P0420 on a 2013 GMC Sierra 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 2013 GMC Sierra 1500
Ranked from 167 NHTSA owner complaints filed for this vehicle.
- 1
Excessive oil consumption fouling/contaminating the catalytic converter (worn piston rings / oil-control issues documented on this 5.3L Vortec)
High severityOne owner reports excessive oil consumption in normal operation and another had faulty piston rings diagnosed requiring engine replacement; burned oil passing through the exhaust coats and poisons the catalyst.
Reported in 2 NHTSA complaints - 2
AFM (Active Fuel Management) lifter/lifter-spring failure causing cylinder misfire that can damage the catalyst over time
Medium severityOwners report broken fuel-management lifter springs causing the engine to "miss" and idle rough, and a separate lifter-clatter complaint tied to the AFM system, a known 5.3L failure mode; sustained misfire overheats and can melt the catalyst substrate.
Reported in 2 NHTSA complaintsParts:valve lifters - 3
Naturally worn/aged catalytic converter
Medium severityEfficiency degrades gradually with mileage/heat cycling even without an underlying engine defect; confirm with a rear O2 sensor waveform comparison to the upstream sensor.
Common causeParts:catalytic converter - 4
Failing downstream (rear) oxygen sensor
Medium severityA lazy or biased rear O2 sensor can mimic reduced catalyst efficiency; compare switching rate to the front sensor before condemning the converter.
Common causeParts:oxygen sensor - 5
Exhaust leak upstream of the rear O2 sensor
Medium severityA leak before the sensor lets in fresh air, skewing the rear sensor's reading and falsely signaling low catalyst efficiency; inspect manifold gaskets and pipe joints.
Common cause - 6
Ignition coil or spark plug misfire
Medium severityAny chronic misfire dumps raw fuel into the exhaust and overheats the catalyst; check for stored/pending misfire codes.
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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