P0420 on a 2013 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 2013 Nissan Altima

Ranked from 2,304 NHTSA owner complaints filed for this vehicle.

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1
Failed or melted catalytic converter
High severity
Multiple owners had shops directly diagnose a failed/melted catalytic converter, including one whose converter was found melted inside on inspection and another whose aftermarket replacement converter failed again 13 months later.
Reported in 4 NHTSA complaints
Parts:catalytic converter
2
Faulty mass airflow (MAF) sensor causing incorrect fuel trim
Medium severity
An owner's check engine light was tied to a Nissan recall for the mass airflow sensor; a bad MAF skews the air/fuel ratio, which over time can degrade converter efficiency.
Reported in 1 NHTSA complaint
Parts:mass airflow sensor
3
Valve cover gasket oil leak fouling ignition components and exhaust
Medium severity
One owner found oil pooling around the coils and spark plugs from a leaking valve cover gasket, with burning oil smell through the exhaust/vents — oil-fouled plugs cause misfires and oil burning can contaminate the converter.
Reported in 1 NHTSA complaint
Parts:valve cover gasket
4
Failed downstream oxygen sensor
Medium severity
A slow or biased rear O2 sensor can make the PCM flag the converter as inefficient even when it's fine; one owner's converter replacement was paired with new O2 sensors.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leaking manifold gasket or pipe joint lets outside air skew the sensor reading used to judge cat efficiency.
Common cause
Parts:exhaust manifold gasket
6
Long-term ignition misfire damaging the converter
High severity
Worn coils, plugs, or injectors causing repeated misfire dump unburned fuel into the exhaust, overheating and melting the catalyst substrate.
Common cause
Parts:ignition coil spark plug

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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