P0420 on a 2018 Honda Odyssey

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 Odyssey

Ranked from 940 NHTSA owner complaints filed for this vehicle.

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1
Oil entering the combustion chamber (rod-bearing/internal wear) contaminating the catalyst
High severity
This platform has a documented pattern of rod-bearing wear with oil finding its way into cylinders and metal shavings in the oil; burned oil passing through the exhaust coats and degrades the catalyst.
Reported in 2 NHTSA complaints
2
Sustained cylinder misfire (e.g., worn spark plugs) overheating the catalyst
High severity
One owner had spark plugs replaced under a Honda warranty/recall repair for a recurring check-engine light; roughly six months later both the front and rear catalytic converters were diagnosed failed, which the dealer tied to the earlier misfire condition cooking the substrate.
Reported in 1 NHTSA complaint
Parts:catalytic converter spark plugs
3
Worn or aged catalytic converter
High severity
Simple mileage-related degradation of the substrate/washcoat reduces conversion efficiency below the OBD-II threshold.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A lazy or biased downstream sensor can make a healthy catalyst look inefficient, or mask/mimic a real fault; compare upstream vs downstream sensor switching on a scan tool.
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 draws in outside air, disturbing the sensor's ability to confirm proper catalyst function.
Common cause
6
Coolant contamination of the catalyst from an internal coolant leak
High severity
A leaking head gasket or intake gasket that lets coolant into the exhaust stream will quickly poison and destroy the catalyst.
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