P0420 on a 2016 Chevrolet Impala Eco Eassist

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 Impala Eco Eassist

Ranked from 786 NHTSA owner complaints filed for this vehicle.

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1
Catalyst damage from a prolonged cylinder misfire (cracked piston)
High severity
A complaint documents a confirmed cylinder misfire traced to a cracked piston described as a known issue for this model-year engine, and a related complaint on the same engine family logged a P0302 misfire eventually attributed to the pistons; sustained misfires like these dump unburned fuel into the exhaust and are a common way of overheating the catalyst.
Reported in 2 NHTSA complaints
2
Oil contamination of the catalytic converter from head-gasket failure
Medium severity
A complaint describes the main head gasket failing on these vehicles and leaking oil; burned oil passing through the exhaust can coat and poison the catalyst, lowering its measured efficiency.
Reported in 1 NHTSA complaint
Parts:head gasket catalytic converter
3
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
4
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
5
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
6
Aftermarket or incorrect-spec replacement catalytic converter
Low severity
A converter that isn't OE-equivalent may never register as efficient enough to the OBD monitor, even when functioning.
Common cause
Parts:catalytic converter

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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