P0420 on a 2017 Toyota Tacoma

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 2017 Toyota Tacoma

Ranked from 237 NHTSA owner complaints filed for this vehicle.

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1
Cylinder misfires damaging/overheating the catalyst
High severity
Multiple owners report cylinder-specific misfires (P0301, cyl #3, cyl #5/#6, cyl #2) that required coil, spark plug, or full engine repair; unburned fuel reaching the converter overheats and degrades the substrate.
Reported in 4 NHTSA complaints
Parts:catalytic converter ignition coil spark plug
2
Coolant intrusion / head gasket failure contaminating the catalyst
High severity
One owner's head gasket leak warped cylinder #1, and several others report the well-known rear coolant bypass joint (crossover pipe) leaking on the 2GR-FKS V6; coolant reaching the exhaust coats and poisons the catalyst.
Reported in 4 NHTSA complaints
Parts:catalytic converter head gasket coolant bypass joint
3
Worn/aged catalytic converter
High severity
Normal high-mileage degradation of the catalyst substrate reduces conversion efficiency below threshold.
Common cause
Parts:catalytic converter
4
Failing downstream (rear) oxygen sensor
Medium severity
A lazy or biased rear sensor can read as reduced catalyst efficiency even when the converter is fine.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leaking manifold gasket or cracked exhaust pipe upstream of the rear sensor introduces fresh air, skewing the efficiency reading.
Common cause
6
Oil consumption/leak contaminating the catalyst
Medium severity
Oil burned past worn valve seals, rings, or from an external leak (several owners report oil-filter-housing and general oil leaks on this engine) coats the catalyst washcoat.
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.

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Matching symptoms

Check Engine Light