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

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

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 and turn the key on (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).

    Once the scan tool activates, go to Live Data mode and scroll down to the PID labeled Coolant.

  2. 2

    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.

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.

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.

TEST 2: The Wiggle Test

The wiggle test will help you to find out if the ECT sensor diagnostic trouble code is due to a false connection (at the connector) or a problem in the wiring.

You'll need a helper to wiggle the wires for you, as you keep your eyes glued on your scan tool. What you're looking for is to see if the wiggling will change the result you got in TEST 1.

To be a little more specific:

  1. If in TEST 1 you got a -40° F reading: Since this usually indicates an ‘open’ short in one of the two ECT sensor's wires... wiggling the sensor's wires could bring them back together and you would see the -40° F go back to a normal temperature reading.
  2. If in TEST 1 you got a 250° F reading: Since this usually indicates that the two ECT sensor's wires have shorted together... wiggling the sensor's wires could get them to separate and you would see the 250° F go back to a normal temperature reading.
  3. If in TEST 1 you got a normal temp reading: Wiggling the wires may cause the intermittent short to pop up and you would see either a -40° F or 250° F on your scan tool.

OK, this is what you need to do:

  1. 1

    With the key On (but with the engine Off), check the coolant temperature with your scan tool.

  2. 2

    Have a helper gently wiggle the coolant sensor's connector and wires.

    Your job is to keep your eyes on the coolant temperature displayed on the scan tool.

  3. 3

    What you're looking for is one of two things:

    The coolant temperature will stay the same (no fluctuations caused by the wiggling).

    Or, the temperature will all of a sudden change to a drastic temperature (either -40° F or 240° F).

Let's take a look at what your test results means:


CASE 1: The wiggle test DID NOT cause changes in the coolant temperature   -This tells you that there are no wiring issues with engine coolant temperature (ECT) sensor.

So if in TEST 1 you had:

  1. A -40° F reading, the ECT sensor is BAD, replace it.
  2. A 250° F reading, the ECT sensor is BAD, replace it.
  3. A ±10° F of ambient temperature reading, go to TEST 3.

CASE 2: The wiggle DID create drastic changes in the coolant temperature (for example -30° F to 240° F) This confirms a malfunction with the engine coolant temperature sensor's connector or its two circuits.

The next step is to visually check that the two wires coming out of the connector are not fused together or at any point in the wiring harness.

Repairing the issue with the sensor's wire or connector will solve the diagnostic trouble codes (P0117, P0118).