There are 2 separate coolant
temperature sensors. 1 is the sender for the gauge cluster. The other is for the engine
computer. The computer only sets codes for the sensor with the Gray/Red stripe, and Light Green/Red stripe wires. There is one sensor right on top of the thermostat
housing (where the upper radiator hose connects to engine). The other sensor is nearby, on the driver's side of the thermostat housing.
If the computer is not seeing at least 250 F from the Engine Coolant Temperature sensor during the self test, the code will set. If the check engine light is not on, this code may just be a result of not satisfying all the self-test criteria. The engine must be idling at full normal temperature before running the self test codes. If a bad thermostat is keeping the temps down, this code could result.
The above wiring diagram shows the sensor circuit that can cause the code. The wires must be checked to ensure that the sensor receives reference voltage and that the signal voltage is present at the computer pin 38.
Circuit 359 is reference voltage of 5 volts, supplied by pin 91 of the PCM (computer). This voltage is shared by many sensors. The voltage drops created by the ECT sensor is read by the PCM monitoring voltageat pin 38 circuit 354. If the voltage is less than .2 volts during the self test, the code sets because low voltage is equivalent to low temperature. The hotter the sensor gets, the closer the voltage gets to 5 volts supplied as reference. So, you need to make sure that the test is carried out properly, and that the voltage at pin 38 does indeed vary with temperature. If you have a scan tool that can monitor the data from the engine sensors, use it to look at ECT sensor voltage signal. If the voltage is stuck low, this means that circuit 354 is shorted to ground, and needs to be repaired.