P0420 on a 2011 Toyota Prius
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 Prius
Ranked from 516 NHTSA owner complaints filed for this vehicle.
- 1
Oil consumption contaminating the catalytic converter
Medium severityThis generation Prius (1.8L 2ZR-FXE) has a well-known oil-consumption pattern (~1 qt/1000 mi); burned oil passing through the exhaust coats and poisons the catalyst, lowering its efficiency.
Reported in 3 NHTSA complaints - 2
Long-term untreated misfire damaging the catalyst substrate
High severitySustained misfire dumps unburned fuel into the exhaust, overheating and melting the catalyst; two complaints trace rough running/misfire to a leaking head gasket on this generation Prius.
Reported in 2 NHTSA complaints - 3
Worn/degraded catalytic converter
High severityReduced oxygen-storage capacity drops efficiency below threshold; one owner had the converter replaced by the dealer but the failure persisted, pointing to a converter (or upstream) issue.
Reported in 1 NHTSA complaintParts:catalytic converter - 4
Failing downstream (post-catalyst) O2 sensor
Medium severityA lazy or biased rear sensor can mimic reduced catalyst efficiency even when the converter itself is still good.
Common cause - 5
Exhaust leak ahead of the downstream sensor
Low severityA leak upstream of the rear sensor skews the efficiency comparison the ECM uses to set the code.
Common cause - 6
Engine oil/coolant contamination of the converter from an internal leak
Medium severityCoolant or oil entering the exhaust stream (e.g., from a head gasket breach) chemically poisons the catalyst.
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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