BCM Code F12: A/C Low Side Temp Sensor Circuit: Notes

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.

NOTE: If this code is not displayed during second pass of diagnostic codes, it is an intermittent failure and cannot be diagnosed using this procedure. Refer to NOTE ON INTERMITTENTS at end of this text.

The A/C low side temperature sensor uses a thermistor to control signal voltage to BCM. The BCM applies and monitors voltage on circuit No. 731 to sensor. When sensor is cold, its resistance is high, therefore, BCM will see a high monitored voltage. As sensor warms, its resistance becomes less and signal voltage is lowered through sensor ground, circuit No. 736. This signal voltage will vary between 5 volts (open circuit) and zero volts (shorted circuit).

Code F12 will set if signal voltage indicates an open circuit, temperature less than -29°F (-34°C); or shorted circuit, temperature greater than 189°F (87°C). These conditions can be observed in BCM data display (parameter P.2.8) as a reading outside the range of -29°F to 189°F (-34°C to 87°C).

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

Test Number 1) If temperature reading changes after disconnecting sensor, BCM and wiring are okay.

Test Number 2) If temperature reading changes after jumpering sensor terminals, BCM and wiring are okay.

Test Number 3) By applying ground to various points in circuits, an open can be isolated when temperature display changes from about -29°F to 194°F (-34°C to 90°C).