Glow plug - voltage and current

The purpose of this test is to examine individual glow plug operation through measurement of voltage and current.

How to perform the test

View connection guidance notes.


The engine must be below normal operating temperature to capture the pre-heat waveform.

  1. Connect a low amp clamp to PicoScope channel A switch on and zero the clamp on the 60 A scale before attaching around the suspect glow plug cable.
  2. Connect PicoScope channel B voltage probe to the same glow plug lead.
  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. Turn on the ignition and wait for the glow plug warning lamp to extinguish before starting the engine and running at idle.
  6. Allow the engine to run until your waveforms return to zero values. (This time will vary across manufacturers).
  7. Turn off the engine.
  8. Stop the scope.
  9. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.


The orientation of the current clamp relative to the wire will determine whether it has a positive or negative output. If a live waveform does not appear on your screen, or appears to be inverted, try reversing the orientation of the clamp.

Example waveform

Waveform notes

Channel A shows the glow plug current. 

The waveform can best be interpreted in 3 sections from left to right:

1 Pre-heat

This initial section shows the switch on current spike. The current falls as the glow plug heats up. This lasts 2.5 seconds, then the pre-heat stage enters a modulation phase in order to prevent overheating damage to the glow plug.

2 Engine cranking

The engine cranks for just over 3 seconds. During this period the control unit switches the glow plug current off.

3 Post Heat

The glow plug is switched on again and settles down to a modulated current/temperature whilst the engine is running. This helps to reduce engine cold running noise and emissions. In this case the glow plugs are switched off after 1.5 mins engine running time. The run time with glow plugs on is dependant on many factors monitored by the Engine Control Module (ECM) and therefore will vary.

Channel B shows the glow plug voltage, again in 3 sections;

1 Pre-heat

As expected the voltage is below 12 V during the switch on and pre-heat phase but increases slightly as the current falls./

2 Engine cranking

The glow plug is switched off 0 V.

3 Post Heat

The modulated control in this example allows just over 13 V to be maintained.

Expanded Waveform

The Expanded Waveform shows the on-off modulation switching controlled by the ECM.

Waveform Library

Go to the drop-down menu bar in the lower left corner of the Waveform Library window and select glow plug current or glow plug voltage.

Further guidance

There are various types of glow plug now available. Most modern diesel engines now use rapid glow systems. These systems tend to use a ceramic type glow plugs which can reduce the glow wait time by more than 50%. They have two coils internally connected in series, a small heating coil at the plug tip and a larger regulating coil within the plug case.

The minimum in cylinder start temperature for diesel engines is ≈850°C, today glow plugs will exceed this temperature within a few seconds. The down side to this is the temperature must now be regulated to prevent overheating, hence the modulated signal. Most plugs are regulated to perform within 1000°C to 1100°C.

Manufacturer specifications do vary but most systems will need an ambient temperature below 9°C to operate and the post heat phase may cut off if the vehicle is driven and engine speed exceeds 2500 RPM.

Now that the glow plug temperature is controlled it is possible to have them operate after the engine has started. This has proved beneficial for emission control and smoother cold running.

It can sometimes be difficult to distinguish standard and rapid glow type glow plugs.

Note. The rapid glow type glow plugs must not be tested with a direct voltage supply as damage will result from such testing.

A good check would be to say that if your waveform has a modulating signal then don’t test by connecting directly to a voltage supply. Another distinguishing characteristic is that because of the nature of operation the rapid glow plugs have a very low resistance. The glow plugs in this test had 0.6 Ω resistance.

Always check manufacturer specifications.

Glow plug failure.

The majority of glow plug failures are due to excessive heat. If the glow plug itself overheats then the casing swells and can split this is likely to be an over voltage issue. Beware there are glow plugs which are designed to operate on 6 volts.

Over voltage is likely to be a control issue rather than a glow plug failure.

The time that the glow plugs are on is dependant on both ambient and coolant temperatures and is controlled by the glow plug control unit or the engine control module.

In general, the glow plugs will reach maximum temperature in just a few seconds and will remain on for around 3 minutes. This is a general guide, please refer to manufacturer specification for actual figures.

A broken tip on a glow plug, without any visible casing swell could be caused by incorrect injection timing or injector dribble.

Check out the control system for the glow plugs and the injector condition/timing for the engine using manufacturer data.

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

  • 30 A (low amps) current clamp


  • 20 A / 60 A DC (low amps) current clamp


  • Multimeter Probes


  • Premium 6-way breakout lead set


  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • PicoScope Battery Clip


Help us improve our tests

We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.

Add comment

Your email address will not be published. Required fields are marked *

Guided test: Glow plug - voltage and current