P0420 on a 2016 Chevrolet Malibu

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 2016 Chevrolet Malibu

Ranked from 859 NHTSA owner complaints filed for this vehicle.

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1
Oil contamination of the catalyst from an internal engine oil leak
Medium severity
Two complaints describe internal oil leaks — a failed main seal requiring engine replacement, and a lubrication-system failure with fluid loss; burned oil reaching the exhaust is a recognized way of poisoning a catalyst's efficiency.
Reported in 2 NHTSA complaints
Parts:catalytic converter
2
Catalyst overload from Stochastic Pre-Ignition misfires (cracked piston)
High severity
A complaint states directly that a cracked piston caused excessive oil consumption and misfires that 'overloaded the converter causing it to fail,' tied to GM's SPI recall/monitoring campaign on the 1.5L LFV engine, a documented cause of catalyst failure on this engine.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Fuel injector leak fouling/overheating the catalyst
Medium severity
A complaint describes an injector O-ring/backing-ring failure leaking fuel onto a hot cylinder and causing misfire; raw fuel reaching the exhaust from a leak like this can overheat and damage the catalyst.
Reported in 1 NHTSA complaint
Parts:fuel injector catalytic converter
4
Worn or degraded catalytic converter (age/mileage)
Medium severity
Catalyst efficiency declines gradually with heat cycling and mileage until it drops below the OBD monitor's threshold.
Common cause
Parts:catalytic converter
5
Failing downstream oxygen sensor
Medium severity
A sluggish rear O2 sensor can mimic a failing catalyst by tracking the upstream sensor's switching too closely.
Common cause
Parts:oxygen sensor
6
Exhaust leak ahead of the downstream O2 sensor
Medium severity
A leak between the catalyst and the rear sensor introduces fresh air that skews the efficiency comparison the monitor relies on.
Common cause
Parts:exhaust gasket

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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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Matching symptoms

Check Engine Light