P0420 on a 2020 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 2020 Honda Odyssey

Ranked from 430 NHTSA owner complaints filed for this vehicle.

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1
Chronic misfire from connecting rod bearing wear damaging the catalyst
High severity
Multiple owners report engine knocking/ticking and confirmed rod bearing failure on this V6 (NHTSA Action PE25008); the resulting rough running/misfire can overheat and damage the catalyst substrate over time.
Reported in 4 NHTSA complaints
Parts:connecting rod bearings catalytic converter
2
Low fuel pressure from a failing high-pressure fuel pump skewing the mixture
Medium severity
One owner's loud fuel pump tripped a low-fuel-pressure code; an incorrect air/fuel mixture reduces catalyst conversion efficiency.
Reported in 1 NHTSA complaint
Parts:high-pressure fuel pump
3
Worn or failing catalytic converter
High severity
Catalysts degrade with age/mileage and heat cycling, reducing conversion efficiency below the OBD threshold.
Common cause
Parts:catalytic converter
4
Failing downstream (rear) oxygen sensor
Medium severity
A degraded rear O2 sensor can mimic reduced catalyst efficiency; compare upstream/downstream switching rates.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leak before the rear sensor admits outside air, skewing the efficiency calculation.
Common cause
6
Oil contamination of the catalyst
Medium severity
Oil reaching the exhaust from an internal engine leak (e.g., the bearing wear noted above) coats and poisons the catalyst, reducing its efficiency.
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