P0420 on a 2016 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 2016 Ford F 150 Super Crew
Ranked from 1,686 NHTSA owner complaints filed for this vehicle.
- 1
Exhaust leak ahead of the downstream oxygen sensor
Medium severityRecurring on this truck — cracked exhaust manifolds and backed-out manifold bolts leak outside air into the exhaust stream ahead of the sensor, skewing the catalyst-efficiency reading; inspect manifold gaskets/bolts for soot and looseness.
Reported in 3 NHTSA complaints - 2
Long-term untreated misfires overheating the substrate
High severityThis truck has multiple documented misfire-producing faults (cam phaser wear, timing chain failure) — unburned fuel dumped into the exhaust from a chronic misfire overheats and can melt the catalyst brick.
Reported in 3 NHTSA complaints - 3
Worn or degraded catalytic converter
Medium severityDirectly reported on this truck — a scan tool pulled a catalytic-converter failure code that recurred roughly every 30,000 miles even after PCM reprogramming, indicating actual converter wear rather than a software issue.
Reported in 1 NHTSA complaintParts:catalytic converter - 4
Coolant contamination of the catalyst from internal coolant intrusion
High severityReported on this truck as coolant entering a cylinder and cracking the head; coolant burned through the exhaust coats and poisons the catalyst washcoat, killing conversion efficiency alongside the underlying engine damage.
Reported in 1 NHTSA complaint - 5
Failing downstream (post-catalyst) oxygen sensor
Low severityA sluggish or biased downstream sensor can mimic a failed catalyst on the monitor even when the converter is fine; compare sensor switching rates to spec.
Common causeParts:oxygen sensor - 6
Chronic rich-running condition overwhelming the catalyst
Medium severityA leaking injector or skewed upstream O2 sensor keeps the mixture rich, saturating the catalyst's oxygen storage capacity; check long-term fuel trims.
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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