P0420 on a 2018 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 2018 Honda Cr V
Ranked from 2,094 NHTSA owner complaints filed for this vehicle.
- 1
Catalyst contamination from head gasket coolant/oil leak
High severityWidely reported head gasket failures on this engine let coolant and combustion byproducts reach the exhaust, coating and poisoning the catalyst's active surface and dropping its efficiency below threshold.
Reported in 11 NHTSA complaints - 2
Turbocharger oil seal failure contaminating the catalyst
Medium severityOwners had a failed turbo diagnosed after head gasket repair didn't resolve the issue; oil passing into the exhaust stream via a failing turbo will foul the catalyst.
Reported in 2 NHTSA complaints - 3
Catalyst substrate damage from prolonged, untreated cylinder misfires
Medium severityRaw unburned fuel from ongoing misfires (documented on this engine) overheats and can melt the catalyst substrate, causing efficiency to drop.
Reported in 2 NHTSA complaintsParts:catalytic converter - 4
Worn or aged catalytic converter
Medium severityCatalyst efficiency naturally degrades with age/mileage; a scan-tool comparison of upstream vs. downstream O2 activity confirms a worn converter.
Common causeParts:catalytic converter - 5
Failing downstream oxygen sensor
Low severityA lazy or degraded post-cat O2 sensor can mimic reduced catalyst efficiency; compare its switching pattern to the upstream sensor.
Common causeParts:oxygen sensor - 6
Exhaust leak upstream of the downstream oxygen sensor
Low severityA leak ahead of the rear sensor allows outside air in, skewing its readings to falsely indicate low catalyst 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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