P0420 on a 2020 Ford F 150 Super Crew

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 2020 Ford F 150 Super Crew

Ranked from 506 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Worn-out catalytic converter
High severity
Multiple owners had their catalytic converter diagnosed as failed and requiring replacement after the CEL appeared.
Reported in 2 NHTSA complaints
Parts:catalytic converter
2
Exhaust leak upstream of the catalyst/downstream sensor
Medium severity
A documented factory turbo exhaust-manifold bolt/warping issue lets exhaust escape before the catalyst, which can both damage the converter and skew the upstream/downstream sensor comparison.
Reported in 1 NHTSA complaint
Parts:exhaust manifold gasket
3
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A lazy or biased downstream sensor makes a healthy converter look inefficient; compare upstream/downstream sensor switching rates.
Common cause
Parts:oxygen sensor
4
Long-term misfires overheating and melting the substrate
High severity
Untreated ignition or fuel misfires dump raw fuel into the exhaust, which can overheat and damage the catalyst brick.
Common cause
5
Oil or coolant contamination of the catalyst
Medium severity
Internal engine oil consumption (documented on this platform) can foul the catalyst with ash/phosphorus over time, reducing conversion efficiency.
Common cause
6
Substandard aftermarket replacement converter
Medium severity
A converter that doesn't match OEM catalyst loading/cell density can pass initial inspection but fail the efficiency monitor within a short time.
Common cause
Parts:catalytic converter

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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