Troubleshooting P0117 and P0118 ECT Sensor Tests (4.6L, 5.4L)

This article will help you to diagnose diagnostic trouble codes P0117 (Engine Coolant Temperature Circuit Low Input) and/or P0118 (Engine Coolant Temperature Circuit High Input).

Ford uses two different temperature sensors. One is called the engine coolant temperature (ECT) sensor and the other is the engine cylinder head temperature (CHT) sensor. This article concentrates on the engine coolant temperature sensor (see photo below).

Here are some specific differences between both of them: The engine coolant temp (ECT) sensor measures the temperature of the coolant, the cylinder head temperature (CHT) sensor measures the temperature of the metal itself (of the cylinder head) and no coolant is involved.

Although you can apply this info to any Ford vehicle with a V6 or V8, this info concentrates on the Ford 4.6L and 5.4L V8 engines.

Important Suggestions And Tips

TIP 1: A malfunctioning engine coolant temperature sensor will NOT make your vehicle overheat.

If your vehicle is overheating, this article won't be of much help since this test article only diagnoses the ECT sensor.

TIP 2: A scan tool (with Live Data capability) is the preferred method to test these two diagnostic trouble codes (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).

TIP 3: 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.

P0117 And P0118 Essentials

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

1. This DTC tells you that the PCM is seeing a temperature reading above 250° F (121° C).
2. This DTC indicates one of several things:
   • 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

1. This DTC tells you that the PCM is seeing a coolant temperature of -46° F (-50° C).
2. This DTC indicates one of several things:
   • 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

One of the things that will help you solve a ECT temperature problem is knowing how it works. In this section, I'll briefly describe how the voltage drop that the sensor creates is used as the basis for a temperature reading.

The coolant temperature sensor on your Ford vehicle is a thermistor, which means that its resistance changes with temperature. Here in a nutshell, is how the engine coolant temperature sensor works:

1. 1

   The engine coolant (ECT) temperature sensor has two wires. One receives 5 Volts from the PCM. The other supplies a Ground path (inside the PCM too) for those 5 Volts.
   
   These 5 Volts are referred to as the 5 Volt Reference signal in all of the service and repair literature.

2. 2

   The sensor's resistance is higher the colder the coolant is. This causes a higher voltage drop.
   
   Specifically and in plain English:

   1. The higher resistance of the ECT sensor blocks some of the 5 Volts that are trying to pass thru' it.
   2. The voltage that does not make it thru' (because of the resistance) is known as a Voltage Drop.
   3. The PCM senses this voltage drop and translates it to a specific temperature.

   You can measure this voltage drop (the values) with a multimeter in Volts DC and with its leads probing the sensor's wires (the sensor must remain connected to its connector).

3. 3

   As the engine warms up, the coolant starts to absorb the temperature, which in turn causes the coolant temp sensor to heat up.

   1. This causes the sensor's resistance to start decreasing.
   2. This lower resistance causes more of the 5 Volts to pass across the sensor.
   3. This means that the voltage drops will be lower. The PCM senses this lower voltage drop and translates it into a higher temperature reading.

1. 4

   The following table will help you to see this temperature to resistance to voltage drop 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

OK, let's turn the page and get testing.