P0420 on a 2021 Ford F 150 Super Crew 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 2021 Ford F 150 Super Crew Diesel

Ranked from 941 NHTSA owner complaints filed for this vehicle.

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1
Catalyst damage from chronic cylinder misfires
Medium severity
This dataset shows repeated spark plug, coil, and injector failures causing misfires on this engine; unburned fuel reaching the converter overheats and damages the substrate.
Reported in 5 NHTSA complaints
Parts:catalytic converter
2
Catalyst fouling from chronic oil consumption (5.0L Coyote V8, Ford TSB 19-2365)
High severity
Multiple complaints on this exact engine document severe, undetected oil consumption; burned oil passing through the exhaust coats and poisons the catalyst over time.
Reported in 4 NHTSA complaints
3
Known catalytic converter defect covered by Ford Customer Satisfaction Program 24E04/24E08
High severity
One owner's P0420-pattern complaint was explicitly diagnosed as a catalytic converter failure covered by a Ford customer service program that also requires a PCM software update for catalyst monitoring, describing it as a widespread issue on this platform.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Worn/aged catalytic converter (normal wear)
Medium severity
Efficiency naturally declines with mileage; confirm with an upstream/downstream O2 sensor switching comparison.
Common cause
Parts:catalytic converter
5
Failing downstream (rear) oxygen sensor
Medium severity
A sluggish or biased rear sensor can make a healthy catalyst look inefficient.
Common cause
Parts:oxygen sensor
6
Exhaust leak upstream of the downstream O2 sensor
Low severity
A leak before the rear sensor lets in outside air, mimicking poor catalyst efficiency.
Common cause
7
Upstream O2 sensor drift affecting fuel trim
Low severity
A slowly failing front sensor can push the mixture off target enough to indirectly affect catalyst readings.
Common cause
Parts:oxygen sensor

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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