Ford Mustang P0117 Trouble Code – Test your ECT Sensor

Problems with your Ford engine coolant temperature sensor, or your ECT can lead to the Ford Mustang P0117 trouble code. This condition will cause a problem with your coolant temperature gauge and can prevent your cooling fans to turn on. More importantly the trouble code for your Ford ECT sensor can create problems with your engine running rich or using too much fuel. This condition is caused because your engine computer no longer knows if your engine is properly warmed up, and will attempt to deliver more fuel to try and warm it up to operating temperatures.

When you see your Ford Mustang check engine light ON, you need a OBDII scan tool. This tool connects to your OBDII diagnostic port and analyzes the stored codes in your engine computer. If you determine that your stored diagnostic trouble code is for the Ford Mustang P0117 trouble code, the specific language for this code is Engine Coolant Temperature (ECT) Circuit Low Input. This means that the ECT sensor in your vehicle is reading or sending a signal that’s far lower than what the engine computer is expecting. Despite popular belief, this translates into a real time temperature that works out well above 250° F or 121° C.

Ford Mustang P0117 trouble code 1

In other words although the ECT sensor is reading a low signal, this really means that the engine coolant temperature it’s reading is actually much higher than what it expects. There’s 2 wires that lead to your Ford Mustang coolant temperature sensor. One is a switched 5 volt signal sent from the engine computer, or the Powertrain Control Module. The other wire is a sensor ground, and using both power and ground the Mustang ECT uses the internal resistance to adjust to the coolant temperature.

As the coolant in your Ford Mustang heats up, the internal resistance of the Mustang ECT lowers. When you have a Ford Mustang P0117 check engine code, you can test the ECT easily using your multimeter. Of course if you have a good OBDII scan tool you can simply plug into your Ford and read the Live Data that’s stored within. This test is easy to conduct because all you will need is an OBDII scan tool that can read Live Data.

Testing the Mustang ECT to correct the Ford Mustang P0117 code

The easy way to test your engine coolant temperature sensor is through a OBDII scan tool. Connect the tool and test your Mustang when your engine is cold. Turn the ignition to the ON position and read the Live Data. Depending on the temperature of your engine, you should read the following values.

  • 50° F 3.51 V
  • 68° F 3.07 V
  • 86° F 2.60 V

The lowest temperature that your Mustang ECT sensor will read is -40°F, so if you read this value this means that there’s definitely an issue with your Ford Mustang engine coolant temperature sensor or wiring.

Symptoms of the Ford Mustang P0117 check engine code

Because your Ford engine computer won’t know what the operating temperature of your engine is, there’s a good chance that the engine may run rich or lean. Here’s a few other common Ford Mustang issues that can stem from a failed ECT sensor.

  • Inconsistent fuel delivery
  • Bad gas mileage
  • Incorrect temperature reading
  • Engine won’t start
  • Check Engine Light ON

Today I’ll be showing you how to address the Ford Mustang P0117 in a 2001 Ford Mustang with a 3.8 liter engine in it. This engine coolant temperature sensor is a two pin unit and you’ll be using a digital multimeter to test the internal resistance.


Testing your Ford Mustang P0117 trouble code through the ECT

Using your multimeter you will be disconnecting the engine harness to your Ford Mustang engine coolant temperature sensor. You will need to turn the dial to read resistance or OHMS, and then you’ll be measuring the resistance through the two pins at the Mustang ECT. This will tell you if there’s an issue with the wiring or the sensor itself to correct your Ford Mustang P0117 trouble code.

Ford Mustang P0117 trouble code 2With the engine coolant sensor unplugged and the engine around room temperature, you can use the multimeter to read the internal resistance of the sensor. This will allow you to see whether or not the Mustang ECT sensor is any good or not.

  • 50° F  58.75 Ω
  • 68° F  37.30 Ω
  • 86° F 24.27 Ω
  • 104° F 16.15 Ω

If you find that the Mustang ECT sensor does not respond with the internal resistance listed above, you can determine whether the sensor is bad or if the wiring is bad. Once you find that the the ECT sensor is bad you can simply replace it and use a OBDII scan tool to erase your scan tool.

When your ECT sensor responds correctly, you need to check the wiring harness to see if there’s a switched power signal and a ground at the connector. This should resolve your Ford Mustang P0117 OBDII trouble code. Have any questions about this guide? Leave us a comment below and let us know!

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