The engine light is on. On cold start-up the engine runs rough, and when driving cold the idle will surge.

The vehicle displayed rough running and surging idle when cold, with a check engine light on. Fault code P0420 – Catalyst system efficiency below threshold (Bank 1) was stored. The code would clear but returned after approximately one week. Initial suspicion was a faulty pre-cat oxygen sensor, with ignition components also showing faults.