P0420 on a 2018 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 2018 Toyota Camry
Ranked from 727 NHTSA owner complaints filed for this vehicle.
- 1
Catalyst contamination from coolant intrusion (failed electric water pump)
Medium severityThis vehicle has repeated reports of the electric water pump failing and coolant entering the engine; coolant burned through the combustion chambers can coat and poison the catalyst.
Reported in 4 NHTSA complaints - 2
Catalyst damage from an unresolved cylinder misfire
Medium severityA confirmed cylinder-4 misfire (P0304) was reported on this vehicle; unburned fuel from a sustained misfire can overheat and damage the catalyst substrate over time.
Reported in 1 NHTSA complaintParts:catalytic converter - 3
Worn or degraded catalytic converter (age/mileage)
Medium severityThe most common root cause of this code as the catalyst's oxygen-storage capacity degrades over time and miles.
Common causeParts:catalytic converter - 4
Failing downstream (rear) oxygen sensor
Low severityA lazy or drifting rear O2 sensor can mimic reduced catalyst efficiency even when the converter itself is still good.
Common causeParts:oxygen sensor - 5
Exhaust leak ahead of the downstream oxygen sensor
Low severityA leak that lets outside air reach the rear sensor skews its reading toward looking like poor catalyst efficiency.
Common cause - 6
Fuel or ignition trim fault overworking the converter
Low severityA rich or lean-running condition from an unrelated fuel-trim or ignition issue forces the catalyst to work beyond its capacity, accelerating efficiency loss.
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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