P0420 on a 2021 Ford F 150 Super Cab

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 2021 Ford F 150 Super Cab

Ranked from 941 NHTSA owner complaints filed for this vehicle.

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1
Long-term untreated misfires melting the catalyst substrate
High severity
This same truck/engine had repeated documented misfires (cracked spark plugs, failed coils, a cracked fuel injector, and piston/valve damage) — raw fuel from those misfires is a well-known way to overheat and melt a catalyst.
Reported in 6 NHTSA complaints
2
Worn or failing catalytic converter
High severity
An owner's dealer diagnosed a catalytic converter failure (service program 24E08) described as a widespread failure across model years, with the dealer also noting a PCM software update tied to catalyst monitoring.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
PCM software calibration affecting catalyst monitor
Medium severity
The dealer in this case specifically flagged a needed PCM software update related to the catalytic converter monitor as part of the diagnosis.
Reported in 1 NHTSA complaint
4
Failing downstream O2 sensor
Medium severity
A lazy or biased post-catalyst sensor reads too close to the upstream sensor and falsely reports low efficiency.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leak lets in outside air that skews the catalyst efficiency comparison.
Common cause
6
Oil consumption fouling/contaminating the catalyst
Medium severity
Oil burned past worn rings or valve seals coats the catalyst and reduces its efficiency over time.
Common cause
7
Exhaust manifold gasket leak
Low severity
A leaking exhaust joint ahead of the sensors introduces false air readings.
Common cause
Parts:exhaust manifold gasket

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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