P0420 on a 2015 Chevrolet Suburban 1500
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 2015 Chevrolet Suburban 1500
Ranked from 428 NHTSA owner complaints filed for this vehicle.
- 1
Chronic misfires (AFM lifter failures) damaging the converter substrate
Medium severityThis truck's well-documented Active Fuel Management lifter failures dump unburned fuel into the exhaust over time, a known way to overheat and damage a catalytic converter and drop its measured efficiency.
Reported in 7 NHTSA complaints - 2
Repeated catalytic converter failure/replacement
High severityOne owner reports replacing the converters three separate times (roughly 30k, 80k, and 165k miles) with a literal P0430 code cited directly, evidence of chronic converter wear on this truck.
Reported in 1 NHTSA complaintParts:catalytic converter - 3
Catalytic converter theft or removal
High severityA separate owner discovered the converter cut out and missing entirely, with a pipe substituted in its place; a missing or gutted converter reads as zero catalyst efficiency immediately.
Reported in 1 NHTSA complaintParts:catalytic converter - 4
Failing downstream (post-catalyst) O2 sensor
Low severityA lazy or skewed downstream sensor reads the exhaust as dirtier than it is, tripping the efficiency monitor even with a healthy converter.
Common causeParts:oxygen sensor - 5
Exhaust leak ahead of the downstream sensor
Low severityA leaking gasket or cracked pipe upstream of the rear sensor lets outside air skew the reading, mimicking converter wear.
Common cause - 6
Oil or coolant contamination of the catalyst
High severityConsumed engine oil or a coolant leak burning through the combustion chamber coats the catalyst bed and kills its efficiency.
Common causeParts:catalytic converter
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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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