Action Taken

WARNING: This page is about a different car, the 1990 Cadillac Fleetwood and 1990 Cadillac DeVille. However, it is still accessible from the selected car via links, so may be relevant.

ECM turns on SERVICE SOON light.
ECM uses MAT sensor value in place of coolant sensor for the first 4 minutes, then the value of 194°F (90°C) is used.

The coolant sensor is a thermistor (temperature-sensitive resistor). Sensor uses a 2-wire harness. Sensor signal voltage comes from ECM to terminal "B" on circuit No. 410. Sensor reference ground comes to terminal "A" from ECM on circuit No. 476.

As temperature of sensor increases, sensor resistance is lower. The monitored signal voltage from ECM to terminal "B" decreases as sensor temperature increases and current flows through sensor element to terminal "A" sensor ground.

Code E15 sets because ECM assumes coolant temperature cannot be -36°F (-38°C) or less when MAT is 23°F (-5°C) or greater. Sensor signal is open to sensor.

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

Test Number 1) If sensor is open, parameter P.0.4 should read -26°F (-32°C) or less. If not, sensor signal is not open. See Note on Intermittents.

Test Number 2) Check for open sensor signal circuit No. 410 from ECM to sensor connector. If parameter P.0.4 reads 298-304°F (148-151°C) with connector shorted, circuit No. 410 is okay.

Test Number 3) Check for open sensor ground circuit No. 476 from ECM to sensor. If terminal "B" to ground causes parameter P.0.4 to read 291-304°F (144-151°C), there is an open in circuit No. 476 between sensor terminal "A" and ECM terminal D13.

Test Number 4) Check for ECM's ability to recognize a short to ground or low voltage on terminal B8, coolant sensor input.

Test Number 5) Fault is at ECM connector or ECM. Before ECM is replaced, perform CHART C-1: ECM REPLACEMENT CHECK .