Products suited to this guided test*
  • Back-pinning Probe Set

  • Large Dolphin/Gator Clips

  • PicoScope Battery Clip

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 The tools used in this guided test may have been superseded and the products above are our latest versions used to diagnose the fault documented in this case study.

Idle speed control valve (solenoid type)

The purpose of this test is to evaluate the Idle Speed Control Valve (ISCV) control signal from the Engine Control Module (ECM).

How to perform the test

View connection guidance notes.

  1. Use manufacturer's data to identify the ISCV switched earth circuit.
  2. Connect PicoScope Channel A to the switched 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 engine and allow it to idle.
  5. Start the scope to see live data.
  6. Whilst observing the screen, switch on electrical loads (AC, heating, lights etc.) to affect the idle speed.
  7. With your waveform on screen stop the scope.
  8. Turn off the engine.
  9. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

  • A typical “sawtooth” pattern with the voltage reducing from 12 V to around 6 V as the ECM switches to earth.
  • Switching frequency will increase and decrease with valve activation.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select Idle Speed Control Valve ISCV voltage.

Further guidance

An ISCV is used by the Engine Control Module (ECM) to regulate engine idle speed according to engine temperature and load when there is no driver demand from the accelerator pedal and the throttle is closed.

For example, during cold start conditions, the ECM will seek to quickly raise engine temperature by increasing engine speed to a fast idle, at around 1200 rpm.

With an ISCV, the ECM can maintain and adjust the idle speed for changes in engine load caused by the air conditioning, power steering, automatic transmission, or charging systems, etc.

The ECM controls the valve opening position by varying the switched earth signal; the greater the average voltage, the greater the average solenoid current and the greater the valve opening.

Due to its location, the ISCV is susceptible to carbon fouling. As such the valve may be electrically functioning with a normal waveform but mechanically faulty. In this situation, the valve must be removed for examination, cleaning or replacement.

Symptoms of a faulty ISCV:

  • Rough idle.
  • Engine cutting out when at or approaching idle.
  • Malfunction Indicator Lamp (MIL) illumination.
  • Idle speed related Diagnostic Trouble Codes (DTCs).
  • Fuel trims out of tolerance with related DTCs.

Causes of ISCV failure:

  • Fuel resin/coke causing actuator to stick.
  • Internal electrical damage due to heat and vibration.
  • External connector damage/contamination.


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: Idle speed control valve - electromagnetic