1999 Cadillac Deville Still Shows P0118 After Replacing Coolant Temperature Sensor
Good afternoon, We recently replaced the coolant temperature sensor on our 1999 Cadillac Deville, but the vehicle continues to display the P0118 diagnostic trouble code (DTC). The engine is also exhibiting erratic behavior—jumping and accelerating unpredictably. Temperature readings from the ECT sensor are fluctuating wildly between -50°F and 225°F, which is clearly abnormal. It's worth noting that we previously replaced the AC compressor at another shop, and they were unable to reset the system—now the AC will not accept refrigerant. Any insights or troubleshooting steps would be greatly appreciated. We suspect a wiring issue or sensor circuit fault despite replacement. Thank you in advance for your help!
First, it's important to understand that a diagnostic code does not confirm a part failure—it indicates a problem within the system. DTC P0118: Engine Coolant Temperature (ECT) Sensor Circuit High Voltage. With an OBD2 scanner, you should check exactly what the Powertrain Control Module (PCM) is detecting from the ECT sensor circuit. A common issue is a wiring fault—inspect the first several inches of the harness near the sensor for pinching, open circuits, or damage. Additionally, the PCM may restrict AC operation under certain conditions—for example, when refrigerant pressure falls below a threshold (typically around 40 psi) or exceeds another limit (around 220 psi). Since the ECT sensor is malfunctioning, this could be disrupting the system's ability to regulate AC function. Correcting the ECT issue should restore normal AC operation.