Products suited to this guided test*
  • 2 Pin AMP connector breakout lead

  • Back-pinning Probe Set

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

  • *At Pico we are always looking to improve our products. The tools used in this guided test may have been superseded and the products above are our latest versions used to diagnose the fault documented in this case study.

Engine coolant temperature sensor - Simtec (GM) type

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

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 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 waveform

Waveform notes

This known good waveform has the following characteristics:

The sensor is a Negative Temperature Coefficient type (NTC), where the output voltage falls as the coolant temperature increases.

At a voltage in the range 40-50°C the Engine Control Module (ECM) increases the sensor supply voltage. The voltage then continues to fall as coolant temperature rises.

Further guidance

The ECT is a small two pin sensor its function is to provide a voltage relative to coolant temperature back to the ECM. It is this signal that determines the engine warm up, enrichment and fast idle duration.

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 manufacturer states that this gives a finer control.

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.

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Guided test: Engine coolant temperature sensor - Simtec (GM) type