Auto Systems and Repair: P0118 - OBD-II Trouble Code
OBD II Fault Code
- OBD II P0118
Fault Code Definition
- Engine Coolant Temperature Circuit High Input
What does Mean?
The Engine Coolant Temperature Sensor measures the rise and fall of the Engine Coolant Temperature. This provides critical data needed for the Powertrain Control Module (PCM) to control the Air Fuel Ratio, the Ignition Spark Timing, the cooling fans, and many components of the Emissions Control Systems. The Engine Coolant Temperature Sensor converts the coolant temperature into a voltage that is high when the engine is cold and low as the engine warms up.
Code P0118 indicates a high voltage input from the Coolant Temperature Circuit to the PCM.
- Check Engine Light will illuminate
- In many cases, no abnormal symptoms may be noticed
- In some cases, the engine may be hard starting and/or get poor fuel economy
Common Problems That Trigger the P0118 Code
- Defective Engine Coolant Temperature Sensor
- Rusty and/or corroded Engine Coolant
- Faulty or corroded Engine Coolant Temperature wiring or connections
- Engine Coolant Temperature Sensor is replaced when the real cause is rusty coolant
- Engine Coolant Temperature Sensor is replaced when the real cause is a poor connection or chafed wiring
- Engine Coolant Temperature Sensor is replaced when the real problem is in the Cooling System
Polluting Gases Expelled
- HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
- CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas
- NOX (Oxides of Nitrogen): One of the two ingredients that, when exposed to sunlight, cause smog
P0118 Diagnostic Theory for Shops and Technicians
When diagnosing a P0118 code, it is important to record the freeze frame information and then to duplicate the code setting conditions with a test drive while paying close attention to engine load, throttle position, RPM, and road speed on a data streaming scan tool. As you drive the vehicle, compare these values to the Coolant Temperature Sensor PID or parameter ID. The Coolant Temperature Sensor voltage values should rise and fall with changes in engine temperature. Compare the Engine Coolant Temperature or ECT readings to the Intake Air Temperature Sensor readings, as they should move in tandem with each other. However, the Engine Coolant Temperature sensor will have a much higher temperature range.
Check the ECT sensor connector with the key on and the engine off. There needs to be a steady 5 volt reference voltage and a very good ground. Find and use the proper engine performance wiring diagram to discern the proper color and position of these wires in the connector.
It never hurts to do a key on-engine off voltage output test of the ECT sensor. Typically, I CAREFULLY use a heat gun to raise the temperature of the area around the sensor and study the change in values of the Coolant Temperature Sensor circuit. It's important to use the scan tool data stream as this will verify the ECT harness and connections.