P0420 on a 2017 Chevrolet Equinox

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 2017 Chevrolet Equinox

Ranked from 270 NHTSA owner complaints filed for this vehicle.

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1
Chronic oil consumption (worn piston rings, 2.4L Ecotec) contaminating the catalyst
High severity
Multiple owners report excessive oil use with the check-engine/reduced-power light on and dealer-confirmed piston-ring wear; burned oil coats and degrades the catalyst over time.
Reported in 3 NHTSA complaints
Parts:catalytic converter piston ring set
2
Exhaust leak (cracked exhaust pipe/manifold) ahead of the sensors
High severity
Two owners on this exact vehicle had a dealer-confirmed exhaust-pipe leak severe enough to enter the cabin; an upstream leak dilutes exhaust gas and commonly trips a false catalyst-efficiency code.
Reported in 2 NHTSA complaints
Parts:exhaust pipe exhaust manifold gasket
3
Worn or degraded catalytic converter
High severity
High-mileage catalyst substrate loses oxygen-storage capacity over time; confirm with a rear O2/AFR sensor switching-rate test versus the upstream sensor.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) O2 sensor
Medium severity
A lazy or biased rear O2 sensor can mimic reduced catalyst efficiency even when the converter itself is fine; compare live rear sensor activity to the front sensor.
Common cause
Parts:oxygen sensor
5
Engine misfire (worn spark plugs/ignition coils) overloading the catalyst
Medium severity
Unburned fuel dumped into the exhaust from a misfire overheats and chemically poisons the catalyst; check for stored misfire codes alongside P0420.
Common cause
Parts:spark plug ignition coil
6
Rich or lean fuel trim from a vacuum leak or dirty MAF sensor
Low severity
Sustained rich or lean operation prevents the catalyst from cycling in its efficient window; check long-term fuel trims.
Common cause
Parts:mass airflow sensor

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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