Error Code P0115 is described as an Engine Coolant Temperature (ECT) Sensor Circuit Malfunction. It’s detected when there is too high or too low a change in the voltage signals that this sensor sends to the PCM (powertrain control module, also called an engine control module or ECM).

It’s a generic trouble code, meaning it applies to all vehicles made since 1996 up to the present. Specifications on description and troubleshooting depends on the make and model of the vehicle.

This code is related to P0116, P0117, P0118 and P0119.


The job of the ECT sensor is to measure the rise and fall of the Engine Coolant Temperature. This provides the PCM the needed data to control different aspects of Emission Control Systems, particularly Air Fuel Ratio, Ignition Spark Timing and cooling fans. The ECT sensor converts coolant temperature into voltage that is high when the engine is cold and low when the engine is warm. Basically, it’s a thermistor that changes resistance with temperature. The sensor usually comes with two wires, one with a 5V reference voltage for the PCM and the other for ground signal. Note that this is entirely different from a temperature sender (which controls the dash temperature gauge and works in a similar way to the sensor).

This code is set when the PCM detects erratic or irrational changes in the voltage signals from the ECT circuit.

Common Symptoms

Aside from activating the check engine light, some people may not notice anything abnormal about their car. In some cases however, the vehicle may experience different drivability issues that can be easily associated with problems in the Emission Control Systems, such as:

  • Hard starting
  • Engine may die
  • Rough running
  • Engine hesitation until it warms up
  • Backfire in tailpipe
  • Cooling fans constantly running when they shouldn’t be, or not run at all when they should be
  • Engine running lean and increase in NOx emission (may require gas analyzer)

Possible Causes

Error Code P0115 can usually be traced to a bad ECT sensor. Other common causes include:

  • Damaged wiring or connector at the sensor
  • Faulty or corroded connector or wirings
  • Corroded or rusty engine coolant
  • Open or short in signal circuit
  • Open or short in reference signal circuit
  • Bad PCM (rare)

How to Check

As with other error codes, you can start your diagnosis by resetting the OBD-II scanner to clear the core, then retest the vehicle to see if the code comes back.

If the code comes back, it is recommended to conduct a test drive. It is important to record the freeze frame data, and then duplicate the conditions when the code was set, from engine load, RPM, throttle position and road speed, through a data streaming tool. Then, compare the values to the Coolant Temperature Sensor parameter ID. The voltage values should rise and fall with the changes in the temperature. Then compare the ECT readings to the readings of the IAT sensor; the two should move in tandem, with ECT a little higher in temperature range.

With the key on, engine off, check the ECT sensor. There should be a steady stream of 5V reference voltage and a good ground. Look for the proper engine performance wiring diagram to determine the proper color and positions of the connectors and wires.

Visual inspection is also very important. Make sure you conduct a thorough inspection of the wiring and connectors, while also looking for any signs of damage. Determine the coolant temperature using a scan tool, and not a dash temperature gauge, as the latter doesn’t offer an effective way to determine the temperature of the coolant. This is because the error code refers to the ECT sensor, and dash gauge is usually operated by a one-wire sender, which is an entirely different sensor not related to the code.

How to Fix

This error code is quite simple to fix. You can start by repairing or replacing the ECT sensor. If the ECT sensor is in good condition, then you should move to the wiring and connectors. Do the necessary repair and replacements.

If that still doesn’t fix the problem, you can try replacing the ECT sensor with a new one.

In most cases, this error code shows up due to corrosion in the connector or shorting in the harness, causing open circuit. So you should check that out as well.