ON BOARD DIAGNOSTICS (OBD)

Dashboard lights

Early versions of Motronic display fault codes by flashing dashboard lights, with set numbers of pulses for each fault. Usually the ignition must be turned on and off rapidly several times to initiate code-reading. Check your car’s workshop manual.

Manufacturers’ OBD

Later systems output fault codes via a plug-in scanner. For the first generation of OBD, each manufacturer used its own unique coding system. For this BMW a specific scanner needs to be connected to a 20-pin OBD port under the bonnet.

OBD 2

From the late Nineties, regulators decreed all manufacturers must use the same OBD system with standardised fault codes. An inexpensive generic OBD 2 scanner will read and clear fault codes from any make or model of car that uses OBD 2.

Service indicator reset

Some versions of Motronic indicate on the dashboard when servicing is required. After servicing, this display needs to be reset. Some cars require dashboard or control column buttons to be pressed in a certain order, others are reset via the OBD port. Plug the scanner into the OBD port and turn on the ignition. It will interrogate the ECU and display any fault codes that have been stored. The scanner can also display live data from each sensor, with or without the engine running.

This is useful to test if a sensor is working as expected. For example, with the ignition on and the engine not running, press the accelerator pedal and see if the throttle position sensor readings change as expected.