P0116 indicates that your Subaru Forester’s engine coolant temperature sensor is sending a signal to the PCM (powertrain control module) that is outside of its normal operating range (in terms of voltage).
While P0116 is a generic OBD2 code (it has the same definition for any car or truck), it has the same basic causes and diagnostic order for the Forester as any other vehicle.
The most common causes of P0116 are a bad ECT (engine coolant temperature) sensor or a bad thermostat. There can be other causes as well. We discuss this in much greater detail below in the “causes + diagnosis” section.
P0116 Definition: ECT Sensor – Range/Performance
Here’s the definition of P0116 for the Subaru Forester is divided into it’s two halves:
ECT (Engine Coolant Temperature) Sensor 1
The ECT is responsible for telling the PCM the temperature of your engine’s coolant. It uses this data to adjust the air-fuel ratio, change engine timing, turn the radiator fan on and off, etc.
Range/Performance
The ECT is typically a two-wire sensor. The first wire supplies voltage from the PCM. The second is the ground wire.
As the engine temperature increases, the resistance lowers. The lower the resistance, the higher the temperature reading. Here are the two most common scenarios where P0116 is thrown in the Forester are:
- The engine is warming up, but the temperature reading is increasing faster than what should be possible.
- The signal isn’t changing at all as the engine warms up.
Subaru Forester P0116 Symptoms
Here are the most common symptoms of P0116:
- Check engine light
- Poor fuel mileage
- Heater not working well (indicates a stuck thermostat)
- Engine not running properly (stalls, black smoke, misfiring)
Subaru Forester P0116 Causes + Diagnosis
While there is around a 50% chance that throwing a new ECT sensor or Thermostat in your Forester and calling it a day will fix P0116, it’s a good idea to do your due diligence before wasting the time and money installing a sensor that you may not need.
Compare the IAT Temp Versus the ECT Temperatures When Cold
If you have access to a good scan tool, you can quickly determine if your ECT is bad or not by comparing it to the IAT sensor readings.
When your Forester’s engine is truly cold (think parked overnight), the ECT and IAT (idle air temperature) readings should be the same. If the ECT shows a temperature higher than the IAT, the ECT will need to be replaced.
If the ECT temp reads much colder than the current temp, that would indicate that it is probably bad.
Clear the codes, take it for a test drive, and see if P0116 comes back.
Test the ECT Sensor
Unplug the ECT sensor. If the temp goes to the negative digits (on a scanner), that would indicate that the circuit is okay and that the ECT sensor needs replacing.
If your Forester is equipped with a temperature gauge, it should dip all the way to cold. A scanner is still more reliable, but sometimes you have to work with what you’ve got.
If the temperature sensor reading is super high, that can indicate that the ground wire is bad.
If everything seemed fine with the ECT sensor, move it’s time to suspect a bad thermostat.
Check the Wiring Harness for Wiring Issues
Now that you’ve unplugged your Forester’s ECT sensor and the temperature reading stayed high, it’s time to inspect the wiring harness. Look for wiring that has been damaged, frayed, or burnt.
The ECT harness connection may be damaged. Make sure that the pins look like they are in good shape. The ECT’s proximity to the hot exhaust will give it a rougher life than many sensors, and damage to the harness is a common cause of P0116.
Check that the voltage going from the PCM is correct. On most vehicles, it’s 5 volts.
Bad Thermostat
A bad thermostat can cause P0116 by not allowing the Forester’s engine to heat up. With coolant constantly cycling through the engine, the reading from the ECT doesn’t increase much and the code is thrown.
A good way to get a read on whether the thermostat is stuck open is if the engine has been running for a while, does the heater blow as hot as it normally would?
Bad PCM
It is possible (but not probable) that the PCM has gone bad. Take it to a skilled mechanic for diagnosis before you go down this avenue.
Conclusion
P0116 in the Subaru Forester is most likely to be caused by a bad thermostat or ECT sensor. Thanks for reading, and good luck with your vehicle!