P0420 on a 2010 Chevrolet Hhr
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 2010 Chevrolet Hhr
Ranked from 515 NHTSA owner complaints filed for this vehicle.
- 1
Worn/failed catalytic converter requiring replacement
Medium severityOne owner had the catalytic converter (and front axle) replaced after an overheating/misfire-type event on this same vehicle.
Reported in 1 NHTSA complaintParts:catalytic converter - 2
Exhaust leak upstream of or near the converter
Medium severityA dealer diagnosed an exhaust leak and installed a 'direct-fit' converter after a stalling complaint, at over $600.
Reported in 1 NHTSA complaint - 3
Failing downstream (rear) O2 sensor
Medium severityA lazy or dead rear sensor reads the same as a dead catalyst to the PCM.
Common causeParts:oxygen sensor - 4
Oil or coolant contamination of the catalyst
Medium severityA leaking valve seal/head gasket coats the substrate and kills conversion efficiency.
Common cause - 5
Long-term/repeated misfires overheating the catalyst substrate
High severityUnburned fuel ignites in the converter and melts the honeycomb.
Common cause - 6
Damaged or corroded exhaust component causing a leak near the sensors
Low severityRust-through ahead of the rear O2 sensor skews its reading.
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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