P0420 on a 2011 Toyota Sienna

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 Sienna

Ranked from 610 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalyst oil contamination from oil cooler hose/pipe failure
High severity
This generation Sienna (3.5L V6) has a widely documented oil cooler hose/pipe failure (Toyota enhanced warranty ZE2) where the line ruptures and sprays engine oil under the hood and onto hot exhaust components; oil reaching the converter fouls the catalyst and drops its efficiency.
Reported in 9 NHTSA complaints
Parts:oil cooler hose catalytic converter
2
Worn or failed catalytic converter
High severity
One owner's complaint specifically lists the catalyst as a failed component alongside the EVAP and EGR/VVT systems.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Failing downstream (rear) oxygen sensor
Medium severity
A degraded rear O2 sensor can mimic reduced catalyst efficiency; compare upstream/downstream switching rates.
Common cause
Parts:oxygen sensor
4
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leak before the rear sensor admits outside air, skewing the efficiency calculation.
Common cause
5
Long-term misfire damaging the catalyst substrate
Medium severity
Unburned fuel from a chronic misfire overheats and melts the catalyst's internal substrate over time.
Common cause
6
High-mileage/aged catalytic converter (normal wear)
Low severity
Catalyst substrate naturally degrades with heat cycling and mileage, gradually dropping conversion efficiency below the OBD threshold even without a specific failure event.
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