P0420 on a 2011 Ford Edge

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 2011 Ford Edge

Ranked from 1,801 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalyst damage from prolonged, unresolved cylinder misfires
High severity
One owner in this batch had recurring cylinder misfires over 6+ months (melted coil packs, repeated spark-plug changes) without the underlying cause being fixed; raw unburned fuel repeatedly washing through the converter is a classic way to cook the substrate.
Reported in 1 NHTSA complaint
2
Worn or high-mileage catalytic converter
Medium severity
Substrates wear out with mileage and thermal cycling; one owner in this batch was quoted for catalytic converter replacement, consistent with normal end-of-life failure.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Failing or lazy downstream (post-cat) oxygen sensor
Medium severity
A sluggish rear O2 sensor makes a healthy converter look inefficient to the PCM; compare rear O2 switching activity to the front sensor's on a scan tool.
Common cause
Parts:oxygen sensor
4
Exhaust leak between the converter and the downstream O2 sensor
Medium severity
A leak lets outside air reach the rear sensor and mimics a converter that isn't scrubbing efficiently.
Common cause
5
Oil or coolant contamination of the catalyst
High severity
Internal engine leaks that let oil or coolant reach the exhaust coat and poison the catalyst substrate, permanently reducing efficiency.
Common cause
6
PCM catalyst-monitor miscalibration
Low severity
Rarely, an outdated PCM calibration flags a functionally good converter as inefficient; a dealer-level reflash can rule this out before replacing parts.
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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light