P0420 on a 2022 Ford F 150 Super Cab Diesel

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 2022 Ford F 150 Super Cab Diesel

Ranked from 559 NHTSA owner complaints filed for this vehicle.

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1
Catalytic converter damage from unresolved cylinder misfires (2.7L EcoBoost intake-valve failure, recall 24S55)
High severity
This platform has a documented intake-valve failure that causes cylinder misfires and raw unburned fuel entering the exhaust; that unburned fuel is a classic way to overheat and damage a catalytic converter, pushing efficiency below threshold.
Reported in 3 NHTSA complaints
2
Exhaust manifold/gasket leak upstream of the catalytic converter or downstream sensor
Medium severity
One owner reported exhaust fumes leaking into the cabin along with an identified exhaust gasket leak; a leak in this location skews the sensor reading the PCM uses to judge catalyst efficiency.
Reported in 1 NHTSA complaint
Parts:exhaust gasket
3
Worn or high-mileage catalytic converter
Medium severity
Substrate efficiency degrades naturally with age/mileage; this is the baseline cause of P0420 on any vehicle.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A lazy or biased rear O2 sensor can make a good converter look inefficient; compare front/rear switching patterns.
Common cause
Parts:oxygen sensor
5
Contaminated catalyst substrate from oil or coolant intake
Medium severity
Oil or coolant reaching the exhaust (from a leaking valve seal, head gasket, etc.) chemically poisons the catalyst.
Common cause
Parts:catalytic converter
6
Rich or lean fuel trim accelerating catalyst wear
Low severity
A sustained fuel-trim imbalance chemically ages the catalyst faster than normal operation.
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