Just got off the phone with a fellow Volvo owner. The battery went dead on his '09 S40. He threw on some jumper cables (in the dark.....you can see where this is going) and inadvertently installed the cables reversed. Older battery was then replaced with a new one, yet the vehicle will not start. The lights and everything else appear to work normally, including the remote door locks. He mentioned that he saw a P0603 code with a code reader, which tells us that the ECU is probably at fault. I had him try resetting the unit by disconnecting the battery and touching the two battery cables together for 30 minutes. Then reinstalling them, cycle the door locks several times (he did 10 times). To no avail. I'm thinking by installing the jumpers backwards, it fried the ECU.
He plans on contacting a local mechanic tomorrow to ask about a replacement and cost of reprogramming the unit. Online I found several sites that carry them, but I'm not certain whether or not the part numbers match. His has four different numbers on it. The closest I see that matches others online are these two, 31268178 or 31254903.
Does anyone have any other suggestions that we may try? I'm also hoping that the way we reset the ECU was the correct method...(that I did not make matters worse).

 

P0603 is a KAM error which sometimes happens when someone runs a bad performance tune on their car.
It may be cheaper to have a Volvo dealer reprogram your ECU with the correct firmware for your car. You may have some other issues you need to fix once you get the car running.

 

check all fuses as well...

 

this is a very good idea before you do anything else, check relays too!!
Be advised that there are more than one computer on most modern cars now, you may have others like BCM and a starter control module, ABS module . These may have been affected as well.

 

Thank you. We'll double check the fuses, relays, etc. Will post follow up once issue has been resolved.

 

Turns out one of the fuses had blown. He had not checked all of them thoroughly.

 

You got off easy this time !! To keep from this happening again the best way I found to NOT reverse the cables is to hook up the 'Black" ground cable on both cars to bare metal and the "Red" cable on the battery post on both batteries then start the car that your jumping from and let run for about 5mins, then turn off the car your jumping from and try and start the car with the dead battery. Turning off the car your jumping from will minimize any damage to the other cars electronics.