Additional Checks
- Check for proper Exhaust Gas Recirculation (EGR) system operation. See DTC P0401: EGR SYSTEM INSUFFICIENT FLOW under DIAGNOSTIC TESTS in SELF-DIAGNOSTICS - 3.8L BONNEVILLE, GRAND PRIX, IMPALA, LESABRE, MONTE CARLO, PARK AVENUE & REGAL article.
- Inspect the Transaxle Range Switch input with the vehicle in Drive and the gear selector in Drive or Overdrive.
- Check Torque Converter Clutch (TCC) operation. TCC applying too soon can cause the engine to spark knock. See appropriate DIAGNOSIS article in TRANSMISSIONS.
- If scan tool readings are normal and there are no engine mechanical faults, fill the fuel tank with a known quality gasoline that has a minimum octane reading of 87 and re-evaluate vehicle performance.