P0420 on a 2018 Ford Expedition
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 2018 Ford Expedition
Ranked from 363 NHTSA owner complaints filed for this vehicle.
- 1
Catalyst damage from repeated cam-phaser/timing-chain-driven misfires
Medium severityThis engine's documented cam phaser and timing chain failures (recall 21N03/24E13) disrupt valve timing and cause misfires; unburned fuel from sustained misfires can overheat and damage the catalyst substrate over time.
Reported in 3 NHTSA complaints - 2
Worn/failed catalytic converter
Medium severityDealer diagnosis in this batch called for catalytic converter replacement at 156k miles alongside other high-mileage wear; converter efficiency naturally degrades with age/mileage.
Reported in 1 NHTSA complaintParts:catalytic converter - 3
Failing downstream (rear) O2/air-fuel sensor
Low severityA degraded rear sensor can falsely indicate poor catalyst efficiency even when the converter itself is healthy; compare rear sensor switching pattern to the front sensor's.
Common causeParts:oxygen sensor - 4
Exhaust leak ahead of the downstream sensor
Low severityA leaking exhaust manifold, gasket, or pipe joint dilutes the sample the downstream sensor sees, mimicking a failed catalyst.
Common cause - 5
Oil or coolant contamination of the catalytic converter
High severityOil or coolant burned through worn rings/valves or a leaking gasket coats and poisons the catalyst; look for blue/white exhaust smoke and check oil/coolant levels.
Common cause - 6
Loose or corroded O2 sensor wiring/connector
Low severityA corroded or loose connector at either O2 sensor produces an erratic signal that the PCM can misread as a catalyst efficiency fault; inspect and clean connectors.
Common cause
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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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