfires reported on this vehicle can overheat and melt the catalyst substrate; a rattling or restricted converter on inspection confirms this.
Common cause

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These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0420 on your car still needs shop confirmation before any repair.

Common symptoms

Check engine light
Usually no drivability issue
Failed emissions test
Sulfur smell in some cases
Reduced power if converter is clogged

What to check before replacing parts

Check for misfire and fuel trim codes first

Look for: Any active or recent misfire, or lean/rich trim codes, means raw fuel or an off mixture has likely already damaged or is currently overworking the converter — fix those before touching the catalyst.

Inspect for exhaust leaks ahead of the converter

Look for: A leak before the converter (cracked manifold, loose flange, blown gasket) lets in outside air that confuses the downstream O2 sensor into reading as if the catalyst isn't working, even when it is.

Compare upstream and downstream O2 sensor activity

Look for: A healthy converter makes the downstream sensor read flat and steady while the upstream sensor swings actively. If the downstream sensor switches nearly as fast as the upstream one, the converter isn't storing oxygen properly and is likely worn out.

Check converter temperature and performance

Look for: A converter outlet noticeably cooler than expected relative to its inlet under load suggests little catalytic reaction is happening inside — supports a genuinely failed converter rather than a false reading.

Hold off on replacing the converter until root causes are ruled out

Look for: If misfires, exhaust leaks, or fuel trim issues caused this code, a new converter will fail again quickly — confirm those are clean first.

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Matching symptoms

Check Engine Light