Info - Normal Characteristics of Malfunction Indicator Lamp (MIL) During On-Board Diagnostic (OBD) Inspection/Maintenance (I/M) Testing

The purpose of this bulletin is to inform dealers and OBD inspection/maintenance testing facilities of normal characteristics of the malfunction indicator lamp (MIL) and the engine or powertrain control module (ECM or PCM).

Some customers may comment on having been rejected or having failed an OBD emission inspection. In some cases, these vehicles do not need repair and may have been rejected or may have failed the OBD emission inspection due to a lack of understanding with regard to the normal characteristics of the MIL and ECM/PCM.

Following an engine run period, the MIL may not illuminate on an ignition OFF to ignition ON transition unless a 30 second delay is observed between key OFF and key ON. The delay is necessary on some control modules to allow the control module, either ECM or PCM, to power down. Most vehicles may require less time to achieve power down, but a 30 second delay will satisfy all known applications. This is normal operation for all GM control modules.

Some OBD I/M emissions test centers may not wait 30 seconds after a vehicle is turned off before proceeding with the MIL bulb check, which could result in a vehicle rejection or test failure.

To confirm proper MIL operation, wait 30 seconds after the ignition is turned OFF before proceeding with any key ON bulb check.