Engine coolant temperature sensor

The purpose of this test is to evaluate the voltage output of an Engine Coolant Temperature (ECT) sensor as the engine coolant changes from cold to warm.

How to perform the test

View connection guidance notes.

  1. Connect PicoScope Channel A to the temperature sensor 5 V supply circuit.
  2. A good earth is required, connecting the PicoScope earth lead to the battery negative terminal will check out the earth circuit.
  3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
  4. Start the scope to see live data.
  5. Start the engine.
  6. Observe the waveform as the engine coolant warms up.

Example waveforms

Waveform with filter applied

Waveform notes

This known good waveform has the following characteristics:

  • The initial voltage depends on the coolant temperature.
  • A gradual voltage reduction as engine coolant temperature rises.

The second example shows the same waveform with a filter added to clean up the image.

Waveform Library

Go to the drop down menu in the lower left corner of the Waveform Library window and select, Coolant temperature sensor.

Further guidance

The function of the ECT sensor is to provide a measure of the engine’s coolant temperature. The signal is used by the Engine Control Module (ECM) as part of the engine load sensing apparatus. It is therefore partly responsible for the determination of fuelling, timing and engine speed requirements.

ECT sensors are manufacturer specific; despite having similar external appearances, their outputs can vary dramatically.

The majority of ECT sensors have Negative Temperature Coefficients (NTCs), meaning their internal resistance decreases as the sensing element temperature increases. Therefore, in the example above, the voltage measured across the ECT sensor drops as its resistance decreases. A Positive Temperature Coefficient (PTC) sensor will have the opposite behaviour.

The temperature sensor circuits are highly resistance sensitive, requiring a good circuit with clean connectors and no extraneous resistances: any poor/corroded connections will falsify the signal at the ECM, causing the engine to operate out of tolerance.

Diagnostic trouble codes

Selection of component-related Diagnostic Trouble Codes (DTCs):

P0115 - Engine Coolant Temperature (ECT) Sensor Circuit

P0116 - Engine Coolant Temperature (ECT) Sensor Performance

P0117 - Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage

P0118 - Engine Coolant Temperature (ECT) Sensor Circuit High Voltage

P0119 - Engine Coolant Temperature Circuit Intermittent

P0125 - Engine Coolant Temperature (ECT) Insufficient for Closed Loop Fuel Control

P1114 - Engine Coolant Temperature (ECT) Sensor Circuit Intermittent Low Voltage

P1115 - Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage

P1116 - ECT Signal Unstable or Intermittent

P1117 - Engine Coolant Temp. Signal Out-Of-Range Low

P1118 - Engine Coolant Temp. Signal Out-Of-Range High

P1119 - ECT Signal Out-Of-Range With TFT Sensor

P1258 - Engine Coolant Overtemperature - above 268°F - Protection Mode Active


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • Premium 6-way breakout lead set


  • 2 Pin AMP connector breakout lead


  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • Large Dolphin/Gator Clips


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Guided test: Engine coolant temperature sensor