P0420 on a 2015 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 2015 Toyota Camry

Ranked from 268 NHTSA owner complaints filed for this vehicle.

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1
Oil consumption from worn piston rings contaminating the catalyst
High severity
This generation Camry's 2.5L engine has a well-known oil-consumption defect (worn oil-control rings); six owners here report burning a quart every 1,000-3,000 miles with no external leak found, and burned oil fouls the catalyst.
Reported in 6 NHTSA complaints
Parts:piston rings catalytic converter
2
Engine misfire/cylinder stumble damaging the catalyst
High severity
Several owners describe the engine 'stumbling' or feeling like it briefly shut off under acceleration, consistent with a misfire dumping unburned fuel into the exhaust and overheating the substrate.
Reported in 4 NHTSA complaints
3
Worn/degraded catalytic converter substrate (normal aging)
Medium severity
Age and mileage alone reduce a converter's ability to keep O2 storage within the ECM's efficiency window.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A biased or slow rear O2 sensor can make a healthy converter appear inefficient.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the sensor
Medium severity
A leaking manifold or gasket lets exhaust escape before the sensor, distorting the efficiency comparison.
Common cause
Parts:exhaust manifold gasket
6
Fuel trim issues from a vacuum or intake leak
Low severity
Unmetered air throws off the fuel trims the ECM uses to judge catalyst performance.
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.

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

Matching symptoms

Check Engine Light