Many years ago…

When the electronic control modules could not be erased and reprogrammed…

A saloon lands in the technical support department. MIL [Malfunction Indication Lamp] staying on. A scan of PCM [Powertrain Control Module] indicates a problem in the Leak Detection Pump circuit. Diagnostic procedure is very clear in the manual.The problem is that this vehicle is not equipped with Leak Detection Pump and the senior technician, an experienced one in this model, is confused.

A little about the leak detection pump…

It is part of the system that detects fuel evaporative leaks. Emission control!

A solenoid operated diaphragm that builds up pressure in the EVAP system and a switch that changes from closed to open position when the pressure build up reaches a predetermined value.

PCM monitors the switch position to find out the time required for a change from close to open and decides whether there is a leak or not.

Now, the scan tool says there is an open circuit in the leak detection pump circuit. True, there are wires going to the pump in the PCM connector, but these wires ends in a dummy connector under the steering column as this vehicle is not equipped with the pump. The PCM is not supposed to detect the open in the leak detection pump circuit as a fault. The vehicle has been running without any problem for quite some time. Why all of a sudden the PCM started thinking differently?

A little into the history of the vehicle…

It came one week ago with starting trouble. The technician has found fuel pump control circuit in the PCM defective and had ordered a new PCM. It took one week for the PCM to arrive. He programmed the blank PCM with the software available in the manufacturer’s website, as usual, and fixed in the car. Everything was fine, except the MIL.

A quick reference of the workshop manual indicated that this model  in Europe and America  has the leak detection pump.


I went to the software download section in the manufacturer’s website. Just above “Middle East Market” lies “Mexico and …”. Things were clear. The technician had selected “Mexico and…” by mistake and had downloaded the wrong software.

Control modules those days were non reprogrammable type. I don’t know whether any manufacturers use those modules now a days. You can program them once only.

The cost of new PCM was very high. We had to find a way out. Otherwise the techncian will be ending up paying one fourth of his salary every month for another six months.

We did find out a way before the customer came to collect the vehicle at the promised time.

We did not harm anything in the system. Just a small thing to cheat the PCM and it started thinking that everything is fine in the leak detection pump circuit.

And the MIL? Gone…

Whaaaat?  You already know what we did?!

Good… You must have completed the OBD Level 2 course and must be clear about the logic behind detection of faults by control modules. Correct?