BMW-North: Thanks for the info. I knew the level sensor was electronic and didn't touch the liquid, but I assumed it detected when the upper float got too low. Does it directly sense the level of the fluid, or is there another bobber inside the ET? I think my Aux fan relay is working, but am not sure. Where is it, in case I want to replace it? Is it just one of the normal relays by the DME or by the passenger compartment fuses?
Tell me if I understand the aux (electric) fan process correctly. DME reads coolant temp from sensor in lower radiator hose. If DME thinks it's too hot, say above 95C (or you turn on A/C), DME turns on aux fan via a relay. If relay is bad, Aux fan runs on low speed instead of high speed?
My car overheated 3 days after replacing my ET in the middle of an AutoX run. Testing suggests the new MTC aluminum body thermostat got stuck shut. Plus after overheating, the heater circuit in the thermostat first threw a P0599 error then burned out with a loud "POP" and some smoke. I put the old thermostat back, added more coolant and rebled the system, and it appears the overheating problem is solved. But I won't know for sure until I can drive somewhere where I can rev the car up to 6000 RPM again, probably the next AutoX.