[77newell]

The high voltage alarm on the coolant level; we are used to thinking of a level sensor as an on-off switch, on ecu engines it doesn't quite work that way. Voltage is applied through through a resistor in the ecu. The sensor has resistance such that it pulls the voltage down a bit under normal circumstances. If it is pulled down the right amount the ecu "knows" the system is electronically functional. With the high voltage alarm indication the ecu is saying the resistance in that sensor circuit, including the sensor itself, is higher than it should be. This means that the ecu cannot reliably determine whether the coolant level is safe. If the sensor circuit resistance is high, when the sensor resistance goes low when the coolant goes low, the ecu may not trigger the low coolant alarm and we end risking engine destruction. Therefore, the ecu system is designed to warn us that there is a problem with the circuit in addition to warning us when the coolant is low.

I'm not sure I have the details exactly right, but I am sure of the engineering approach used that verifies the integrity of all the sensor systems used by the ecu.