P0420 on a 2017 Ford F 150 Crew 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 2017 Ford F 150 Crew Cab

Ranked from 1,066 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Persistent misfire from cam phaser/timing chain wear damaging the catalyst
High severity
This truck series shows a heavy pattern of cam phaser/VCT and timing chain wear causing rough running and stalls; sustained misfire dumps unburned fuel into the exhaust and can foul or overheat the catalyst.
Reported in 5 NHTSA complaints
Parts:cam phaser timing chain catalytic converter
2
Head gasket failure allowing coolant into the exhaust
Medium severity
One complaint documents a cracked cylinder head gasket; coolant reaching the catalyst chemically poisons it and drops efficiency.
Reported in 1 NHTSA complaint
Parts:cylinder head gasket catalytic converter
3
Worn or failing catalytic converter
High severity
Reduced oxygen-storage capacity drops efficiency below threshold; this is the baseline cause the code is designed to detect.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) O2 sensor
Medium severity
A lazy or biased rear sensor can mimic reduced catalyst efficiency even when the converter itself is still serviceable.
Common cause
5
Exhaust leak ahead of the downstream sensor
Low severity
A leak upstream of the rear sensor skews the efficiency comparison the ECM uses to set the code.
Common cause
6
Oil consumption/leak contaminating the converter
Low severity
Burned oil passing through the exhaust (several complaints describe oil leaks/smoke on this truck series) can coat and poison the catalyst over time.
Common cause

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