P0118 OBD II Trouble Code Diagnostics (1996-2003 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer)

OBD II trouble code P0118 ECT Sensor Circuit High Voltage is set when the PCM in your Ford sees a fixed engine coolant temperature of -4 to -46°F (-20 to -43°C) that doesn't correspond to actual engine operating conditions.

In this tutorial, I'm gonna' help you troubleshoot the P0118 lighting up the check engine light (CEL) on your 4.0L equipped Ford pickup, SUV or mini-van.

If a trouble code P0117 is stored in your Ford's PCM's memory, the following tutorial will help:

• How To Test: P0117 OBD II Trouble Code (1996-2003 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).

ES You can find this tutorial in Spanish here: Cómo Probar El Sensor ECT -Código P0118 (4.0L Ford) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 4.0L V6 Ford Aerostar: 1996, 1997.
• 4.0L V6 Ford Explorer: 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003.
• 4.0L V6 Mercury Mountaineer: 1998, 1999, 2000, 2001, 2002, 2003.

Symptoms Of A P0118 Diagnostic Trouble Code

When the fuel injection computer sees and extreme cold temperature from the engine coolant temperature (ECT) sensor, your Ford is not gonna' run or idle at its best.

You may see one or more of the following symptoms when the engine coolant temperature (ECT) sensor fails:

1. Check engine light (CEL) is on.
2. DTC P0118 is present.
3. Won't pass the state mandated emissions test.
4. Gas mileage will suffer.
5. Hard start and/or extended cranking time (after shut off).
6. Black smoke coming out of the tailpipe.

P0118 -What Does This Code Really Mean?

In a nutshell, when a code P0118 ECT Sensor Circuit High Voltage lights up the check engine light (CEL), the PCM is letting you know that it sees a continuous engine coolant temperature of -4 to -46°F (-20 to -43°C).

The PCM in your Ford knows that as the engine starts and runs, it's gonna' heat up and that the ECT sensor is gonna' report just how much it's heating up. So when the PCM sees an extreme cold engine coolant temperature (that doesn't change), it knows the ECT sensor has either failed or there's a problem with the sensor's circuits.

Why is the engine coolant temperature so important for the fuel injection computer? This is due the fact that the colder the engine is, the more fuel the fuel injection computer (known as the PCM = Powertrain Control Module) needs to inject.

As the engine runs and warms up, the less fuel the PCM is needs to inject to keep the engine running happily and polluting less.

So when the PCM gets the wrong engine coolant temperature, it can no longer control fuel injection to maximize performance and decrease emissions.

How The ECT Sensor Works

Knowing the basics of the engine coolant temperature (ECT) sensor will help diagnose the P0118 ECT Sensor Circuit High Voltage trouble code lighting up the check engine light.

Without getting overly technical, this is how the engine coolant temperature (ECT) sensor works:

1. The engine coolant temperature (ECT) sensor is a 2-wire sensor.
   • One wire (circuit) supplies power.
     • Power is in the form of 5 Volts DC and is provided only with Key On Engine Off (KOEO) or Key On Engine Running (KOER).
     • Power is provided by the PCM.
   • The other wire (circuit) supplies Ground.
     • This Ground is fed to the ECT sensor by the PCM (internally).
2. The ECT sensor is a thermistor. Its resistance changes in response to changes in the engine coolant's temperature.
   • The cooler the engine coolant, the higher the resistance of the ECT sensor.
   • The warmer the engine coolant, the lower the resistance of the ECT sensor.
3. When the 5 Volts pass through the engine coolant temperature (ECT) sensor, the resistance of the sensor produces a voltage drop. This in plain English means that only a certain amount of the 5 Volts are let through to the Ground wire.
   • The cooler the engine coolant is, the less voltage is let thru' onto the Ground wire (due to the ECT sensor's higher resistance).
     • The amount of voltage that doesn't make it thru is the voltage drop.
     • The PCM translates this higher voltage drop into a colder temperature value.
   • The warmer the engine coolant, the more voltage is let thru' onto the Ground wire (due to the ECT sensor's lower resistance).
     • The PCM translates this lower voltage drop into a warmer/hotter temperature value.
4. The following chart shows the temperature-resistance relationship:

   Temperature	Resistance    (K Ω)	Voltage Drop
   50° F (10° C)	58.75 Ω	3.51 Volts DC
   68° F (20° C)	37.30 Ω	3.07 Volts DC
   86° F (30° C)	24.27 Ω	2.60 Volts DC
   104° F (40° C)	16.15 Ω	2.13 Volts DC
   122° F (50° C)	10.97 Ω	1.7 Volts DC
   140° F (60° C)	7.70 Ω	1.33 Volts DC
   158° F (70° C)	5.37 Ω	1.02 Volts DC
   176° F (80° C)	3.84 Ω	0.78 Volts DC
   194° F (90° C)	2.80 Ω	0.60 Volts DC
   212° F (100° C)	2.07 Ω	0.46 Volts DC

Common Causes Of A P0118 Trouble Code

The 2 most common cause of trouble code P0118 are:

1. A bad engine coolant temperature (ECT) sensor.
2. A problem in the sensor's 2 wires. Usually an open-circuit problem circuit (think ECT sensor unplugged from its connector).

Although extremely rare for this to happen, a bad PCM can also cause a false P0118 trouble code.

In this tutorial, I'll help you troubleshoot all three of the above. With this basic info under our belts, let's turn the page and get testing!

START HERE: Troubleshooting DTC P0118

Remember, when the P0118 ECT Sensor Circuit High Voltage trouble code sets, your Ford's PCM thinks that the engine coolant temperature is at -4 to -46°F (-20 to -43°C), no matter how long the engine has been running.

With a scan tool that has Live Data capability, you and I can easily test the ECT sensor and troubleshoot the cause of the P0118 DTC.

Now, if you don't have a scan tool with Live Data capability and need to buy one, check out my recommendation here: ANCEL AD530 OBD2 Scanner w/ Live Data (Amazon affiliate link).

Here's a brief description of the tests you'll be doing with the help of this tutorial:

1. Confirm the ECT sensor's temperature value.
   • Using the scan tool, we'll see if the ECT sensor is reporting a temp that's -4 to -46°F (-20 to -43°C).
   • TEST 1: Checking The Engine Coolant Temperature Value.
2. Check that the ECT sensor connector's wiring is not shorted together.
   • This involves removing the hard plastic protector that's over the wires and inspecting the condition of the two ECT sensor wires.
   • We'll also do a wiggle test of the ECT sensor's 2 wires.
   • TEST 2: Checking The Condition Of The ECT Sensor's 2 Wires.
3. Jumper the ECT sensor connector's terminals with a jumper wire.
   • In this test step, we'll jumper the ECT sensor connector's terminals and then, via the scan tool's Live Data mode, see if the PCM now reads 250 to 300°F (121 to 150°C).
   • This test will help you to eliminate a bad PCM and/or a hidden electrical short somewhere in the wiring between the PCM and ECT sensor.
   • TEST 3: Jumpering Together The ECT Sensor Circuits.
4. Testing the ECT sensor with the engine running.
   • In this test step, we'll see if the ECT sensor can report a rising coolant temperature as the engine idles and warms up.
   • TEST 4: Checking The Temperature With The Key On Engine Running.

TEST 1: Checking The Engine Coolant Temperature Value

The very first thing we need to do, is see the coolant temperature the PCM is seeing.

If the engine coolant temp is at -4 to -46°F (-20 to -43°C), then we know the problem is currently present and that trouble code P0118 ECT Sensor Circuit High Voltage is reporting a true condition.

In Live Data mode, the scan tool's PID (Parameter Identification) that will let us see what temperature the PCM is seeing, is labeled as: COOLANT (see the photo in the image above).

If you don't have a scan tool and you need to buy one, check out my recommendation: ANCEL AD530 OBD2 Scanner w/ Live Data (Amazon affiliate link).

OK, this is what you need to do:

1. 1

   Connect your scan tool to your Ford and turn the key to the on position.
   
   NOTE: This test is done with the Key On Engine OFF (KOEO).

2. 2

   Go to its Live Data mode once the scan tool has powered up.
   
   Scroll down to the PID labeled COOLANT (°F).

3. 3

   The COOLANT PID should register a temperature that's ±10 °F of ambient temperature (if all is normal).

   1. So let's say that it's 50 °F outside, then the ECT sensor PID should register something between 40 to 60 °F.
4. 4

   You'll see one of three possible temperature results:

   1. -4 to -46°F (-20 to -43°C).
   2.        OR
   3. 250 to 300°F (-121 to -150°C).
   4.        OR
   5. The current outside coolant temperature.

Let's interpret your test results:

CASE 1: Your scan tool shows a -4 °F (-20°C) reading. This confirms that you do have a problem with the engine coolant temperature (ECT) sensor or its circuits and that the P0118 DTC is reporting a true condition.

Seeing an extreme cold temperature of -4 °F (-20°C) indicates one of two things, either that the ECT sensor is bad or the ECT sensor connector's wires have an open-circuit problem (think unplugged sensor).

The next step is to physically check the condition of the wires and do a simple wiggle test. Go to: TEST 2: Checking The Condition Of The ECT Sensor's 2 Wires.

CASE 2: Your scan tool shows an extreme hot 250-300°F reading. This high temperature (when the engine is completely cold) confirms that there's a problem with the ECT sensor or its circuits. You'll also see a diagnostic trouble code P0117 (ECT Sensor Circuit Low Voltage Input) stored in the PCM's memory.

The most likely cause will be that the ECT sensor circuits (wires) are shorted together or the sensor has failed (internally).

For further testing specifics, go to this tutorial: How To Test: P0117 OBD II Trouble Code (1996-2003 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).

CASE 3: Your scan tool shows a temperature reading that's ±10 °F of ambient temperature. This tells you that at the moment the ECT sensor and its circuits are OK and more importantly, that the problem is intermittent.

But, since your scan tool retrieved a DTC P0118 the problem may just be hiding at the moment, I recommend doing the next three tests in this tutorial. Go to: TEST 2: Checking The Condition Of The ECT Sensor's 2 Wires.

If the ECT sensor passes TEST 2, continue on to TEST 3 and TEST 4 (or until one test is failed).

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: Jumpering Together The ECT Sensor Circuits.

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.

TEST 4: Checking The Temperature With The Key On Engine Running

If you've reached this point, then all of the ECT sensor tests you've done so far (TEST 1, TEST 2 and TEST 3) show the ECT sensor behaving normally.

There's a good chance, that once the engine warms up, the sensor could start malfunctioning, so in this test you'll start the engine and let it warm up and make sure that the coolant temperature your scan tool is reporting actually rises.

IMPORTANT: You must start this last test with the engine completely cold, or your test results won't be accurate.

This is what you need to do:

1. 1

   Check and take note of the coolant temperature your scan tool is displaying.
   
   The coolant temperature that your scan tool is showing, should be at or around ambient temperature.
   
   Now, touch the upper radiator hose with your hand. The hose will be cool to cold (ambient temperature).
   
   NOTE: The ECT sensor must be connected to its electrical connector for this test to work.

2. 2

   Start the engine and let it warm up.
   
   As the engine runs, keep your eyes on the engine coolant's temperature (on your scan tool). The temperature displayed should rise.
   
   Every now and then, recheck the temperature of the upper radiator hose. It should still be cool to the touch.

3. 3

   Once the coolant temperature has reached 190° F, check the temperature of the upper radiator hose by lightly touching it with your hand.
   
   The hose should now be hot to the touch since at 190° F the thermostat opens and lets hot coolant start to flow to the radiator. OK, let's interpret your test results below:

CASE 1: The coolant temperature displayed by your scan tool rose. This tells you that the engine coolant temperature (ECT) sensor is working correctly and that the ECT sensor problem, causing the ECT sensor code, is intermittent.

Here's why: In this test step, we've used the opening of the thermostat, which occurs at 190 to 195° F as a cross reference source for the temp sensor's accuracy.

Although this test is not 100% accurate, it is a good indicator of the ECT sensor's performance.

CASE 2: The coolant temperature displayed by your scan tool jumped between one negative number to a positive number (for example -30° F to 240° F) This confirms a malfunction with the engine coolant temperature (ECT) sensor. Replace the ECT sensor.

More Diagnostic Test Tutorials

You can find a complete list of tutorials for the 4.0L V6 Ford Explorer, Aerostar, and Mercury Mountaineer in this index:

• 4.0L V6 Ford Explorer And Mercury Mountaineer Index of Articles.

Here's a sample of the tutorials you'll find there:

• Fuel Pump Driver Module Tests (2004-2010 4.0L V6 Ford Explorer And Mercury Mountaineer).
• How To Test For A Blown Head Gasket (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
• How To Test The Fuel Pump (1993-2000 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
• How To Test Engine Compression (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
• How To Bench Test A Starter Motor (Step By Step) (at: easyautodiagnostics.com).