2009 B8 Quattro Sedan. My rear defroster is shutting off after turning on after.

[scottay5150]

A few seconds. What gives?

TIA.

[Smac770]

Scan for DTCs.

The rear defroster is direct connected to battery power, through a control relay and a 40A fuse, both in the right rear. The other relay is the 12v sockets relay. So there's no real way for the car to know anything about the state of operation of the defroster directly.

The only thing the car will know is the control line to the relay, from the J393 (46 - central convenience). Maybe there's an issue on the control line and that's cutting out the control. Or the car believes there's a low voltage situation and disallowing the defroster. You're pressing the button the J255 (8 - hvac), but it's then telling the J393 via CAN to "flip the relay". But the J533 is monitoring the battery voltage and energy management, and can also negate that command, using CAN comms also.

Could be the relay is just bad. Swap the two relays and see if the 12v sockets stop working and the defroster starts working.