P0420 on a 2021 Honda Cr V Hybrid

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 2021 Honda Cr V Hybrid

Ranked from 410 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalytic converter contamination from chronic coolant intrusion via head gasket failure
High severity
The same documented head-gasket failure (NHTSA PE25016) that lets coolant into the cylinders also sends coolant/combustion byproducts through the exhaust, poisoning the catalyst and dropping its efficiency.
Reported in 7 NHTSA complaints
Parts:catalytic converter head gasket
2
Catalyst substrate damage from repeated/prolonged cylinder misfires
High severity
The recurring flashing-check-engine, loss-of-power misfire events reported across this fleet dump unburned fuel into the exhaust, which can overheat and melt the catalyst over time.
Reported in 6 NHTSA complaints
Parts:catalytic converter
3
Worn/aged catalytic converter (normal wear-out)
Medium severity
Catalysts lose efficiency with age/mileage independent of any other fault.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A degraded rear O2 sensor can falsely indicate reduced catalyst efficiency even on a healthy converter.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the sensors
Low severity
Any leak ahead of the catalyst/sensors introduces outside air that skews the efficiency calculation.
Common cause
6
Fuel injector fault upsetting air-fuel ratio
Medium severity
A leaking or clogged injector (repeatedly replaced on this model) can push the mixture rich or lean enough to affect catalyst efficiency readings.
Common cause
Parts:fuel injector

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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