The onboard diagnostic (OBD) code P0118 is an Engine Coolant Temperature Circuit – High Input malfunction. This code refers to the engine coolant temperature (ECT) sensor. The ECT is a thermistor (an electrical resistor whose resistance is greatly reduced by heating) and is located in a coolant passage in the cylinder head. Its primary job is to monitor the rise and fall of engine coolant temperature. Normal operation of the sensor means that resistance will be high when the coolant temperature is low and the resistance drops when the coolant temperature increases.

The ECT works with the Powertrain Control Module (PCM). The PCM monitors voltage drop to determine the coolant temperature. When the engine has been running for more than a few minutes and the ECT is reading a less than freezing temperature, the PCM thinks there is a circuit fault and sets the P0118 code. The code is also set if the PCM determines that the ECT sensor resistance is out of specification.

 Common Symptoms

  • Check Engine Light will be lit on the dashboard.
  • Poor fuel economy or hard starting may be present in some instances.
  • No abnormal symptoms may be noticed in other cases.

Possible Causes

  • Defective ECT sensor
  • Corroded or dirty engine coolant
  • Faulty or corroded ECT wiring or connections

What to Check

Using a scan tool, check the reading of the coolant sensor. If it reads a number that makes sense, the problem might be intermittent. Wiggle the connection at the connector while using the scan tool and watch for any drop-offs. A drop-off would indicate a bad connection.

If the scan tool reads an illogical number, check the resistance of the temperature sensor. If it is out of specifications for the vehicle, you have found your problem and it will need to be replaced.

If it is within specifications, unplug the sensor and jumper the two terminals of the connector together using a fused jumper wire. The ECT reading should now be above 250F. If not, there is likely a problem with a bad connection.

Also, check the ECT sensor connector with the key in the on position but the engine off. There should be a steady 5 volt reference voltage and a good ground connection. If you do not have these things, check for these same items at the PCM connector.

How to Fix It

How to fix the P0118 OBD code will depend on the make and model of your vehicle and what you were able to find in terms of readings with the scan tool when used with the ECT sensor and connector.

The ECT sensor will need to be replaced if the resistance is out of specification for the vehicle. If it is within specifications, the connector will need to be checked for voltage and ground connection. If you do not have those things, the ECT connector will need to be replaced. The ECT sensor can not function properly without a good connection.

The following repairs will fix the P0118 code:

  • Repairing or replacing the ECT connector
  • Repairing or replacing the wiring open circuit as necessary
  • Replacing the ECT with a new sensor

Additional Tips

Typically, the P0118 code means the ECT sensor is bad or the connection to it is bad because it’s shorted out and needs to be replaced. Occasionally, but much more rarely, the problem could exist with the PCM connector or PCM itself.