P0420 on a 2016 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 2016 Honda Accord

Ranked from 591 NHTSA owner complaints filed for this vehicle.

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1
Catalyst damage/overheating from chronic misfire (rod-bearing/timing-related engine failures)
High severity
Owners report rod-bearing/connecting-rod engine failures with knocking, which drives excess unburned fuel into the exhaust and can overheat and damage the catalyst.
Reported in 2 NHTSA complaints
Parts:catalytic converter
2
Catalyst contamination from oil consumption
Medium severity
Owners report needing to add oil every 1,000-3,000 miles; combusted oil ash passing through the exhaust can coat and poison the catalyst washcoat over time.
Reported in 2 NHTSA complaints
Parts:catalytic converter
3
Worn or aged catalytic converter
Medium severity
This vehicle has a prior report of catalytic converter replacement; normal thermal aging reduces oxygen storage capacity below the efficiency threshold over time.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Failing downstream (post-cat) O2 sensor
Medium severity
A lazy or degraded rear oxygen sensor reads exhaust nearly identical to the front sensor, tripping a false efficiency code even with a good converter.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the rear O2 sensor
Low severity
A leaking manifold, gasket, or flex-pipe ahead of the converter draws in outside air, skewing the rear sensor's efficiency reading.
Common cause
6
Fuel delivery fault (weak fuel pump) causing rich/lean swings that degrade the catalyst
Low severity
Fuel pump pressure irregularities documented across this model can cause the mixture to swing rich or lean, accelerating catalyst wear over time.
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