Macca

The good news is that the engine MIL problem is sorted and the car rushed through a successful MOT. I wish I could claim full credit for analysing the system and explicitly tracking down the fault - but I cannot.

Given the scenario described in my original narrative, it was clearly linked to the alarm system. What I did not say in my narrative was that, as the car only comes out of the garage occasionally, I have fitted quick connectors to the battery and disconnect it when it goes away (might be a drain to earth somewhere as the battery fully discharges within a 1-2 weeks). I know disconnecting the battery can cause some issues with the alarm / immobiliser system so I left the battery connected, used the OBD to look at any fault codes (of which there none), but did a 'Clear all Codes' anyway. The engine MIL operation was fully compliant with the design thereafter, so the answer seems to be don't go disconnecting the battery!

There are lots of posts on the forum about the Amber engine Malfunction Indicator Light (MIL) illuminating (which is a Major Fault leading to an MOT failure). My problem is that the engine MIL light is not coming on (which is also an MOT failure, presumably as a safeguard against unscrupulous owners talking the MIL bulb out to hide a major engine fault). My 2004 MGTF Spark has been running sweet as a nut with no warnings or malfunctions on either the Dashboard or OBD and I assumed it would fly through its MOT – it failed due to the MIL Light not illuminating. When I picked the car up after the MOT, the MIL light came on for me. I dragged out the Inspector and point out his error, whereupon the MIL light would not come on during repeated ignition on – start-up events; the car remained an MOT failure. Since then, a 100% repeatable pattern has emerged. For the first start after the Alarm/Immobiliser has been deactivated, the MIL light comes on (albeit it flickers on, goes off for a quarter of a second, and then comes back solid and goes off when the engine starts (i.e. normal function). For all subsequent starts until the Alarm/Immobiliser has been cycled, the MIL light remains off. Does anyone know if this is a known behaviour or an interface issue between the Engine Management Computer and the security system? I don’t fancy my chances of telling the MOT Inspector that he has to cycle the Alarm System before he checks the MIL light operation in the re-test!