Hi Nick, OK maybe the quickest way forward is for me to email the 4 pages in the FSM that comprise the process for determining the cause of the failure. If the garage has a good auto electrician, or access to one, they could sort it. Yes P1518 can be caused by a low voltage in the battery although that is not something I have ever seen in my 19 years with my C5, but the code can also be set by a short to the TAC module's ground circuit. Also if the TAC module's ignition feed circuit is shorted to a B+ supply circuit the TAC module will remain powered up when the ignition is tuned off. So when the ignition is turned on again the TAC module is powered up before the PCM and P1518 is set because the TAC sees no communication from the PCM which has not started up yet (normally the PCM powers up first). Also checking the fuses for the cruise control and brake circuits which share the TAC module harness would be useful. Let me know if you want the diagnostic pages, send me a pm with your email. Out of interest are you seeing P0606 too, as well as reduced engine power message and the MIL turning on? Oh also you say the car starts fine - does that mean there is no problem with the battery?