P0420 on a 2019 Toyota Highlander

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 2019 Toyota Highlander

Ranked from 467 NHTSA owner complaints filed for this vehicle.

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1
Catalyst substrate damage from a confirmed multi-cylinder misfire
High severity
This vehicle had a dealer/AutoZone-confirmed misfire across cylinders 1, 3, 5, and 6; unburned fuel from a sustained misfire overheats and can melt the catalyst's honeycomb substrate.
Reported in 2 NHTSA complaints
2
Oil/coolant contamination of the catalyst from internal engine damage
High severity
One owner's engine was found with a scored cylinder burning both oil and coolant; either fluid reaching the exhaust coats and poisons the catalyst.
Reported in 1 NHTSA complaint
3
Worn or aged catalytic converter
Medium severity
Catalyst efficiency degrades naturally with mileage and heat cycling until it falls below the OBD-II threshold.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A sluggish or biased post-catalyst O2 sensor can mimic a dead converter even when the catalyst is still good.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leaking joint or gasket before the sensor dilutes the exhaust sample with outside air.
Common cause
6
Worn spark plugs or failing ignition coil
Medium severity
Even a mild, easy-to-miss misfire sends extra unburned fuel to the catalyst over time, accelerating its wear.
Common cause
Parts:spark plugs ignition coil

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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