CODE 15 (Open Coolant Sensor Circuit)

1. Disconnect coolant sensor. See Fig 1 . With ignition on and engine stopped, check voltage between sensor connector terminals. Voltage should be greater than 4 volts. If voltage is less than 4 volts, go to step 3). If voltage is okay, check resistance of coolant sensor with engine warm.
2. If resistance is greater than 1000 ohms, replace sensor. If resistance is less than 1000 ohms, sensor connection is poor or coolant is low.
3. Check voltage from ECM terminals No. 17 to 22 with sensor disconnected. If voltage is less than 4 volts, ECM or connection is faulty. If voltage is greater than 4 volts, check for open in wire to ECM terminals No. 17 and 22.

Fig 1: Wiring Diagram for CODE 15

Courtesy of GENERAL MOTORS CORP.