Technical Description of OBD-II Trouble Code
Malfunction in the Circuit of the Engine Coolant Temperature Sensor
What does that signify?
This diagnostic trouble code (DTC) is considered generic because it applies to all vehicles equipped with OBD-II from 1996 onwards. Specific troubleshooting and repair steps may vary slightly depending on the make and model of the vehicle.
The ECT (Engine coolant temperature) sensor is essentially a thermistor that changes its resistance with temperature. It is usually a two-wire sensor, with a 5Volt reference from the PCM (Powertrain Control Module) and a ground signal to the PCM. It is important to note that this is different from a temperature SENDER, which typically operates the dash temperature gauge and functions in a similar way as the SENSOR, but is a different circuit from what P0115 refers to.
As the coolant temperature changes, the resistance on the ground signal to the PCM also changes. When the engine is cold, the resistance is high, and when the engine is warm, the resistance is low. If the PCM detects a voltage condition that appears to be abnormally low or high, the P0115 trouble code will be set.
An example of an Engine Coolant Temperature ECT sensor
Symptoms
The symptoms of a P0115 DTC code may include:
Causes
Usually, the cause can be attributed to a faulty ECT sensor, although the following factors should not be ruled out:
Possible Solutions
First, visually inspect the sensor for any damage to the wiring or connector, and repair as necessary. Then, if you have access to a scanner, determine the temperature of the engine. (If you do not have access to a scan tool, using the dash temperature gauge may not be an effective way to determine coolant temperature. This is because the P0115 code refers to the ECT SENSOR, while the dash gauge is usually operated by a one-wire SENDER. Essentially, it is a different sensor that the code does not refer to.)
2. If the engine temperature is abnormally high, around 280 deg. F, that is abnormal. Unplug the sensor on the engine and observe if the signal drops to, for example, negative 50 deg. F. If it does, then it is likely that the sensor is internally shorted and causing a low resistance signal to be sent to the PCM. However, if you want to be certain that it is the sensor and not the wiring, there are a couple of tests you can perform. With the ECT sensor unplugged, check for a 5-volt reference on the reference circuit with KOEO (Key on engine off). Additionally, you can check the resistance of the sensor to ground using an Ohm meter. The resistance of a normal sensor to ground may vary slightly depending on the vehicle, but generally, if the engine temperature is around 200 deg. F., the resistance will be approximately 200 Ohms. If the temperature is around 0 deg. F., the resistance will be over 10,000 Ohms. With this test, you should be able to determine if the resistance of the sensor matches the temperature of the engine. If it is not accurate according to your engine’s temperature, then you likely have a faulty sensor.
3. Now, if the temperature of the engine according to the scanner is around 280 deg. F. and unplugging the sensor does not cause the reading to drop to negative 50 deg. F., but it remains at the same high temperature reading, then you will need to repair the short on the signal circuit (ground) to the PCM. It is shorted directly to ground somewhere.
4. If the temperature reading of the engine according to the scanner is showing negative 50 deg. F or so (and you do not live in the arctic!), unplug the sensor and check for a 5V reference present at the sensor.
5. If there is no 5V reference, then check at the PCM connector for the proper 5V reference. If it is present at the PCM connector, then repair the open or short on the 5V reference from the PCM. If there is no 5V reference present at the PCM connector, then your diagnosis is complete and you may have a fault in the PCM. 6. If the 5V reference circuit is intact, then check the ground signal to the PCM using the previous ground resistance test. If the resistance is not normal for the temperature of the engine, then measure the resistance of the ground signal to the PCM by removing the ground signal wire from the PCM connector. The wire should have no resistance when unplugged from the PCM to the sensor. If it does, repair the open in the signal to the PCM. If it does not have any resistance on the ground signal wire and the resistance test of the sensor is normal, then suspect a faulty PCM.
Other coolant engine light codes: P0115, P0116, P0117, P0118, P0119, P0125, P0128
