"If you ever get stuck, check that the exhaust is clear and run the vehicle just long enough to warm the interior and then shut it off for at least twenty minutes. While warming the vehicle, slightly open two windows, say about 3mm each, unless you detect exhaust fumes coming in through either window. Shut the upwind window if you detect fumes. NEVER heat the vehicle with all windows shut !!!!!! Two young people died near hear just last week by running their vehicle continuously wit the windows shut. And open one window when using candles."

The above advice from gnu is worth repaeting.