P0420 on a 2022 Honda Civic Hatchback
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 2022 Honda Civic Hatchback
Ranked from 880 NHTSA owner complaints filed for this vehicle.
- 1
Prolonged/unresolved misfire damaging the catalyst
High severityA leaking fuel injector on this vehicle caused fuel wash-down, fouled spark plugs, and a blown head gasket — the kind of sustained raw-fuel event that overheats and damages a catalyst.
Reported in 1 NHTSA complaint - 2
Oil leak contaminating the exhaust/catalyst
Medium severityThis vehicle developed a rear-main-seal oil leak that owner reports was dripping directly onto the hot exhaust, a documented way to foul/degrade a catalytic converter.
Reported in 1 NHTSA complaint - 3
Internal engine damage/metal debris affecting combustion and exhaust output
Low severityA separate 2022 Civic on this same engine family suffered an early engine failure from manufacturing debris, which can also carry into and damage the catalyst.
Reported in 1 NHTSA complaint - 4
Worn/aged catalytic converter (mileage-related)
Medium severityCatalysts lose efficiency with age and mileage even without a specific failure event.
Common causeParts:catalytic converter - 5
Failing downstream oxygen sensor
Medium severityA degraded rear O2 sensor can report reduced catalyst efficiency even when the converter itself is still serviceable.
Common causeParts:oxygen sensor - 6
Exhaust leak upstream of the sensors
Low severityA leak in the manifold or front pipe dilutes the exhaust sample and can present as reduced 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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