Coolant temperature sensor

The purpose of this test is to evaluate the correct operation of a Coolant Temperature Sensor (CTS) responding to a change in coolant temperature from cold to warm. 

How to perform the test

  1. Plug a premium BNC test lead into Channel A on the PicoScope.
  2. Place a large black clip on the black (negative) plug on the test lead.
  3. Place a Back-pinning Probe on the colored (positive) plug on the test lead.
  4. Place the black clip on the battery negative terminal.
  5. Probe the coolant temperature sensor with the Back-pinning Probe.

The two connections on the sensor are the voltage supply at about 5 volts and the earth return. It is the first of the two connections that needs to be made.

Alternative connections can be made using the TA012 two-pin test lead adapter. Connect as above, but without an acupuncture or multimeter probe, and monitor both sides of the shrouded plugs to identify which wire is the sensor return. Connection to the coolant temperature sensor is illustrated in Figure 1.

Example waveforms

Waveform notes

The Coolant Temperature Sensor (CTS) is a two-wire device with a voltage supply at approximately 5 volts.

The sensor alters its resistance with engine temperature change. The majority of sensors have a negative temperature coefficient (NTC), which means that the resistance of the component decreases as the temperature increases. The resistance change alters the voltage output from at the sensor, and this voltage can be monitored for any discrepancies across its operational range. Select a time scale of 500 seconds, connect the oscilloscope to the sensor and observe the output voltage. Start the engine, and in the majority of cases the voltage will start in the region of 3 to 4 volts. However, this voltage depends on the temperature of the engine. As the temperature increases the resistance decreases and so does the voltage.

The voltage change is usually smooth. If the CTS has a fault at a certain temperature, using a scope is the only reliable way of detecting it.

NOTE: The GM/Vauxhall/Opel Simtec system has a point at which the voltage alters suddenly during the warm-up period. This is described in the notes section of the Coolant Temperature Sensor (GM/Vauxhall/Opel) waveform page.

The first waveform was captured without filtering, and shows a large amount of noise picked up from the engine by the wiring loom. The second waveform was captured using lowpass filtering (set in the Channel Options menu) with a cut-off frequency of 10 Hz.

coolant temperature sensor

Figure 2 - A typical coolant temperature sensor

Technical information

The Coolant Temperature Sensor (CTS) is a small two-terminal device whose job is to report the engine's temperature back to the Engine Control Module (ECM). This signal determines the engine's warm up enrichment and fast idle speed.

The sensor normally has a negative temperature coefficient (NTC) which means that the component's resistance drops as the temperature rises. A positive temperature coefficient (PTC) sensor is not as common as the NTC and its resistance reacts to temperature in the opposite way.

To increase the vehicle's drivability and performance on pre-1992 cars without a catalytic converter, the resistance can be altered by inserting a resistor in series with the coolant temperature sensor,

The resistor's value has to be calculated before its insertion. This modification cannot be implemented on engines equipped with a catalytic converter as the extra fuelling will upset the corrective operation of the lambda or oxygen sensor.

The sensors are manufacturer-specific and the outputs vary dramatically, although the units may look identical. Any poor connections in the circuit introduce an extra resistance in series and falsify the readings that the ECM sees. Reading the resistance at the ECM's multi-plug will confirm this.


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

  • 2 Pin AMP connector breakout lead


  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • Small Crocodile/Gator Clips


  • PicoScope Battery Clip


  • Large Dolphin/Gator Clips


  • Premium Test Lead: BNC to 4 mm, 3 m


  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)


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Guided test: Coolant temperature sensor 5 V