ECT Sensor Test (P0117, P0118) (GM 4.3L, 5.0L, 5.7)

This article will help you to troubleshoot the engine coolant temperature (ECT) sensor on your GM 4.3L, 5.0L, or 5.7L equipped pick up, van, mini-van or SUV.

You can either use a multimeter or a scan tool (w/ Live Data capability) to diagnose the ECT sensor and/or trouble codes.

Important Suggestions And Tips

TIP1: A bad engine coolant temperature sensor will not cause your pick up (van, mini-van, or SUV) to overheat. This article does not cover any type of overheating issue.

If your vehicle is overheating, I suggest taking a look at these two tests:

• How To Test The Thermostat (GM 4.3L, 5.0L, 5.7L).
• Troubleshooting A Blown Head Gasket (GM 4.3L, 5.0L, 5.7L).

TIP2: You'll need a scan tool with Live Data capability to be able to follow the test steps in this article (although you can resistance test the ECT sensor with a multimeter).

You don't need a professional $5K (US) scan tool to do it. A simple generic scan tool will do (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).

TIP3: Some of my testing suggestions involve tests with the engine running, this means you've got to be on your toes and alert. Think safety all of the time.

Symptoms Of A Bad Engine Coolant Temp (ECT) Sensor

The are several symptoms a bad ECT sensor causes, and they are:

1. Check engine light is on and one of the following codes is stored:
   • P0117: ECT Sensor Circuit Low Voltage.
   • P0118: ECT Sensor Circuit High Voltage.
2. Bad gas mileage.
3. Extended cranking. In other words, a hard start.
4. Your vehicle may crank but not start.

When there's a problem with the ECT sensor or its circuits, the PCM will see one of the three distinct conditions:

• It will receive input from the ECT sensor indicating an extremely high coolant temperature. Usually around 250° F, even tho' the engine is cold. This condition leads the PCM to register a P0118 DTC.
• It will receive input from the ECT sensor showing an extremely low temperature. This temp is usually around -40° F. This condition leads the PCM to set a P0117 DTC.
• Reports a 'normal' temperature, but this temperature does not correspond to the actual coolant temperature.

P0117 And P0118 Essentials

You've probably seen the official OBD-II description of these two diagnostic trouble codes but in plain English what are these two codes saying? Well, this section will shed some light on this (by the way, you're gonna' see the term voltage drop and I'll explain this term in the next section):

P0117: Engine Coolant Temperature (ECT) Circuit Low Input.

• This code indicates that the PCM is seeing a low voltage drop across the ECT sensor, which it's translating as an extremely hot coolant condition.
• When the PCM senses this low voltage input, it will translate this voltage input to a 250° F temperature.
• This code is caused by one of the following:
  • Coolant temp sensor's wires are shorted to Ground.
  • Coolant temp sensor's wires are shorted to each other.
  • Coolant temp sensor is bad.
  • PCM is bad.

P0118: Engine Coolant Temperature (ECT) Circuit High Input

• This code indicates that the PCM is seeing a high voltage drop across the ECT sensor, which it's translating as an extremely hot coolant condition.
• When the PCM senses this high voltage input, it will translate this voltage input to a coolant temperature of -46° F (-50° C).
• This code is caused by one of the following:
  • Coolant temp sensor's wires have an 'open-circuit' problem (usually due to a cut wire).
  • Coolant sensor's connector has become unplugged from the sensor itself.
  • Coolant temp sensor is bad.
  • PCM is bad.

Coolant Temperature Sensor Basics

Using a scan tool with Live Data capability is the fastest and easiest way to diagnose an ECT sensor but sometimes, you've got to verify the sensor's output directly at the sensor, this section will shed some light on how the ECT sensor works so that, if you have to, you can test it directly with a multimeter.

The coolant temperature sensor's resistance changes with temperature (this type of sensor is called a thermistor).

Here are more specifics:

1. The engine coolant (ECT) temperature sensor has two wires in the connector.
   • One is the power wire and this power is in the form of 5 Volts. These 5 Volts are created inside the PCM.
   • The other wire is the Ground wire. This Ground is provided internally by the PCM too.
2. When the coolant is cold (or ambient temperature), the sensor's internal resistance is higher.
   • This higher resistance has a direct impact on the 5 Volts passing thru' it since it blocks some of it from passing thru' to Ground (-this action of blocking voltage from passing thru' is called a voltage drop) .
   • The PCM senses this voltage drop and translates the specific amount into a temperature reading.
   • Now, in case your wondering., this voltage drop can be measured with a multimeter in Volts DC since it (the meter) would act as a bridge (to Ground) for the voltage being blocked.
3. As the engine warms up, the coolant starts to heat up, which in turn causes the coolant temp sensor's resistance to decrease.
   • This decreasing resistance now lets more of the 5 Volts to pass thru' the sensor onto Ground.
   • The PCM is able to sense this lower voltage drop and translates the numbers as a higher temperature.
   • So, if you were using a multimeter, this would mean that the voltage numbers, displayed by the meter, would decrease as the engine's coolant heats up.

The following table will help you to see this temperature to resistance relationship:

OK, enough of the working theory, let's get testing.

TEST 1: Checking The Temperature With The Key On Engine Off

OK, the first thing you have to do, after pulling the diagnostic trouble codes, is to check the coolant temperature.

Before I go any further, I need to tell you that to test the ECT sensor, it's super important that you start out with a completely cold engine (at ambient temperature). Why? 'Cause this will ensure an accurate test result.

How will you know that the engine is at ambient temperature? Because when the engine is completely cold, the coolant temperature (as registered on your scan tool) will be within ±10° F of the outside temperature (ambient temperature).

OK, this is what you need to do:

1. 1

   Connect your scan tool to the diagnostic connector.

2. 2

   Turn the key on but don't crank or start the engine.

3. 3

   Once the scan tool activates, go to Live Data mode

4. 4

   In Live Date mode, scroll down to the PID labeled Coolant.

5. 5

   Now, take note of the coolant temperature. There are only three results possible and they are:
   
   1. The temperature will be within ±10° F of ambient temperature (outside temperature).
   
   2. The temperature will read at or near -40° F.
   
   3. The temperature will read 250° F.

Let's take a look at what your results mean.

CASE 1: The coolant temperature was within the ±10° F of ambient temperature. So far so good, but the ECT sensor or its wiring could still be bad.

There's a good chance, that even tho' you got a normal temperature result, the ECT sensor is behaving good at this point in time but once the engine warms up or the vehicle is on the road, the problem manifests itself. So, the next step is the wiggle test. Go to: TEST 2: The Wiggle Test.

CASE 2: The coolant temperature reading was at or near -40° F . This confirms that there is a problem with the coolant temperature sensor and also confirms the veracity of the P0118 diagnostic trouble code.

Reading such an extreme negative temperature usually indicates that the ECT sensor has become disconnected somehow. To make sure, go to: TEST 2: The Wiggle Test.

CASE 3: The coolant temperature reading was at or near 240° F . This confirms diagnostic trouble code P0117 and that there's a problem with the engine coolant temp sensor or its circuits.

It's been my experience, that when this code pops up, it's usually due to the ECT sensor's wiring shorting together.

The next step is to do a wiggle test on the ECT sensor's connector and wiring. For this, go to: TEST 2: The Wiggle Test.