I just got my X5 back from BMW for maintenance on air conditioning problem. Dealer advised me that thermostat is what causing the "engine service soon" light. I drove for a week. I've asked if it's safe to drive back on that weekend and the service manager couldn't give me a definite answer. Well, I was on a bumper-to-bumper traffic for 25-30min (running 10mph) when I noticed that the temp gauge was reaching high. I immediately pulled over and shut the engine down. A minute thereafter was followed by a bang (expansion tank) burst. X5 towed to a safe place. Local mechanic removed and replaced the thermostat, expansion tank and the clutch fan. Flushed the coolant while he replaced the stuck position thermostat.