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

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 (Ford 4.0L).

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

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 V DC
   68° F (20° C)	37.30 Ω	3.07 V DC
   86° F (30° C)	24.27 Ω	2.60 V DC
   104° F (40° C)	16.15 Ω	2.13 V DC
   122° F (50° C)	10.97 Ω	1.7 V DC
   140° F (60° C)	7.70 Ω	1.33 V DC
   158° F (70° C)	5.37 Ω	1.02 V DC
   176° F (80° C)	3.84 Ω	0.78 V DC
   194° F (90° C)	2.80 Ω	0.60 V DC
   212° F (100° C)	2.07 Ω	0.46 V 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!