Phantom Misfire Fault Codes

Today we had a 2002 Impreza Bugeye WRX in the workshop with the Engine Management Light (EML) on, for diagnostics and repair.

An initial read of the ECU showed 3 ‘historic’ codes saved into memory – P0301, P0302 and P0000, as well as some Learned Knock Correction, and some Negative Airflow Range trim which is carried through Open Loop.

The car was showing no signs of a misfire (or two!), so the ecu was reset, and some data logging carried out on the road.

This quickly revealed an intermittent fault with the Neutral Position Switch

The above log file shows a gear change from 3rd to 4th, followed by a false Neutral indication immediately after, whilst at 100% throttle in 4th! Then 3 more false indications whilst on the ‘coast down’.

The car was returned to the workshop, where a replacement switch was fitted.

Another ECU reset and a road test showed the new switch to be operating correctly, and the phantom misfire codes cured.

This is a fairly common fault, and easily rectified with a new switch assembly. – The bugeye ECU has a tendency to suffer from misfire related codes, which can also be caused by a faulty cam sensor, coil pack or spark plug.