Hi Paul thanks for the reply, Trawled all the C5 faults one night but couldn't find a resolution on the fan issue, both the dealer and my indy say they do it, but it shouldn't be staying on long enough for the battery to drain. Lots of probs on the forum with sensors reading high outside temperatures but couldn't find anything on it showing two dashes, however for the price of a sensor it may be worth a try.Sensor on the nearside mirror and window cluster on the offside so doubtful if it would be a common earth there, but anything possible.Windscreen replaced for mot and still happens with lights.