Glow plugs - current

The purpose of this test is to evaluate the condition of the glow plugs based on the total current draw, while also confirming their operation time.

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

  1. Connect the high amp current clamp to PicoScope channel A, switch on and zero the clamp.
  2. Place the current clamp onto one of the battery cables (+) or (-), or directly onto the glow plug supply cable from the timer relay access permitting. Observe the current directional arrow, if incorrect the waveform will be inverted.
  3. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  4. Select Go or press the space bar to see live data.
  5. Switch on the ignition to activate the glow plugs. Wait a few seconds before starting the engine.
  6. Allow the engine to idle and observe your waveform. After a short time it is usual for the glow plugs to be switched off.
  7. Stop the engine and turn off the ignition.
  8. Use the Waveform Buffer and Zoom tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

There is a current spike to around 100 A as the glow plugs are switched on. The current then immediately falls to around 70 A, followed by a downward curve indicating the time taken, 1.5 s, to heat the glow plugs to maximum. The operating current then remains stable around 50 A until control switches the glow plugs off. Total on time is 11 s.

Waveform Library

Go to the drop down menu in the lower left corner of the Waveform Library window and select, Glow Plug Current.

Example glow plug

Further guidance

In order to ignite diesel fuel the air temperature during compression must reach the critical temperature of 850° C.

With cold ambient temperatures, intake air and engine components, this may not be achieved.

In these circumstances glow plugs are used to heat up the combustion area to aid starting.

Glow plugs are also used to improve the cold running of a diesel engine thereby improving emissions.

They are designed to operate within a glow temperature of 850° to 1100° C.

Glow plugs may be controlled either by the Engine Control Module (ECM) or by a dedicated glow plug control unit/timer.

Use the vehicle data to identify system switching and on time characteristics.

One of the main causes of glow plug failure is overheating.

In order to prevent overheating and allow operation during engine running the majority of glow plug systems now use Pulse Width Modulation (PWM) as a method of control.

It is important to be able to identify “control” failure as a cause of glow plug failure.

Other causes of glow plug failure can be;

  • Improper injector spray pattern.
  • Incorrect engine timing.
  • Application of 12 v directly onto a glow plug outside of the engines control, note; some systems operate at 6 v. (incorrect testing).

Characteristics of failed glow plugs;

  • Engine will not start.
  • Excessive cranking time.
  • Rough running when cold.
  • Excessive emissions when cold.

The operating current can be calculated using the wattage rating of the glow plug. This rating is available in the appropriate diesel data books.

Once you know the wattage, multiply it by the number of cylinders and then divide by the voltage to calculate the expected final current.


  • One glow plug = 150 watts
  • 4 glow plugs    = 600 watts
  • Current             = watts divided by volts
  • 600 watts divided by 12 volts = 50 amps.

Diagnostic trouble codes

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

P0380 - Glow Plug/Heater Circuit "A" Malfunction

P0381 - Glow Plug/Heater Indicator Circuit Malfunction

U0106 - Lost Communication with Glow Plug Control Module

U0307 - Software Incompatibility with Glow Plug Control Module

U0407 - Invalid Data Received from Glow Plug Control Module


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

  • 200 A / 2000 A (high amps) DC current clamp


  • 600 A DC (high amps) current clamp


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Guided test: Glow plugs - current