1997 Audi A4 Air and coolant system

If you suspect air under the thermostat, which WILL keep it from opening, look for a sensor or a threaded plug on or near the thermostat housing. Those can be removed to bleed the system as you fill it. If you remove a sensor, keep the ignition switch off while it's unplugged so you don't falsely set a diagnostic fault code. On a lot of engines you can also remove the radiator hose, then poke the spring-loaded center plate of the thermostat to let the air out.

If you continue to find air under the thermostat, have your mechanic perform a chemical test to check for a leaking cylinder head gasket. That involves drawing air from the radiator through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow. That air will also prevent the thermostat from opening. Thermostats have to be hit with hot liquid. Hot air won't do it.