Illustration courtesy Delphi Illustration courtesy Delphi Figure 8: The voltage dropped by the varied resistance of the ECT (right) goes up as the sensor's resistance goes up. The ECM measures the remainder of the 5 volt dropped across its internal resistor and sends that voltage to an A-D converter to be turned into a digital input the ECM's main processor can use. That internal resistor is covered in depth in the April 2015 MSR. It may actually be two resistors in series. Both are used when the coolant is cold while only one is used when the coolant is warmer to increase the ECM's accuracy in measuring coolant temperatures. This will result in a shift in analog voltage (when measured by a voltmeter) when the coolant temperature hits that temperature. Figure 9: The "Dead Band" is that OEM defined voltage level that is very close to 0 volts (grounded) and very close to 5 volts (reference voltage). Disconnecting most sensors including the cooling system's ECT sensor will result in entering the dead band on the high side setting a P0118 and / or P0119 DTC and jumping ref low (sensor ground) to the signal wire (typically pulled up to 5 volts) should set a DTC. July 2016 6 MACS Service Reports