P0420 on a 2020 Honda Cr V

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 Cr V

Ranked from 335 NHTSA owner complaints filed for this vehicle.

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1
Catalyst contamination from coolant intrusion (failing head gasket)
High severity
Multiple owners in this batch had a dealer-diagnosed head gasket allowing coolant into a cylinder (accompanied by a 'Cooling System Failure' message); coolant burned through the engine coats and poisons the catalytic converter.
Reported in 4 NHTSA complaints
Parts:catalytic converter head gasket
2
Catalyst damage from prior sustained misfires/oil dilution
Medium severity
This engine's documented fuel-into-oil dilution issue and related misfire events (flashing CEL, limp mode) can send unburned fuel/oil residue through the exhaust, overheating and fouling the catalyst over time.
Reported in 2 NHTSA complaints
3
Worn/failed catalytic converter (age/mileage)
Medium severity
Catalyst efficiency degrades naturally with age and mileage independent of any other fault.
Common cause
Parts:catalytic converter
4
Failing downstream (rear) O2/air-fuel sensor
Low severity
A degraded rear sensor can mimic poor catalyst efficiency; compare its switching pattern to the front sensor's.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Low severity
A leaking manifold or pipe joint dilutes the sample the downstream sensor reads, mimicking a failed catalyst.
Common cause
6
Loose or corroded O2 sensor wiring/connector
Low severity
A corroded or loose connector at either O2 sensor produces an erratic signal that the PCM can misread as a catalyst efficiency fault; inspect and clean connectors.
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.

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

Matching symptoms

Check Engine Light