P0420 on a 2011 Toyota Corolla

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 2011 Toyota Corolla

Ranked from 590 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalyst fouled by an oil-burning engine
High severity
Three owners reported significant oil consumption (some tied to a misfire/check-engine event needing cylinder work); oil burned through worn rings or valve seals coats and degrades the catalyst over time.
Reported in 3 NHTSA complaints
2
Prior or ongoing engine misfires damaging the catalyst substrate
High severity
One owner's random-misfire code (P0303) traced to fuel-flooded, cleaned spark plugs is exactly the kind of unburned-fuel event that overheats and can melt catalyst substrate over time.
Reported in 2 NHTSA complaints
3
Worn or degraded catalytic converter
Medium severity
The most common reason a catalyst falls below the efficiency threshold is simple age/mileage degradation of the substrate.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) O2 sensor
Low severity
A degraded downstream sensor can misreport catalyst performance, causing a false efficiency-below-threshold code.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Low severity
A manifold, flange, or pipe leak upstream of the sensor lets in outside air that skews the efficiency reading.
Common cause
6
Coolant contamination of the catalyst (e.g., from an internal leak)
High severity
Coolant burned through a failing gasket or seal coats the catalyst substrate and permanently reduces its efficiency, separate from an oil-burning issue.
Common cause

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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