P0420 on a 2019 Ford Expedition El
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 2019 Ford Expedition El
Ranked from 464 NHTSA owner complaints filed for this vehicle.
- 1
Chronic cam-phaser-driven misfire/rough running accelerating catalyst wear
Medium severityThis platform's widely reported cam phaser/VCT failures cause repeated rough running and timing-related misfire; sustained misfire dumps unburned fuel into the exhaust, which is a known way to prematurely degrade catalyst efficiency.
Reported in 5 NHTSA complaints - 2
Catalytic converter failure/low efficiency
High severityMultiple owners had a catalytic converter directly diagnosed as failed or reporting low efficiency, in one case recurring shortly after an emissions recall repair and affecting several other Expeditions at the same dealer.
Reported in 3 NHTSA complaintsParts:catalytic converter - 3
Failing downstream (post-cat) O2 sensor
Medium severityA lazy or degraded downstream sensor can mimic the upstream sensor's switching and falsely trip a low-efficiency code.
Common causeParts:oxygen sensor - 4
Exhaust leak ahead of the downstream sensor
Medium severityA leaking manifold or flex-pipe joint introduces outside air that confuses the catalyst monitor.
Common causeParts:exhaust gasket - 5
Oil or coolant contamination of the catalyst
Medium severityInternal oil or coolant consumption reaching the exhaust coats and poisons the catalyst substrate.
Common causeParts:catalytic converter - 6
Stuck-open fuel injector causing a rich condition
Medium severityA leaking or stuck-open injector over-fuels a cylinder, and the resulting rich exhaust overloads and degrades catalyst efficiency over time.
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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