P0420 on a 2020 Honda Pilot
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 Pilot
Ranked from 472 NHTSA owner complaints filed for this vehicle.
- 1
Worn or failed catalytic converter
High severityAn owner's emissions light led to a dealer diagnosis that the catalytic converter itself needed replacement.
Reported in 1 NHTSA complaintParts:catalytic converter - 2
O2 sensor lean-bias / fuel-injector wear pattern documented on this engine
Medium severityOne owner's scan showed a bank-2 sensor stuck lean together with the catalyst-efficiency code, matching Honda's own service bulletins tying P030x/P219x/P0430 to worn injectors on this 3.5L engine.
Reported in 1 NHTSA complaint - 3
Failing downstream (post-catalyst) O2 sensor
Medium severityA degraded rear sensor can mimic reduced catalyst efficiency even when the converter itself is still good.
Common cause - 4
Exhaust leak upstream of the sensor
Medium severityAn exhaust leak ahead of the catalyst or sensor lets in outside air, corrupting the efficiency comparison.
Common cause - 5
Oil or coolant contamination of the catalyst
High severityOil or coolant reaching the exhaust stream chemically poisons the catalyst coating and drops its efficiency.
Common cause - 6
Long-term misfires melting or fouling the substrate
High severityUnburned fuel repeatedly reaching a hot converter damages the honeycomb substrate 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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