Pcm Code EO15, Open Coolant Sensor Circuit: Action:

PCM turns on SERVICE ENGINE SOON light and uses MAT sensor value in place of coolant sensor value all calculations 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 PCM 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 PCM to terminal "B" increases as sensor temperature decreases and less current flows through sensor element to terminal "A" (sensor ground). Code EO15 sets because PCM assumes coolant temperature cannot 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.

If sensor is open, PCM parameter ED04 should read -38°C or less. If not, then sensor signal is not open. See NOTE ON INTERMITTENTS.
This checks open sensor signal in circuit No. 410 from PCM to sensor connector. If parameter ED04 reads 148-151°C with connector shorted, then circuit No. 410 is okay.
This checks open in the sensor ground (circuit No. 1076 from sensor pin "A" to ground splice). If shorting pin "A" to ground causes parameter ED04 to read 148-151°C, then there is an open in circuit No. 976.
This checks if PCM can recognize a short to ground or low voltage on terminal E16, coolant temperature signal. If grounding terminal E16 causes ED04 to read 148-151°C, then there is an open circuit in circuit No. 410 from the PCM to the coolant sensor.
Fault is most likely at PCM connector or PCM. See PFI CHART C-1, PCM REPLACEMENT CHECK before replacing PCM.