P0420 on a 2015 GMC Yukon

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 GMC Yukon

Ranked from 546 NHTSA owner complaints filed for this vehicle.

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1
Untreated misfire from AFM/DFM lifter collapse overheating the converter
High severity
This 6.2L generation has widespread Active Fuel Management lifter/camshaft collapse in this complaint set causing rough idle and shaking; one owner reported a flashing check-engine light (the classic sign of an active misfire that cooks the catalyst) alongside the rough-running complaints.
Reported in 1 NHTSA complaint
2
Oil consumption fouling the catalyst
High severity
One owner reported heavy oil consumption with metal debris in the oil from lifter/AFM failure; oil burned through a failing valvetrain can coat and prematurely degrade the catalyst.
Reported in 1 NHTSA complaint
3
Worn/degraded catalytic converter
Medium severity
Normal aging of the ceramic substrate reduces oxygen storage capacity; confirm with a post-cat O2 sensor waveform that tracks too closely with the pre-cat sensor.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A lazy or biased rear O2 sensor reports poor efficiency even on a healthy converter; compare rear sensor switching activity to the front sensor on a scan tool.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of or near the downstream O2 sensor
Medium severity
A leaking manifold gasket or pipe joint lets outside air reach the rear sensor, mimicking low catalyst efficiency.
Common cause
6
Coolant intrusion into a cylinder (e.g., head gasket or intake gasket leak) contaminating the catalyst
High severity
Coolant burned in the combustion chamber deposits silicates on the catalyst substrate and destroys efficiency quickly; check for coolant loss with no visible external leak.
Common cause
7
Damaged or collapsed catalyst substrate from a prior backfire/engine event
Medium severity
A physically broken honeycomb (rattling on start, exhaust restriction) will fail the efficiency monitor outright; verify with a back-pressure test or visual/CT scan of the converter.
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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light