P0420 on a 2023 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 2023 Honda Cr V

Ranked from 506 NHTSA owner complaints filed for this vehicle.

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1
Oil/coolant contamination of the catalyst from head gasket failure
High severity
The same head-gasket failure documented repeatedly on this generation CR-V lets coolant reach the combustion chamber and pass through the exhaust, coating and poisoning the catalyst; several of these owners needed an engine replacement or rebuild.
Reported in 6 NHTSA complaints
2
Catalyst damage from a chronic engine misfire
High severity
A mechanical failure severe enough to shake the block (broken piston rod, misfiring cylinder) sends unburned fuel into the exhaust, overheating and melting the catalyst substrate over time.
Reported in 2 NHTSA complaints
3
Worn or aged catalytic converter
Medium severity
Catalyst efficiency naturally degrades with mileage and heat cycling until it falls below the OBD-II threshold.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A sluggish post-catalyst O2 sensor can read like a dead converter even when the catalyst is still good.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leaking gasket or joint before the sensor dilutes the sample with outside air, mimicking a failed catalyst.
Common cause
6
Worn spark plugs or failing ignition coil causing intermittent misfire
Medium severity
Even a mild, non-obvious misfire sends extra unburned fuel to the catalyst over time, accelerating efficiency loss.
Common cause
Parts:spark plugs ignition coil

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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