V70 preheating lamp stays on

Sounds like a pain! Before you tear everything apart, have you checked the glow plugs themselves? If one's shorted, it could confuse the preheating unit and cause those symptoms. Also, is the smoke black, white, or blueish? That can point to different problems.

Thanks! I haven't checked the glow plugs directly. How would I test them to see if they are shorted? The smoke appears to be black.

Black smoke usually means too much fuel or not enough air. Since it started with the preheating issue, and you haven't messed with the engine's fuel settings, my guess is the preheating unit is indeed the culprit. It's probably sending bad signals to the engine management system. I'd suggest taking it to a workshop to confirm. Faulty cables can be tricky to diagnose properly.

Thanks for the insights. I took it to a workshop, and they confirmed it was a defective cable connection on the preheating control unit. The repair cost me 95€. Problem solved!