P0420 on a 2016 Chevrolet Malibu Eco

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 Eco

Ranked from 810 NHTSA owner complaints filed for this vehicle.

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1
Catalyst substrate damage from prolonged, unresolved misfires
High severity
Unburned fuel from repeated cylinder misfires (cracked pistons documented on this vehicle) overheats and melts the catalyst substrate.
Reported in 4 NHTSA complaints
Parts:catalytic converter
2
Catalyst contamination from blown head gasket (oil/coolant intrusion)
High severity
Oil or coolant reaching the catalyst poisons the substrate and drops efficiency; this vehicle's grounding complaints document a blown head gasket with no oil in the engine.
Reported in 2 NHTSA complaints
Parts:catalytic converter head gasket
3
Exhaust leak ahead of the catalyst/downstream sensor
Medium severity
A leaking turbo clamp or exhaust joint upstream of the converter admits extra oxygen, skewing the efficiency reading; diagnosed on this vehicle as a turbo clamp/exhaust failure.
Reported in 1 NHTSA complaint
Parts:exhaust clamp exhaust gasket
4
Worn/degraded catalytic converter (age and mileage)
Medium severity
Catalysts naturally lose efficiency with age and high mileage until they fall below the OBD-II threshold.
Common cause
Parts:catalytic converter
5
Failing downstream oxygen sensor
Low severity
A sluggish or biased rear O2 sensor can mimic a degraded catalyst reading even when the converter itself is fine.
Common cause
Parts:oxygen sensor
6
General exhaust system leaks
Low severity
Loose flange gaskets or corroded joints anywhere ahead of the rear sensor let in extra air and trip the code.
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