P0117 is a generic OBD2 code. It indicates that your Subaru Legacy’s PCM (powertrain control module) has detected the signal coming from the engine coolant temp (ECT) sensor has a voltage level below its normal operating range.
With the ECT sensor, as the voltage level drops, it will appear that your Legacy is overheating.
While P0117 is often caused by a bad sensor or wiring issue, you should treat it like the engine is overheating to avoid doing severe damage to the engine (and your wallet).
Here’s the meaning of P0117 for the Subaru Legacy, divided into its two main parts:
ECT (Engine Coolant Temperature) Sensor
The ECT is responsible for telling the PCM the temperature of your engine coolant. It uses this data to adjust the air-fuel ratio, change engine timing, turn the radiator fan on and off, etc.
Circuit 1 – Low Input
The voltage signal coming from the engine coolant sensor is below the normal operating range. The lower the signal, the higher the temperature reading.
Subaru Legacy: P0117 Symptoms
Here are the most common symptoms of P0117:
- Fail-safe Mode: If your vehicle has a fail-safe mode, P0117 will usually activate it. Your PCM is trying to keep your engine from overheating and cracking a head or blowing a head gasket.
- Check Engine Light: Any other codes that came up with P0117 can give helpful insight into what’s going on with your engine.
- Engine Running Rough: There may be black smoke coming from the exhaust (suggesting the engine is overheating).
Subaru Legacy: P0117 Causes & Diagnosis
With the way that the ECT works, your Legacy’s PCM will see a signal indicating that the engine is overheating A LOT. There are two ways to go with diagnosing P0117.
- If the engine is overheating, you need to determine why.
- If there are no signs the engine is overheating, it’s time to determine if the sensor is bad or not.
The cooling system of a vehicle is relatively simple. Here are the most common reasons your Subaru Legacy can overheat:
- Low Coolant: Check the overflow tank. If it’s empty, fill it up. Don’t fill a radiator when it’s hot. Running a hot engine with cold fluid can cause problems. Taking the radiator cap off with a hot engine can melt your face with steam.
- Bad Thermostat Or Water Pump: Feel the radiator hoses if the engine is overheating. If they don’t feel hot, that would indicate that coolant isn’t cycling through the engine. A bad thermostat or water pump can cause this.
- Leaks: Look under the vehicle. Is there coolant leaking anywhere? You can usually see where a leak is even if it’s dry. There’ll be a white chalky buildup where it’s been leaking if it has been leaking for a while.
For more on overheating, check out our guide to reasons why your Subaru Legacy is overheating.
2. Engine Not Overheating P0117 Diagnosis
Here’s how to go about diagnosing P0117, assuming that your Subaru Legacy does not appear to be overheating:
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 Legacy’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.
Clear the codes, take it for a test drive, and see if P0117 comes back.
Test the ECT Sensor
If it’s reading an unnaturally high temperature, unplug it. If the temp goes to the negative digits, that would indicate that the circuit is okay and that the ECT sensor needs replacing.
After unplugging the sensor, if the temperature STAYS high, you’ll need to look at the wiring harness.
Check the Wiring Harness for Wiring Issues
Now that you’ve unplugged your Legacy’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 P0117.
Check that the voltage going from the PCM is correct. On most vehicles, it’s 5 volts.
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.
P0117 in the Subaru Legacy is most likely caused by a bad ECT sensor or wiring issue. However, it can also occur if the vehicle overheats. Good luck!