P0420 on a 2010 Chevrolet Impala
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 Impala
Ranked from 441 NHTSA owner complaints filed for this vehicle.
- 1
Oil consumption contaminating the catalytic converter
High severityTwo complaints describe significant oil consumption/burning on this vehicle; oil passing into the exhaust stream coats and degrades the catalyst over time.
Reported in 2 NHTSA complaintsParts:catalytic converter - 2
Secondary air injection (AIR) check/shutoff valve failure
Medium severityOne owner had this valve fail three separate times, a GM-acknowledged corrosion-durability issue on this engine family; a malfunctioning AIR system disrupts catalyst warm-up and efficiency monitoring.
Reported in 1 NHTSA complaint - 3
Exhaust leak at the catalytic converter/tailpipe joint
Medium severityA reported loose tailpipe at the back of the catalytic converter is a direct exhaust leak near the monitored sensor location, which can trigger an efficiency-below-threshold code.
Reported in 1 NHTSA complaint - 4
Worn or degraded catalytic converter (age/mileage)
Medium severityCatalyst media naturally degrades with age and thermal cycling, a standard cause of reduced efficiency.
Common causeParts:catalytic converter - 5
Failing downstream oxygen sensor
Medium severityA slow or inaccurate downstream sensor can misreport true catalyst efficiency.
Common causeParts:oxygen sensor - 6
Long-term engine misfire degrading the catalyst substrate
High severityUnburned fuel from a sustained misfire overheats and can melt catalyst media, a standard secondary cause.
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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