Q: I have a 99 Nissan Quest, was overheating replaced thermostat.now van will not turnover or start. what could be wrong?

Continuous overheating can damage the valves and ultimately cause a no start condition. A diagnostic in your circumstance would likely start with a compression test to determine if the cylinders even have compression. Also, the engine should be turned over by hand to see if it is freely moving. If there is normal compression and the engine is not binding, an ordinary no start diagnostic (fuel, ignition, air induction) would be carried out. Note that starter tests, such as those performed at AutoZone are essentially useless because during the test absolutely no load is put on the starter. A starter can pass with flying colors at AutoZone and still be useless in starting an actual engine (often due to a failed bendix drive). If you want these diagnostic steps performed by a certified Mechanic, dispatched by YourMechanic right to your location, please request a no start diagnostic and the responding certified mechanic will get this taken care of for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.