P0420 on Ford EcoBoost – Catalyst Efficiency Below Threshold

How P0420 Normally Appears

• Check engine light with no obvious drivability issue.
• Sometimes appears after a long period of misfires or rich/lean running.
• Often intermittent at first – passes self‑tests sometimes, fails at other times.

Common EcoBoost‑Specific Causes

• Age‑ and mileage‑related catalyst wear, especially after years of hard use.
• Aftermarket sports cat that doesn't quite meet OEM efficiency thresholds.
• History of misfires or over‑fuelling that has overheated and damaged the catalyst substrate.
• Exhaust leaks near the front of the system affecting oxygen sensor readings.

Suggested Test Plan

1. Check for any active or pending misfire or fuelling codes and address those first.
2. Inspect for exhaust leaks near the manifold, flexi and front pipe.
3. Review long‑term fuel trims and O2 sensor switching behaviour during a drive cycle.
4. Confirm whether the car has a sports cat or decat section fitted.
5. Only condemn the catalyst once upstream root causes (misfire, rich/lean running) are fixed.