Star (which is actualy a Snowflake) just means the external temperature has reached a point where ice is possible....
If you have differential temps from Drives and Pax sides that is more than likely Blend Motor issues, and trust me, almost 90% of P38 owners will have/will do come across this.
Do a search for blend motors and you'll find loads of adivce on how to change them, not easy but doable.
Diagnsotics will tell you what is wrong with the Blend motors, be it stuck, short circuit, no feedback etc....but 99% of the time, it means you'll have to change them out!
You have to follow the correct method to disconnect the battery, this is detailed in the hand book but the bones of it is:
Loosen - but do not remove the Negative lead
Put key in ignition turn to Position 2 then to 0 again
Within 17 seconds remove the negative lead completely
On reconnection, never leave the keys in the car and leave a door open, on reconnection, sometimes the vehicle will lock the doors, and if the keys are in it, your fecked!!
Sometimes (never happened to me) but you may be required to enter the EKA code to remobilise the car, resync the key fobs rolling code set and put the radio code in.
Disconnectiong the battery will not reset the HEVAC unit, you will still need to investigate the Blend Motor issue!
