[CharlesJones]

Thanks very much for that advice Ingo. That was exactly the information I needed, and the other day I finally got round to testing the pins on the harness connector with a multimeter (I've been snowed under at work recently). I actually couldn't see the numbers you describe on the DME connector, but I counted the pins from the upper right on the DME like you described, and found the corresponding locations on the harness connector where I needed to measure.

The voltage and grounds I was looking for were all good. Stupidly though I didn't think to measure the resistance across the appropriate pins to check the reference and speed sensors - I didn't know you could do that until I read more about trouble shooting afterwards. However, I did check the resistance of these sensors at the connectors in the engine bay, and sure enough the reference sensor showed infinite ohms! What I don't understand is that when I checked resistance of these two sensors before they were both fine - just over 1K ohms each - and it was after I made that measurement that the car started and ran fine. I thought I had read somewhere that these two sensors were called "light bulb" sensors because when they fail they fail for good, but maybe that isn't the case, or maybe it depends how they fail. Anyway, I definately need a new reference sensor, so I've ordered a pair and will replace both sensors this weekend (assuming my new sensors arive on time). I have ordered the cheaper BMW sensors, so I hope I ordered the right ones!

This was an intermittant problem, Ingo, although it only intermitted once, if you see what I mean. The car wouldn't start, so I tried a fiddled with a few things - replaced the DME relay, measured the resistance of those sensors - then the car was suddenly able to start and ran fine, but then at the next attempt it wouldn't start again and it hasn't started since.

Thanks again for your help. I will report back when I have changed those sensors - hopefully it will all be good after that.