P0420 on a 2011 Toyota Camry

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 Camry

Ranked from 620 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalytic converter contamination/degradation from chronic oil consumption (2AZ-FE/2AR-FE oil-ring defect)
High severity
The same widely-reported oil-burning defect on this generation Camry (owners adding a quart every few hundred to ~2,000 miles) coats and poisons the catalyst with oil ash over time, and one owner was separately told the exhaust needed replacement, degrading the efficiency the P0420 monitor checks.
Reported in 14 NHTSA complaints
Parts:catalytic converter
2
Exhaust component leak/damage upstream of the sensors
Low severity
One owner was told the exhaust needed replacement while diagnosing an oil-consumption-related complaint, consistent with a leak upstream of the catalyst skewing the O2 readings.
Reported in 1 NHTSA complaint
3
Worn/aged catalytic converter
Medium severity
Catalyst efficiency degrades naturally with heat-cycling and mileage, the default explanation once oil contamination and misfires are ruled out.
Common cause
Parts:catalytic converter
4
Failing downstream (post-cat) O2 sensor
Low severity
A sluggish downstream sensor reads too similarly to the upstream sensor, making the PCM conclude efficiency has dropped even if the converter is fine.
Common cause
Parts:oxygen sensor
5
Long-term multi-cylinder misfires melting the substrate
High severity
Unburned fuel from repeated misfires ignites inside the converter, overheating and eventually melting the honeycomb substrate.
Common cause
6
Aftermarket or non-OEM catalytic converter
Low severity
A lower-flow or mismatched replacement converter commonly fails the efficiency monitor even when mechanically sound.
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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light