My Diesel Won’t Start When its Cold – How to Fix Glow Plugs.

Diesel engines start using the compression of the air fuel mixture. Diesels don’t like starting when cold. Hence diesel engines are fitted with glow plugs to heat the air fuel mix, to assist starting when cold. Whereas petrol engines start with the assistance of a spark plug to ignite the fuel.

If a diesel engine is running well and starts easily when the engine is warm, but won’t start when cold, (or takes ages to get it started when cold). It could be an issue with the glow plugs, the glow plug system (module, relay, or voltage supply).

Video: It Won’t Start When the Engine is Cold. Is it the Glow Plugs?

Video – Diesel Won’t start when cold?


List of checks that can be carried out:

With a Multimeter:

Without a Multimeter:

This page contains all the detail on glow plug fault finding, their fault symptoms, testing and repair.. If you just want to view the videos, they can be found on this page ‘Diesel Won’t Start When Cold – The Video’s‘. – Opens in a new tab

This article is for guidance only…

Diesel Cold Starting – Fault Symptoms

  • The engine turns over at a reasonable speed (it’s not slow cranking), but the engine won’t start?
  • The engine starts, but only after some time (and multiple attempts)?
  • White smoke is seen from the exhaust (unburnt fuel).
  • When the engine is warm, it starts easily.

You may see white smoke from the exhaust whilst trying to start the engine and for a short while when the engine starts (unburnt fuel). The video above shows an example of what can happen:

Diesel Cold Starting Fault Symptoms – White Smoke

Diesel Cold Starting – The Causes

If you see white smoke coming from the exhaust (unburnt fuel), after several starting attempts…

  • Some, or all of the glow plugs may have failed?
  • There maybe no voltage supplied?
  • The glow plugs may be switching off too soon, or not switching on at all? The timer (or control module) could be defective?
Glow plugs removed from the engine cylinders.

Why Glow Plugs Fail:

There could be a number of causes why they’ve failed. Often, the cause may be just down to failure due to old age and wear and tear! A glow plug is stressed throughout it’s life. It can go from frozen to glowing red hot in a matter of seconds. There is expansion and contraction, oil, rust, corrosive gases, pressure, combustion (explosions), etc, etc. Glow plugs have a hard life! The materials used and quality of manufacture probably have a lot to do with their lifespan, plus… Where you live (the engines starting temperature), number of times the engine is started, how long the engine is run for…

Philosophical statement here – Glow plugs may be like human beings. Some have a long and happy life whilst others… There can also be other factors too. My daughter decided to drive through a deep puddle where water was sucked into the engine. She naturally tried to start the car, but it wouldn’t start (hydraulic water lock in the cylinders). When we cleared the cylinders and got the car started. All the glow plugs started to fail. I presume it was because the glow plugs had been trying to boil the water in the cylinders? So, there could be a number of causes why glow plugs fail…

How do Glow Plugs Work?

Why do we have have them?

Diesel fuel is not as volatile as petrol. It doesn’t ignite so easily, especially when the air temperature (engine starting temperature) is cold. Diesel engines ignite the air/fuel mixture injected into the engine cylinders, by using the compression created by the pistons. Unlike petrol engines, diesel engines don’t have spark plugs. Diesel engines also needs a bit of warmth to help the process…

As part of an explanation, a few years back, I took part in a demonstration to show if diesel fuel would ignite easily (when there was a diesel spill on water). This was in controlled conditions where diesel was added to a water filled tray. The diesel floated on top of the water. A torch flame was used to try and ignite the diesel. However hard we tried, the diesel would not ignite. However, when we heated the diesel in one corner of the tray, and it began to give off a vapour. We were able to ignite this diesel vapour easily. The heat from the flames in the corner of the tray, then started to heat the rest of the diesel floating on top of the water. This created more vapour, and we soon had a raging fire on our hands (with the whole tray alight). We then had fun putting out the test fire with foam fire extinguishers!

Hopefully this explanation shows how glow plugs can help igniting the diesel when the engine is cold to aid starting (more so in colder temperatures).

Glow Plug Fault Finding?

Diesel Cold Starting – Things to Check

  • The relay:
  • The timer (or control module):
  • The glow plugs:


  • The relay is sometimes a separate item, but it can also be incorporated inside the control module?
  • Glow plugs come on in hot and cold weather, to aid starting and help whilst the engine warms up. However, they may be on for different lengths of time.

What Should I Check First (three easy steps)?

For the initial testing, you will need a voltmeter, or a simple method of checking for a voltage. However, it’s not essential. If you don’t have a voltmeter, just go straight to the section on Testing Without a Voltmeter (Multimeter)…

* If you are unsure about working on vehicle electrics, seek the advice of an electrical engineer *

Three Easy Steps:

  1. Is there a voltage?
  2. How long is the voltage applied?
  3. Checking the glow plugs?

1. Is there a voltage at the glow plugs?

If there is a voltage supply (the correct voltage), then we know the relay and control module are probably working (also see 2. below).

2. How long is the voltage applied?

There is a timer that applies a voltage for a set amount of time. Therefore you need to check how long the supply is switched on.

Glow Plug Supply Voltage Measurement

3. Check the glow plugs?

If the correct voltage is applied (and for the correct amount of time), then the actual glow plugs will need to be checked.

Checking the Glow Plugs With or Without a Multimeter (Test Meter):

A New Glow Plug

It’s easy to check the timer and relay with a voltmeter. If you don’t have a voltmeter, you can still check them with a car battery and a jump lead (see the two options below).

Short video on how to remove glow plugs:

Video – Removing Glow Plugs + Where are they Fitted

With a Multimeter (Voltmeter):

There isn’t much you can check on the relay and glow plug control module, other than voltages. Therefore, all you need is a simple voltage meter, voltage tester (or you could use a vehicle lightbulb and some wire). If you have replacements, you can always swap the units over to check their operation?

Please refer to the vehicle manual, as each vehicle may be slightly different.


  1. Checking the Supply Voltage (the Relay):
  2. Checking the Timer (Control Module):
  3. Checking the Actual (Resistance):


  • To turn on the ignition (without starting the engine).
  • The ignition may need to be turned on and off when testing each glow plug, as the timer will operate (the supply may be switched off after approx 10 to 20 seconds depending on the vehicle).
  • To save time, tasks 1 & 2 can be done together…

1. Checking the Supply Voltage (and the relay):

Remove (unclip) each of the connectors and check the voltage to each of the glow plugs when the ignition is switched on. See the video below (at the end of section 2). ‘Testing the Supply Voltage & Timer‘.

What Voltage Should my Glow Plugs Work On?

Measuring the Glow Plug Voltage

There is a large number of different voltage specifications for vehicles, ranging from 4.4v all the way up to 23v. The majority appear to be rated at 11v. However, it should be noted that the 11v glow plug will work perfectly fine on 12v, which is the case in the videos shown here. I guess they are rated at 11v, in case the output from the vehicle’s electrical system is low. Plus they will have a + or – operating tolerance.

You could assume that:

  • 5v glow plugs will operate on a 6v system. Or, a12v system (where the voltage is regulated down to 5v by the vehicles control unit).
  • 11v glow plugs will operate on a 12v system.
  • 23v glow plugs will operate on a 24v system.

You may find a manufacturer’s colour coding system, to indicate the operating voltage (i.e. yellow or orange for 5v). However, I would only use a colour indicator as a guide (past experience, where manufacturer’s don’t strictly adhere to helpful indicators).

Always check your part numbers on the manufacturers website to ensure the correct part is being fitted. I.e. If your vehicle system is 12 volts and you find 5 volts is supplied (and you don’t have a manual showing this type of detail). Check the specifications on the manufacturers website, to check the voltage is correct. The voltage may be regulated down (by the vehicles control unit) to a different voltage…

2. Checking the Glow Plug Timer (Control Module):

Check how long the timer (control module) voltage is supplied to each of the glow plugs. It should be for around 10 to 20 seconds dependant on the vehicle.

Other things to check:

  • If fitted, check that there are no blown fuses in the circuit.
  • Sometimes vehicles are fitted with more than one identical relay (fitted for other tasks). If so, you could use the identical relay to test the circuit. Make sure it’s an identical relay, otherwise there is risk of damage (there should be a part number printed on the relay, so you can check).
  • The relay is sometimes included inside the control module?

Short video on testing the supply voltage and glow plug timer:

Video – Measure the glow plug supply voltage and test the timer operation

3. Checking the Glow Plug Resistance:

With a voltmeter (multimeter), we can check the resistance. This will tell us if they are:

  • Working (low resistance).
  • Not working very well (elevated resistance).
  • Or have failed (high resistance).

They can be tested either when fitted, or removed. However, a less accurate reading may be obtained if testing whilst still fitted (additional resistance in the circuit).


At the end of section 3. is a table of ‘Typical Resistance Values‘, and whether they pass or fail.

3.1. Testing the Resistance (Fitted):

We can measure the resistance whilst they’re still installed. This may only give an indication of their condition, as there may be additional resistance in the circuit. However, it can prove that there is a circuit and that corrosion is not a factor in the glow plug failing to work.

Short video on how to measure the glow plug resistance (when fitted):

Video – How to Measure the resistance (when fitted).

3.2. Testing the Resistance (Removed):

What can we check..?

Testing Glow Plugs – Where to Connect a Multimeter (Voltmeter)

The condition?

Carry out a visual check for general condition, such as corrosion, or any obvious damage etc.

The resistance?

The resistance check is a more detailed check on the condition of the heater element. It can indicate if the glow plug is worn, or on the way out (see the table of Typical Resistance Values below).

Short video on how to measure the glow plug resistance (when removed):

Video – How to Measure the resistance (when removed).

The continuity?

A continuity check will test if there is an open circuit (it’s broken internally). This can be just a simple ‘buzzer’ test. If you don’t get a ‘buzz’, (open circuit) then it’s dead!

Short video on how to test the continuity of glow plugs:

Video – How to test glow plug continuity.

How to Test From Start to Finish (With a Multimeter) – The Complete Guide

Video – How to test glow plugs with a multimeter – The complete guide.

3.3. Typical Resistance Values:

The table below shows typical resistance values and whether they would pass or fail. Note the difference in values between measurements taken when installed and removed. This could vary greatly if there is any corrosion present, or there is a ‘bad contact’ when they’re inserted.

Typical Glow Plug Resistance Values

Glow Plug Condition Resistance (Fitted) 1 Resistance (Removed) 1 Pass or Fail
1 Old 3.031 MΩ 2.181 MΩ
2 Old 16.24 KΩ 15.24 KΩ
3 Old 47.26 KΩ 45.77 KΩ
4 Old 211.2 Ω 208.9 Ω
5 New 1-6 Ω 2 1 Ω
6 New 1-6 Ω 2 1 Ω
Note: 1 The difference in resistance for the same glow plug (when fitted and removed)…
2 The resistance (fitted) may vary and is approximate…


  • A typical resistance value for a working glow plug is usually between 1 and 6 ohms.

3.4. The Glow Plug Resistance Explained:

Low resistance is good. It means the glow plug will heat up…

“The total energy dissipated by the circuit is proportional to current, so the resistance of the heating element has to be low enough to draw sufficient current to generate enough heat”.


“The resistance of the heating element has to be high enough so that most of the energy is dissipated by the heating element”.

In other words…

You want a low resistance, so the energy is dissipated in the glow plug and it heats up. But… Not a really, really low resistance (like a dead short). Otherwise, the energy will be dissipated in the glow plug, and in the rest of the circuit. Blowing the fuse and potentially melting the wires (dependant on the fuse rating and how quickly the fuse reacts!).


  • Each manufacturer, or circuit design may have slightly differing resistance readings, due to their design parameters (and the actual condition of the circuit will also have an effect).

Testing Glow Plugs Without a Voltmeter (Multimeter):

There are some easy checks you can carry out, using a:

  • Vehicle battery, or suitable earth (-ve).
  • A jump lead.
  • A piece of insulated wire.

The following tests can be carried out:

You can test when fitted or removed, as indicated below:

  1. Is there a Circuit (When Fitted)?
  2. Does they Work (When Removed)?
  3. Does the Relay and Timer Work?

Safety Precautions:

Do not short the battery connections, (or yourself across the battery, it could give you a shock!) i.e. Between the live (+ve) and the earth, or bodywork (-ve). 12 volts is not usually enough to kill you, but batteries larger than 12v may do? Hydrogen gas given off by the battery can ignite, causing an explosion. Sending bits of battery and acid everywhere…

1. Is there a Circuit (When Fitted)?

The glow plugs can be tested for a working circuit (when fitted), by using a wire from a live 12v (or 24v depending on the system) supply and touching the end of the exposed glow plug (with the connector removed). If a spark is present when the end of the glow plug is touched, we know current is passing through the glow plug back to the battery. This indicates the glow plug is not open circuit.


  • Do not use a jump lead for this test! A jump lead is usually too big (and you can’t easily see what you’re doing).
  • You are less likely to make the wrong connection with a piece of wire, (i.e. Shorting in the wrong place).
  • Shorting can cause wire to melt (with the potential for a fire!).

2. Does the Glow Plug Work (Testing When Removed)?

When removed, we can carry out a visual check whilst testing to see if it works.

For safety reasons:

In this case, use a jump lead for testing, not a thin piece of wire…

  • The jump lead will hold the glow plug.
  • The jump lead has an insulated handle…


  • Shorting can cause wire to melt and it could cause a fire!
  • For negative earth vehicles, always connect the negative jump lead first (or vice versa).
Testing Glow Plugs – Where do I Connect to?

Two Methods:

a. Connect directly to the battery: (not recommended)
b. Connect to the actual glow plug supply:

a. Connect directly to the battery:

Testing the glow plugs by connecting directly to the battery is not recommended...

  1. Batteries can give off hydrogen gas.
  2. The glow plug can provide a heat source for the hydrogen, causing an explosion.

b. Connect to the actual glow plug supply:

To save time, two test’s can be carried out the same time. Goto 3. Does the Relay and Timer Work? below for more detail.

  1. With the glow plug removed, connect to one of the electrical connectors.
  2. Connect a jump lead to the negative (-ve) of the battery and the other end of the jump lead to the negative (-ve) end of the glow plug.
  3. Make sure the glow plug is not near anything that could be affected by heat.
  4. Switch on the ignition. If you have a working glow plug, the heater element will start to glow (see the image below).
Testing Glow Plugs Removed from a Vehicle With a Jump Lead

3. Does the Relay and Timer Work (Testing when Removed)?

This test is similar to ‘Connect to the actual glow plug supply‘ above, but has an additional test. You can time how long the glow plug works for, (before the timer switches the supply off).

  1. With the glow plug removed, connect to one of the electrical connectors.
  2. Connect a jump lead to the negative (-ve) of the battery and the other end of the jump lead to the negative (-ve) end of the glow plug.
  3. Make sure the glow plug is not near anything that could be affected by heat.
  4. Prepare to time how long it glows for?
  5. Switch on the ignition.
  6. Watch to see if it starts to glow, whilst also timing the duration.
  7. How long did it glow for? It should be for around 10 to 20 seconds dependant on the vehicle.
  8. If it doesn’t glow, it’s probably defective (check the connections?).

Short video on how to test without a test meter:

Video – How to test glow plugs without a multimeter.

How to Test From Start to Finish (With Jump Leads & Wire) – The Complete Guide

Video – How to test glow plugs without a multimeter – The complete guide.

Ordering Parts (Glow Plugs, Test Meter, Tools, etc):

The following table gives examples of the test equipment, tools, etc you can buy to fix your vehicle. If you’re not sure what you need, sometime it’s worth having a look? The list below may give you an idea of what’s what’s available, what to look for and how much it costs…

Ordering Parts – Examples of What You Can Buy:

Glow Plug Prices
Item Location
UK   Glow Plugs
Test Equipment
Glow Plug Tools
Jumper Cables
USA Glow Plugs
Test Equipment
Glow Plug Tools
Jumper Cables
AUS Glow Plugs
Test Equipment
Glow Plug Tools
Jumper Cables
CAN Glow Plugs
Test Equipment
Glow Plug Tools
Jumper Cables
IRL Glow Plugs
Test Equipment
Glow Plug Tools
Jumper Cables
Disclosure: The links in this table are “affiliate links” This means we may garner a small commission (at no cost to you), if you choose to make a purchase.
Thank you for your support.

Replacing Glow Plugs:

Is the same as for the removal, except…

  • It’s in reverse!
  • It may be a good idea to lubricate the threads to prevent corrosion (use some high temperature copper grease, or something similar).
  • Don’t overtighten. See the workshop manual for the correct torque. It may be printed on a replacement suppliers box (see the image below).
  • Check all the electrical connectors are properly connected and not just resting on top of the glow plugs (it’s easily done!).
The Manufacturer’s Suggested Glow Plug Torque Value

Glow Plug Dashboard Indication:

I have seen vehicles where the warning indicator on the dashboard only appears very briefly. So briefly, you can miss it. Whereas on other vehicles, the indicator stays on a lot longer? As a guess, it’s probably down to the circuit design for that particular vehicle (if the vehicle starts and runs fine, with no white smoke). The periodicity the symbol displays may vary from vehicle to vehicle… However, always assume the glow plugs are needed, and wait for them to heat the cylinders, to aid the engine starting process (see the notes below for more info…).

If you need to check they are actually working:

With a test meter:

Without a test meter:


  • Operating them when removed from the vehicle? Best to test the glow plugs one at a time, save melting anything nearby!
  • Glow plugs come on in hot and cold weather, to aid starting and help whilst the engine warms up. However, the length of time they’re on may vary (if this is built into the vehicles electronics by design).

– click or tap any image to view full size –

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