occurred due to the low voltage input. Out of range does not mean the signal is incorrect, it means it has seen voltage reach an unrealistic value. Sensors have what is called a Dead Band. The band is in an area of voltage levels at the far ends of each end of the 0-5 volt range. Each manufacturer set this level for each sensor. Using a temperature sensor for our example: The manufacturer may determine that anything from: 0.001V to 0.175V is out so range. At the top it may be 4.75V to 4.99V is out of range. This is the dead band and is usually caused by an intermittent problem. Anything that would reach 5.0V is open while anything at 0V is shorted. (Fig. 9) Thermostat Monitor Operation My first encounter with a thermostat monitor DTC was on a 2009 Dodge Avenger. The fix was repairing a coolant leak and filling the cooling system. That's quite common. They aren't all that easy. For the true thermostat monitor failure algorithm (ECM's plan of action in software) the vehicle service manual is the next step. Thermostat monitors analyze the performance of the thermostat by comparing the measured ECT to modeled ECT using information from the following inputs: * * * * * Practical Testing The ECM uses the start-up ECT temperature and ambient air temperature to start it's on board diagnostic calculation. The thermostat monitor calculates the amount of energy going into and out of the engine cooling system during the warm up process to determine if the ECT does increase normally as well as maintain the coolant temperature within a calibrated range of the modeled ECT. As mentioned previously the monitor run criteria is set in the calibration. Make sure other DTCs are not set; ensure the coolant reaches a specific start temperature, etc. Illustration courtesy Delphi A practical circuit test anytime you suspect an ECT (or any other analog sensor for that matter) is to simply unplug the ECT sensor to see if you set a P0118 voltage high DTC and then jumper the sensor output to the ECM's reference ground to see if the sensor sets a P0117 voltage low DTC. You may or may not have to drive the vehicle depending on that monitor 's criteria. On some later model vehicles that run evap testing after the key is off, keep in mind you may set a DTC just unplugging the ECT to begin your testing if the ECM is awake and preparing to run the key off Evap small leak monitor. (Fig. 10) ECT Engine load Intake air temperature (IAT) Mass air flow (MAF) Vehicle speed (VS) Figure 10: These simple schematics show the various conditions that can occur with a coolant sensor circuit. Voltage measured with a meter at the ECM or monitored with a scan tool with the properly working circuit (left) should show a value between .5 and 4.5 volts. The same measurement with an open (center) would read 5 volts. A short (right) would measure close to 0 volts. Actual voltage will go up as temperature goes up. July 2016 7 MACS Service Reports