P0420 on a 2015 Honda Accord Plug In Hybrid

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 2015 Honda Accord Plug In Hybrid

Ranked from 609 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalyst damage from prolonged untreated cylinder misfire (VCM/rod-bearing defect)
High severity
Several owners' engines suffered knocking and cylinder failure under Honda campaign 15V121000; unburned fuel from a misfiring cylinder is a well-known way to overheat and damage a catalyst's substrate.
Reported in 5 NHTSA complaints
Parts:catalytic converter
2
Worn or failing catalytic converter
High severity
One owner had the catalytic converter directly diagnosed as faulty by a mechanic after the check engine light appeared.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Oil contamination of the catalyst
Medium severity
One owner had a persistent oil filter/leak issue; oil reaching the exhaust stream can coat and poison the catalyst.
Reported in 1 NHTSA complaint
Parts:oil filter housing catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A slow or biased post-catalyst sensor can make a healthy converter appear inefficient.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the sensors
Low severity
A leak before the converter dilutes the sample and skews the efficiency comparison.
Common cause
Parts:exhaust gasket
6
Natural aging / high-mileage substrate wear
Medium severity
Catalyst efficiency gradually declines with age and mileage even without another root cause.
Common cause
Parts:catalytic converter

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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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light