P0420 on a 2016 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 2016 Ford F 150 Supercab

Ranked from 1,686 NHTSA owner complaints filed for this vehicle.

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1
Exhaust leak ahead of the downstream O2 sensor
Medium severity
Cracked exhaust manifolds and sheared manifold bolts (a documented issue on these trucks) let extra oxygen reach the downstream sensor, mimicking poor catalyst efficiency.
Reported in 2 NHTSA complaints
Parts:exhaust manifold manifold bolts manifold gasket
2
Worn/degraded catalytic converter
Medium severity
Age and mileage reduce the catalyst's oxygen storage capacity below the efficiency threshold; one owner's own scan confirmed a catalytic-converter-related code on this truck.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Long-term misfires damaging the catalyst substrate
High severity
Repeated unburned fuel reaching the converter overheats and melts the substrate; this truck's cam-phaser rattle/stutter complaints describe a chronic misfire condition that can accelerate cat wear.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Oil or coolant contamination of the converter
High severity
Internal oil or coolant leaks (e.g., a cracked cylinder head from coolant intrusion) can coat and poison the catalyst, killing its efficiency.
Reported in 1 NHTSA complaint
Parts:catalytic converter
5
PCM software calibration issue
Low severity
Ford has issued PCM reflashes for some catalyst-monitor false triggers; one owner's catalyst-related code was tied to a powertrain control module software reprogram recall.
Reported in 1 NHTSA complaint
6
Failing downstream (post-cat) O2 sensor
Low severity
A lazy or drifted downstream sensor no longer shows the expected switching difference from the upstream sensor, tripping efficiency monitors even with a healthy converter.
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