P0420 on a 2015 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 2015 Ford F 150 Super Crew
Ranked from 1,425 NHTSA owner complaints filed for this vehicle.
- 1
Exhaust manifold leak ahead of the downstream sensor
Medium severityThis truck has documented exhaust manifold bolt/gasket failures (broken bolts, warped manifolds venting exhaust); a leak between the converter and rear sensor lets in outside air and skews the efficiency reading.
Reported in 3 NHTSA complaints - 2
Worn/degraded catalytic converter and downstream O2 sensor
High severityOne owner had the catalytic converter and oxygen sensors replaced for a persistent check-engine light, and the failure recurred — direct evidence of converter/sensor degradation setting this code on this exact vehicle.
Reported in 1 NHTSA complaint - 3
Long-term ignition misfires melting the substrate
High severityA documented misfire on this vehicle from melted ignition coils can dump unburned fuel into the exhaust and overheat/damage the converter if left unrepaired.
Reported in 1 NHTSA complaint - 4
Failing downstream oxygen sensor (independent of converter wear)
Medium severityA lazy or biased rear sensor alone can trigger a false efficiency code; verify switching speed/voltage range before condemning the converter.
Common causeParts:oxygen sensor - 5
Oil or coolant contamination of the converter
High severityAn internal leak (turbo oil seal, head gasket) pushing oil or coolant into the exhaust stream will poison and quickly degrade the catalyst.
Common causeParts:catalytic converter - 6
Upstream fuel-trim O2/A-F sensor drift
Low severityA slow or biased front sensor feeds inaccurate data into the PCM's catalyst monitor even when the converter is healthy; compare live upstream/downstream switching counts.
Common causeParts:oxygen sensor
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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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