I've recently purchased a second hand 2013 XR6 FG from a dealer in Melbourne and every now and then when I start up the car the air con will turn itself on full blast, despite it not actually being on (if that makes sense).

I have to turn the air con on manually myself and then turn it off again to get it to stop.

This doesn't happen every single time I start the car, but it has happened probably 9 or 10 times in the month that I've had the car.

I've read up on some issues relating to batteries in the XR6 and apparently a battery on its "last legs" can potentially cause random faults with the air con, but before I go and fork out money for a new battery, I want to know if anyone else has had this similar issue and if so, how did you go about fixing it?

I should also note that this weekend just gone I received a 'System Comms Error' message which popped up randomly, but was gone when I started my car up 2 hours later, so I'm not sure if this is possibly related to the battery/air con issue.

Do a load test on the battery, hook up a digital multimeter to the battery and crank it over when cold and see what the voltage drops to, If it is dropping below nine volts it may be as simple to much voltage drop at the bem.