P0420 on a 2017 Nissan Altima

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 Nissan Altima

Ranked from 384 NHTSA owner complaints filed for this vehicle.

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1
Failing oxygen sensor(s) requiring repeat replacement
High severity
Three separate complaints on this model year describe an O2 sensor being replaced and the warning light returning, including one that resolved to a scanned P0130 O2 sensor code — a recurring, vehicle-specific pattern rather than a one-off.
Reported in 3 NHTSA complaints
Parts:oxygen sensor
2
Misfire-related substrate damage (spark plugs/ignition coils/head gasket)
High severity
One complaint describes spark plug and ignition coil replacement followed by a failed head gasket and dead cylinders — unburned fuel and combustion byproducts from that failure sequence are consistent with cat-killing contamination.
Reported in 1 NHTSA complaint
Parts:spark plugs ignition coils
3
Oil consumption/contamination reaching the catalyst
Medium severity
The same complaint that reported failing cylinders also noted continually low oil level — oil burned past worn rings/valve seals coats and poisons the catalyst.
Reported in 1 NHTSA complaint
4
Worn/aged catalytic converter
High severity
Baseline high-mileage catalyst degradation; confirm with a cat-efficiency scan tool test rather than assuming from the O2 code alone.
Common cause
Parts:catalytic converter
5
Exhaust leak ahead of the catalyst or O2 sensors
Medium severity
A leaking joint or gasket ahead of the sensors lets in outside air that skews the efficiency comparison.
Common cause
Parts:exhaust manifold gasket
6
Aftermarket or incorrect-spec replacement O2 sensor
Low severity
A sensor that isn't OEM-equivalent can read incorrectly even when new, re-triggering the code shortly after a repair.
Common cause
Parts:oxygen 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