Coolant temperature sensor - GM/Vauxhall/Opel Simtec

The purpose of this test is to evaluate the correct operation of a Coolant Temperature Sensor (CTS) responding to a change in coolant temperature during the engine warming phase. 

Connection Guidance

How to perform the test

  1. Connect PicoScope Channel A to the 5V supply at the coolant sensor, or access permitting, to the same cable at the Engine Control Module (ECM).
  2. A good earth is required connecting to the battery negative terminal will check out the earth circuit.
  3. Start the engine and run at idle.
  4. Minimise the help screen and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  5. Select GO on your screen or press the space bar to see live data.

Example waveform

Waveform notes

You can see in the Example Waveform that the sensor is a Negative Temperature Coefficient type (NTC), the voltage falls as the coolant temperature increases.

At a point relative to around 40 to 50°C the engine control unit increases the sensor supply voltage. The voltage then continues to fall as coolant temperature rises. The manufacturer states that this gives a finer control.

coolant temperature sensor

Figure 2 - A typical coolant temperature sensor

Further Guidance

Technical Information

The Coolant Temperature Sensor (CTS) is a small two pin sensor whose job is to report the engine temperature back to the ECM. It is this signal that determines the engine warm up, enrichment and fast idle duration.

NTC sensors are predominantly made from a sintered semi-conductor material which will decrease electrical resistance in reaction to temperature rise. When installed into an engine cooling system it will provide an accurate voltage change as the coolant heats up.

The ECM is therefore provided with a signal which will allow accurate control throughout the operating temperature range.

Coolant temperature sensors are manufacturer specific and although the units may look identical the outputs vary dramatically.

All temperature sensors are resistance sensitive. Which means they require a clean circuit. Any poor/corroded connections will introduce an extra resistance in series and will falsify the readings that the ECM sees. Even though the sensor input may be within ECM parameters, not recording any faults, the engine will not operate as designed.

The General Motors Simtec Sensor on this vehicle has a different voltage characteristic to that on the majority of vehicle systems.

The GM/Vauxhall/Opel Vectra 1.6 litre engine uses this sensor and therefore has a distinctive waveform when viewed on the oscilloscope. The voltage displays a conventional drop until the engine reaches 40° to 50°C, at which point the voltage rises suddenly due to switching inside the ECM.

The manufacturer states that this gives a finer control.


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


  • Multimeter Probes


  • 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: Vauxhall Simtec