P0420 on a 2018 Ford F 150 Supercab

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 Supercab

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

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1
Long-term misfires from cam phaser/timing chain wear degrading the converter
High severity
This F-150 generation has widespread cam phaser rattle and rough-running complaints; sustained misfire from that fault is a known way to overheat and damage the catalyst substrate.
Reported in 5 NHTSA complaints
2
Oil or coolant contamination of the catalyst
Medium severity
Owners on this truck reported a blown head gasket and abnormal oil consumption, both of which can poison/clog the catalyst.
Reported in 3 NHTSA complaints
3
Worn or failed catalytic converter
High severity
A dealer diagnosis on this exact truck pointed to catalytic converter failure during a stall investigation.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Downstream (rear) oxygen sensor failure
Medium severity
A lazy or failed rear O2 sensor can falsely indicate poor catalyst efficiency.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leak dilutes the rear sensor's reading, mimicking low catalyst efficiency.
Common cause
6
Damaged or aftermarket exhaust component near the sensor
Low severity
Physical damage or non-OEM exhaust parts can disturb sensor placement/readings.
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