ECM Code EO15: Open Coolant Sensor Circuit: Action:

ECM turns on SERVICE ENGINE SOON light and uses MAT sensor value in place of coolant sensor value for all calculations for the first 4 minutes of operation. Then a value of 194°F (90°C) is used.

The coolant sensor is a variable thermistor. The coolant sensor is a 2-wire sensor with a signal voltage coming from the ECM to sensor terminal "B" (circuit No. 410), and a sensor reference ground on terminal "A" (circuit No. 976).

As temperature of sensor decreases, sensor resistance increases. Signal voltage from ECM to terminal "B" increases as sensor temperature decreases and less current flows through sensor element to terminal "A" (sensor ground). Code E015 sets because ECM assumes coolant temperature can't be -36°F (-38°C) or less when MAT is 23°F (-5°C) or greater.

NOTE: Test numbers refer to test numbers on diagnostic chart.

Test Number 1) If sensor is open, parameter ED04 should read -38°C or less. If not, then sensor signal is not open. See NOTE ON INTERMITTENTS.

Test Number 2) This checks for open sensor signal in circuit No. 410 from ECM to sensor connector. If parameter ED04 reads 148-151°C with connector shorted, then circuit No. 410 is okay.

Test Number 3) This checks for open ground (circuit No. 976) from ECM to sensor. If shorting terminal "B" to ground causes parameter ED04 to read 148-151°C, then there is an open in circuit No. 976.

Test Number 4) This checks if ECM can recognize a short to ground or low voltage on terminal E16, coolant temperature signal.

Test Number 5) Fault is most likely at ECM connector or ECM. See CHART C1, ECM REPLACEMENT CHECK before replacing ECM.