P0420 on a 2021 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 2021 Honda Cr V
Ranked from 311 NHTSA owner complaints filed for this vehicle.
- 1
Catalyst damage/fouling from head-gasket coolant intrusion and chronic misfire (documented CR-V 1.5T defect)
High severityOwners of this exact vehicle document coolant entering the cylinders from a failed head gasket alongside oil dilution and recurring misfires, both of which can overheat or coat the catalyst and drop its efficiency.
Reported in 5 NHTSA complaints - 2
Fuel pump/injector failure contributing to catalyst damage
Medium severityOne owner was told directly that an unrepaired fuel pump leak on this model can lead to needing the catalytic converter replaced, and this vehicle also has documented injector-related misfire events (including a cylinder-1 misfire and a full fuel-injector replacement) that can pass unburned fuel into the exhaust.
Reported in 3 NHTSA complaints - 3
Worn or aged catalytic converter
Medium severityNormal high-mileage catalyst aging reduces oxygen storage capacity below the efficiency threshold.
Common causeParts:catalytic converter - 4
Failing downstream oxygen sensor
Medium severityA lazy or biased rear sensor can mimic poor catalyst efficiency even with a healthy converter.
Common causeParts:oxygen sensor - 5
Exhaust leak ahead of the catalyst or sensors
Medium severityA leak lets in outside air that skews the sensor signals the ECU uses to judge catalyst efficiency.
Common causeParts:exhaust gasket - 6
Failing upstream air-fuel ratio sensor
Medium severityA drifting front sensor can feed the ECU a bad baseline, making a healthy catalyst look inefficient.
Common causeParts:air-fuel ratio sensor
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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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