Hi there,
Look for the codes I used a simple tool as Carsoft 6.5 interface and old laptop with Serial Port and Carsoft 6.5 Software running on Windows XP.
It's not a GT1 stuff obviously,but it's more helpful in investigating the faults than the Generic OBD II tools.
I haven't replaced anything yet,but I am getting into it,because winter is coming,next Tuesday it's going to the dealer for some suspension maintenance and drain diagnosis,will write back as soon as I find out the cause,but as I understand,the causes can be so varied that my experience may not be useful for you.
There's one more thing,is you heater blower working right? under right I mean speed is not jumping up and down.
I know that yours is 2005 and it seems that they updated the hedgehog module,but I had to replace it in my 318i before,due to speed jumps and battery concerns.