P0420 on a 2017 Chevrolet Traverse
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 2017 Chevrolet Traverse
Ranked from 358 NHTSA owner complaints filed for this vehicle.
- 1
Worn or degraded catalytic converter
High severityNormal aging/mileage reduces the converter's oxygen storage capacity; one owner's MIL was traced directly to a plugged/deteriorated catalytic converter.
Reported in 1 NHTSA complaintParts:catalytic converter - 2
Prior untreated cylinder misfire damaging the catalyst substrate
High severityThis vehicle's own misfire complaint (spark plugs/boots replaced for a hesitation/jerk condition) is exactly the kind of unburned-fuel event that can overheat and melt a catalyst over time.
Reported in 1 NHTSA complaint - 3
Oil contamination of the catalyst from an internal engine oil leak
Medium severityOne owner's engine oil pan was found faulty and leaking; oil reaching the exhaust can coat and poison the catalyst, dropping its efficiency.
Reported in 1 NHTSA complaint - 4
Failing downstream (post-catalyst) oxygen sensor
Medium severityA slow or biased downstream sensor makes an otherwise healthy converter look inefficient to the PCM.
Common causeParts:oxygen sensor - 5
Exhaust leak upstream of the sensors
Low severityA leak before the converter dilutes the exhaust sample with outside air, throwing off the efficiency comparison.
Common causeParts:exhaust gasket - 6
Loose or damaged exhaust hangers/connections
Low severityExhaust leaks from loose joints ahead of the sensors are a common cause of a false low-efficiency 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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