Running the engine for heat in a stranded vehicle is sometimes necessary — overnight in deep cold, in extended weather waits. The risk is carbon monoxide accumulation.
Carbon monoxide builds up if the exhaust pipe is blocked by snow, mud, or debris. The CO routes back through the engine bay and into the cabin. CO is odorless and colorless — you don't notice it until you're feeling lightheaded.
Before running the engine for heat: check that the exhaust pipe is clear. In snow, you may need to dig around the tailpipe periodically.
Crack a window slightly when running the engine for heat — this helps ventilate any CO that does enter and ensures fresh air for occupants.
Run the engine intermittently rather than continuously — 10–15 minutes per hour is usually sufficient to keep the cabin survivable in cold weather. Continuous running depletes fuel faster and isn't necessary for survival.