S40 Rear Defroster Issue

Whoa, sounds familiar! I've got an S40 too. I had a similar issue with my rear defrost, and it turned out to be a surprisingly simple fix. Did you happen to check your battery recently? Like, really check it? What's the voltage looking like both at rest and while cranking? Also, when you say 'error code,' can you tell me which one it is? It might give us a better clue.

Good shout on the battery! I did a quick check, but didn't go super in-depth. I'll put a meter on it tonight and see what's what. As for the error code, I need to hook up my OBD reader again and will report back with the exact code. But you really think a dodgy battery could kill the rear defrost but also affect the windshield defrost? Seems a bit odd, no?

Trust me, these cars do weird things when the voltage drops too low. I chased ghosts for weeks before someone suggested checking the battery more thoroughly. It wasn't completely dead, but it was weak enough to cause all sorts of electrical gremlins. The rear defrost was the first to go, then weird stuff with the climate control. Honestly, if the battery checks out, you're probably looking at something more involved, and I'd bite the bullet and go to a workshop. But check the battery first!

Right, I finally got around to properly testing the battery. Turns out it was indeed heavily discharged and likely defective. Replacing it seems to have sorted out the rear window heating and improved the windshield defrost as well, just like you said. Total cost was 290€ for a new battery and installation. Thanks a million for the tip! I was already envisioning spending way more on diagnostics and chasing down phantom wiring issues.