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

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

This article will help you to troubleshoot the engine coolant temperature (ECT) sensor on your GM 4.3L, 5.0L, or 5.7L equipped pick up, van, mini-van or SUV.

You can either use a multimeter or a scan tool (w/ Live Data capability) to diagnose the ECT sensor and/or trouble codes.

Here are the main points of this article:

  1. Important Suggestions and Tips.
  2. Symptoms of a BAD Engine Coolant Temp Sensor.
  3. P0117 and P0118 Essentials.
  4. Coolant Temperature Sensor Basics.
  5. TEST 1: Checking the Temperature with the Key On Engine Off.
  6. TEST 2: The Wiggle Test.
  7. TEST 3: Checking the Temperature with the Key On Engine Running.
  8. Related Articles.

Important Suggestions and Tips

TIP1: A BAD engine coolant temperature sensor will not cause your pick up (van, mini-van, or SUV) to overheat. This article does not cover any type of overheating issue.

If your vehicle is overheating, I suggest taking a look at these two tests:

  1. How to Test the Thermostat (GM 4.3L, 5.0L, 5.7L).
  2. Troubleshooting a Blown Head Gasket (GM 4.3L, 5.0L, 5.7L).

TIP2: You'll need a scan tool with Live Data capability to be able to follow the test steps in this article (although you can resistance test the ECT sensor with a multimeter).

You don't need a professional $5K (US) scan tool to do it... a simple generic scan tool will do (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).

TIP3: 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.

Symptoms of a BAD
Engine Coolant Temp (ECT) Sensor

The are several symptoms a BAD ECT sensor causes, and they are:

  1. Check engine light is on and one of the following codes is stored:
    1. P0117: ECT Sensor Circuit Low Voltage.
    2. P0118: ECT Sensor Circuit High Voltage.
  2. BAD gas mileage.
  3. Extended cranking. In other words... a hard start.
  4. Your vehicle may crank but not start.

When there's a problem with the ECT sensor or its circuits, the PCM will see one of the three distinct conditions:

  1. It will receive input from the ECT sensor indicating an extremely high coolant temperature. Usually around 250° F, even tho' the engine is cold. This condition leads the PCM to register a P0118 DTC.
  2. It will receive input from the ECT sensor showing an extremely low temperature. This temp is usually around -40° F. This condition leads the PCM to set a P0117 DTC.
  3. Reports a ‘normal’ temperature, but this temperature does not correspond to the actual coolant temperature.

P0117 and P0118 Essentials

You've probably seen the official OBD-II description of these two diagnostic trouble codes... but in plain English what are these two codes saying? Well, this section will shed some light on this (by the way, you're gonna' see the term voltage drop and I'll explain this term in the next section):

P0117: engine coolant temperature (ECT) Circuit Low Input.

  1. This code indicates that the PCM is seeing a low voltage drop across the ECT sensor, which it's translating as an extremely hot coolant condition.
  2. When the PCM senses this low voltage input, it will translate this voltage input to a 250° F temperature.
  3. This code is caused by one of the following:
    1. Coolant temp sensor's wires are shorted to ground.
    2. Coolant temp sensor's wires are shorted to each other.
    3. Coolant temp sensor is BAD.
    4. PCM is BAD.

P0118: Engine Coolant Temperature (ECT) Circuit High Input

  1. This code indicates that the PCM is seeing a high voltage drop across the ECT sensor, which it's translating as an extremely hot coolant condition.
  2. When the PCM senses this high voltage input, it will translate this voltage input to a coolant temperature of -46° F (-50° C).
  3. This code is caused by one of the following:
    1. Coolant temp sensor's wires have an ‘open’ short (usually due to a cut wire).
    2. Coolant sensor's connector has become unplugged from the sensor itself.
    3. Coolant temp sensor is BAD.
    4. PCM is BAD.

Coolant Temperature Sensor Basics

Using a scan tool with Live Data capability is the fastest and easiest way to diagnose an ECT sensor... but sometimes, you've got to verify the sensor's output directly at the sensor... this section will shed some light on how the ECT sensor works so that, if you have to, you can test it directly with a multimeter.

The coolant temperature sensor's resistance changes with temperature (this type of sensor is called a thermistor).

Here are more specifics:

  1. 1

    The engine coolant (ECT) temperature sensor has two wires in the connector.

    1. One is the power wire and this power is in the form of 5 Volts. These 5 Volts are created inside the PCM.
    2. The other wire is the ground wire... which is provided internally by the PCM too.
  2. 2

    When the coolant is cold (or ambient temperature), the sensor's internal resistance is higher.

    1. This higher resistance has a direct impact on the 5 Volts passing thru' it since it blocks some of it from passing thru' to ground (-this action of blocking voltage from passing thru' is called a voltage drop) .
    2. The PCM senses this voltage drop and translates the specific amount into a temperature reading.
    3. Now, in case your wondering.... this voltage drop can be measured with a multimeter in Volts DC since it (the meter) would act as a bridge (to ground) for the voltage being blocked.
  3. 3

    As the engine warms up... the coolant starts to heat up, which in turn causes the coolant temp sensor's resistance to decrease.

    1. This decreasing resistance now lets more of the 5 Volts to pass thru' the sensor onto ground.
    2. The PCM is able to sense this lower voltage drop and translates the numbers as a higher temperature.
    3. So, if you were using a multimeter... this would mean that the voltage numbers, displayed by the meter, would decrease as the engine's coolant heats up.
  4. 4

    The following table will help you to see this temperature to resistance relationship:

    Temperature Resistance
      50° F (10° C) 5670 Ω
      68° F (20° C) 3520 Ω
      86° F (30° C) 2238 Ω
    104° F (40° C) 1459 Ω
    122° F (50° C)   973 Ω
    140° F (60° C)   667 Ω
    158° F (70° C)   467 Ω
    176° F (80° C)   332 Ω
    194° F (90° C)   241 Ω
    212° F (100° C)   177 Ω

OK, enough of the working theory...let's get testing in the next page.

Chevrolet Vehicles:

  • Astro
    • 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
  • Blazer
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
  • Blazer TrailBlazer
    • 1999, 2000, 2001

Chevrolet Vehicles:

  • C1500, C2500, C3500 Pick Up
    • 1996, 1997, 1998, 1999, 2000
  • Express Van 1500, 2500, 3500
    • 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007
  • G30 Van
    • 1996
  • K1500, K2500, K3500 Pick Up
    • 1996, 1997, 1998, 1999, 2000

Chevrolet Vehicles:

  • K1500, K2500 Suburban
    • 1996, 1997, 1998, 1999
  • P30 Van
    • 1996, 1997, 1998, 1999
  • S10 Blazer
    • 1994
  • S10 Pick Up
    • 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004

Chevrolet Vehicles:

  • Silverado 1500 Pick Up
    • 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2007
  • Tahoe
    • 1996, 1997, 1998, 1999, 2000

GMC Vehicles:

  • Sierra C1500, C2500, C3500
    • 1996, 1997, 1998, 1999, 2000

GMC Vehicles:

  • Suburban C1500, C2500
    • 1996, 1997, 1998, 1999
  • G3500 Van & Vandura
    • 1996
  • Jimmy & Envoy
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
  • K1500, K2500, K3500 Sierra
    • 1996, 1997, 1998, 1999, 2000

GMC Vehicles:

  • P3500 Van
    • 1996, 1997, 1998, 1999
  • S15 Jimmy
    • 1994
  • Safari
    • 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005

GMC Vehicles:

  • Savana Van 1500, 2500, 3500
    • 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007
  • Sonoma
    • 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004

GMC Vehicles:

  • Yukon
    • 1996, 1997, 1998, 1999, 2000, 2001, 2002

Isuzu Vehicles:

  • Hombre
    • 1997, 1998, 1999, 2000

Oldsmobile Vehicles:

  • Bravada
    • 1996, 1997, 1998, 1999, 2000, 2001

Cadillac Vehicles:

  • Escalade
    • 1999, 2000

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