Fault code ‘P0501’

I would welcome your suggestions for the following problem on my 2002 Audi TT 1.8. I bought the car several years ago and have always had a persistent problem where, at random intervals anywhere between 25-200 miles, I get an engine management fault warning light. In all cases, the fault code stored is ‘P0501’. There are no other issues that arise – for example, the speed reading does not blip, probably because the error signal is of such a short duration. I seem to recall that, on a different code reader, the message had slightly more information and displayed: ‘Vehicle speed sensor A range/performance’.

My garage has replaced the speed sensor twice – the second time with a genuine Audi unit – and the sensor wiring has been checked and is OK. I bought a secondhand dashboard, which was reprogrammed for the vehicle, but this made no difference so the original was replaced. As a final experiment, the sensor wiring route was altered from its usual course from the speed sensor to the speedometer, then to the radio, then to the central locking system, then to the ECU. Since the speed signal had no effect on the radio or central locking, the routing was revised to go from the sensor to the speedometer to the ECU.

At this point, the garage and I ran out of ideas and since I had already spent quite a bit of cash, I bought an OBDII reader to carry in the car, so that I can easily confirm that the issue is not a major engine management fault and can reset the code. John Sykes

The vehicle speed sensor gives a four-pulse signal for each revolution of the driveshaft. The signal is converted by the waveform circuit inside the combination meter before being transmitted to the engine control module, which determines the speed of the vehicle by processing the received pulse signals.

The possible causes of the ‘P0501’ code are the sensor, the wiring or the ECM. As you have replaced the sensor and fitted a genuine unit, and as you have checked and confirmed that the problem is not in the wiring loom, this only leaves the ECM. However, before having the control module checked, I would first inspect the condition of the sensor again and ensure that there are no metal particles present.

Contamination from within the gearbox can cause problems and could be in the form of congealed sludge containing metal particles within the recess that the sensor should be reading from. Although you mention that the speedometer does not blip, this could be the problem. Also, even though the wiring has been checked, I would closely examine the terminals on the engine control module and connecting plug.