How To Test: P0118 OBD II Trouble Code (Ford 4.0L)

TEST 2: Checking The Condition Of The ECT Sensor's 2 Wires

In this test section, we're gonna see if the ECT sensor's wires have an open-circuit problem.

In plain English, this means we're gonna' see if one or both wires are cut.

The photo in the image above (although the connector doesn't belong the ECT sensor), is an example of the kind of problem we're looking for.

Don't worry, this a very simple test. You don't have to check the entire length of the wiring between the PCM and ECT sensor connector, just the length nearest to the ECT sensor's connector (about 6 inches away from ECT sensor's connector).

OK, this is what you need to do:

1. 1

   Remove the hard plastic protector that's over the wires.
   
   You need to expose the ECT sensor's wires starting from the connector to about 6 inches away (from the connector).
   
   IMPORTANT: Perform this test with a completely cold engine. This is a safety precaution to avoid burns on your hands or arms, in case the engine is hot.

2. 2

   Check for dry-rot and/or insulation peeling off the 2 ECT sensor wires and/or broken wires.
   
   The most likely place you'll find this condition (dry-rot and/or insulation peeling off) is right near the ECT sensor's connector.

3. 3

   Have a helper wiggle the 2 ECT sensor wires while you observe the ECT sensor's PID labeled COOLANT (on your scan tool).
   
   What you're looking for is to see if wiggling the wires has any effect on the temperature reported on your scan tool.

Let's interpret your test results:

CASE 1: The wires have dry-rot and/or insulation peeling off and/or broken. This is probably the reason behind the P0118 lighting up the check engine light (CEL) on your Ford.

Your next step is to replace the engine coolant sensor's connector or repair/replace the affected wires.

After the repair, road test your Ford to make sure the P0118 DTC doesn't come back.

CASE 2: Wiggling the 2 ECT sensor wires caused a change in the temperature reading. This test result tells you that the wires do have a problem.

You need to carefully inspect the connector and the 2 wires and replace and/or repair what is damaged or shorted.

To give you some more specifics: Gently wiggling the ECT sensor connector's 2 wires should have no effect on the ECT sensor reading displayed on the scan tool, unless the connector is bad or one of the wires has an ‘open’. Since wiggling the connector did have an effect, you now know that replacing the ECT sensor connector or repairing the problem in the wires will solve the ECT sensor and P0118 Code problem.

CASE 3: The wires DO NOT HAVE dry-rot and ARE NOT shorted together and wiggling the ECT sensor connector's wires DID NOT cause the temperature to change. This tells you that the ECT sensor connector and its wires are OK.

This test result also tells you that the ECT sensor is the one that's malfunctioned and needs to be replaced. Before you do, I suggest one more test.

And this is to test to make sure that the PCM is not fried. This is a very simple test and it requires that you disconnect the ECT sensor from its connector and then checking, with your scan tool in Live Data mode, that the engine coolant temperature (ECT) sensor's PID is now reading -4 °F (-20°C).

For this test, go to: TEST 3: Disconnecting The ECT Sensor From Its Connector.

TEST 3: Jumpering Together The ECT Sensor Circuits

So far, you've verified that the ECT sensor is reporting an extreme cold temperature (TEST 1). You've also wiggle tested and checked the ECT sensor's wires for an open-circuit problem (TEST 2).

Now, we're gonna' unplug the ECT sensor from its connector and jumper the connector's 2 terminals together with a jumper wire.

The purpose of this test is to see if the PCM can react to this (jumpering of the ECT sensor's connector) by displaying an extreme hot temperature of 250 to 300°F (121 to 150°C) on your scan tool.

If it doesn't then we can surmise that one of two things are wrong: Either that the PCM is bad or there's a hidden electrical short in the wires between the PCM and ECT sensor. Both of these conditions could fool the PCM into thinking the ECT sensor is bad.

OK, here's what you'll need to do:

1. 1

   Check the current COOLANT value on your scan tool's display.

2. 2

   Disconnect the ECT sensor from its connector and using a jumper wire jumper the two metal terminals.
   
   IMPORTANT: Be careful and don't insert a jumper wire that will damage the connector's terminals or you'll cause a false contact that'll require a new connector.

3. 3

   PID COOLANT should now read 250 to 300°F (121 to 150°C) for the engine coolant temperature (ECT) sensor reading after you've jumpered the 2 circuits.

   1. You'll also see DTC P0117.

Let's examine your test result:

CASE 1: The PID labeled COOLANT registered 250 to 300°F (121 to 150°C). This test result tells you that the wiring between the PCM and ECT sensor connector is OK and that the PCM is not fried.

You have now confirmed 3 very important things:

1. That the PCM is seeing an extreme cold temperature around -4 to -46°F (-20 to -43°C) (TEST 1).
2. That there are no shorts in the sensor's wiring or in its connector (TEST 2).
3. That the PCM is OK (TEST 3).

Therefore, you can confidently conclude that the ECT sensor is bad and needs to be replaced.

CASE 2: The PID labeled COOLANT DID NOT register 250 to 300°F (121 to 150°C). Make sure that you're testing the correct wires, that your connections are OK, and repeat the test.

Then this tells you that you have a problem in the wiring between the ECT sensor and the PCM or that the PCM is fried (although a bad PCM is rare).

Although testing the wiring between the PCM and the ECT sensor is beyond the scope of this tutorial, you now have eliminated the ECT sensor as the source of the P0118.