A customer booked in her mini for mot, the lady had not used our services before and informed me that the airbag light was illuminated and had being for the past two years or so. Which begs the question how it had passed previous mot examinations. (Section 5.4 reason for rejection "a srs malfunction indicator lamp indicating a system malfunction" The lady was recommended to us after numerous other garage failures to rectify the fault. First step was to carry out a diagnostic check. I connected our autologic diagnostic platform which quickly indicated a comms problem. Before the srs fault could be found i had to establish communications with the srs module. Firstly I checked the system fuse (fuse 18 facia) which was ok and displayed battery voltage, next step was to check the module itself which involved removal of the front seats and carpet to gain access. Once accessed the fusebox feed at module (pin 5) was checked as was the module ground (pin 6) both were ok.
Now to check the communication line (pin 9) of the module which is the dedicated K bus line and would normally display near battery voltage, our K line when checked indicated zero volts. As the K bus line is distributed from the body controller this was the next logical place to check, the module is located in the base of the o/s A pillar. When I removed the trim I could see there had clearly being a water ingress issue as the wiring harness was damp and I was immediately drawn to the tell-tale sign of verdigris. Upon separating the cables I could see a clear break in the K bus line (coloured white, red and yellow, see photo). Once repaired communications were established and an error code could be read (battery voltage below threshold) this fault was likely generated from an historical battery issue. The fault code could now be erased and the vehicle taken on a driving cycle (after refitting the seats of course) the light was now extinguished and no further faults were apparent, all that was left to do was the mot test. Upon collecting her mini the lady to say the least was delighted that the fault was finally rectified.