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P0420

P0420 on a 2020 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 2020 GMC Sierra 1500

Ranked from 438 NHTSA owner complaints filed for this vehicle.

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  1. 1

    Catalyst damage from chronic cylinder misfire

    High severity

    The dominant failure mode in this batch is AFM/DFM lifter collapse causing rough running and misfire (e.g. odiNumbers 11728804, 11745292); sustained unburned fuel reaching the converter overheats and damages the substrate.

    Reported in 2 NHTSA complaints
  2. 2

    Worn or failed catalytic converter

    High severity

    This batch documents an owner whose catalytic converter (and O2 sensors) needed replacement on this exact vehicle shortly after an engine replacement (odiNumber 11748595).

    Reported in 1 NHTSA complaint
    Parts:catalytic converter
  3. 3

    Oil consumption/contamination reaching the exhaust

    Medium severity

    This batch documents an owner reporting white smoke from the exhaust alongside a burning-oil smell on this same engine family (odiNumber 11734729), consistent with oil fouling the catalyst.

    Reported in 1 NHTSA complaint
  4. 4

    Failing downstream O2 sensor

    Medium severity

    A degraded sensor can report a false efficiency reading even with a healthy converter.

    Common cause
    Parts:oxygen sensor
  5. 5

    Exhaust leak ahead of the downstream sensor

    Medium severity

    Allows outside air into the exhaust stream, skewing the reading toward 'inefficient.'

    Common cause
  6. 6

    Normal high-mileage catalyst aging

    Low severity

    Efficiency naturally declines with age and mileage.

    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

1

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.

2

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.

3

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.

4

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.

5

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