P0420 on a 2018 Ford F 150 Super Crew

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 Ford F 150 Super Crew

Ranked from 1,835 NHTSA owner complaints filed for this vehicle.

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1
Catalyst degradation from chronic misfire (cam phaser failure / low compression)
Medium severity
This engine has an extensively documented cam-phaser rattle causing rough running, plus at least one case of a cylinder losing compression entirely — sustained unburned fuel from either overheats and can melt the catalyst substrate.
Reported in 16 NHTSA complaints
Parts:catalytic converter
2
Catalyst contamination from documented excessive oil consumption
Medium severity
Multiple owners report abnormal oil consumption on this engine (TSB 19-2365); oil burned past worn rings coats and poisons the catalyst over time.
Reported in 3 NHTSA complaints
Parts:catalytic converter
3
Worn/degraded catalytic converter
High severity
High-mileage substrate loses conversion efficiency on its own; confirm with pre/post-cat O2 sensor waveform comparison.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A lazy or biased rear sensor mimics poor catalyst efficiency even with a healthy converter; compare switching activity to the front sensor.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the sensor
Medium severity
A leak ahead of the catalyst monitor admits outside air and skews the efficiency reading; inspect manifold, turbo, and flex-pipe joints.
Common cause
6
Rich-running condition from a fuel-metering fault
Medium severity
A dribbling injector or high fuel pressure pushes excess unburned fuel into the exhaust, overheating and damaging 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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Matching symptoms

Check Engine Light