Home Diagnosis and TroubleshootingEngine Problems Glow Plug: Working Mechanism And Why It’s Needed

Glow Plug: Working Mechanism And Why It’s Needed

by Jordan Harris
Glow Plug

By their very design, diesel engines sometimes need help to start, particularly in cold temperatures. Glow plugs are added to diesel engines which heat the fuel and air mixture within the combustion chamber to start your car’s engine efficiently. Diesel glow plugs are regularly needed to generate the necessary heat to make diesel-powered vehicles function in cold climates. A glow plug in a diesel engine makes it easy to start the vehicle in cold weather. Bad diesel glow plugs are a hazard for anyone.

Moreover, the cylinder head and cylinder block will absorb a copious amount of heat as well. As the glow plug is placed in the internal ignition chamber, this component delivers heat to the engine.

A glow plug is a vital element of an engine. To learn more about this engine part, keep reading.

Glow Plug Function

On diesel engines, ignition is affected by the fuel’s self-ignition sprayed into the highly heated and highly compressed combustion air. However, in a cold engine, compression alone cannot achieve the self-ignition temperature, and thus, a pre-glow system is needed.

A pre-glow slow increases the compressed air’s temperature to further help fire the cold engine by using a glow plug. How long the pre-glowing lasts depends on ambient temperature and the engine’s temperature.

Pencil element glow plugs are developed for 12 volts of current and are assembled in parallel. They essentially consist of a pencil element attached to the housing and screw-in threads. Non-releasable round aluminum nut glues the single-pole connecting pin to its housing.

Glow plugs on some older diesel engines work on a 6 volts current and a dropping resistor reduces the voltage from 12 to 6 volts. The glowing period lasts 9 seconds and a 1,652°F “Quick-Start” pencil element temperature is attained. The top temperature rises to 1,976°F after 30 seconds.

A heater element indirectly heats the pencil element. Constructed of resistance wire, this heater element is insulated and embedded in a ceramic composite. Once the glow system switches on, every glow plug gets around 20 amps, with the peak impulse hitting 40 amps (approx.). As the heat increases, the glow plug’s inherent resistance grows and limits the current to around 8 amps.

After approximately a 20-second glowing period, the heater pencil element’s temperature reaches 1,652°F while it rises to its peak of 1,976°F after 50 seconds.

Diesel Glow Plug Location

Diesel glow plugs are situated in the engine’s cylinder head. They are attached with screws to each cylinder. The battery powers all glow plugs with 12 volts of current. Glow plugs are relatively easy to spot as the top protrudes and connects to a wire delivering the power. You might have to take off the valve cover to access the glow plugs in some cases.

Glow Plug Control Module

A glow plug control module is to a glow plug network as a brain is to a living body. It controls and monitors the glow plugs in an engine. The control module is tasked with analyzing information received from the ECU, or engine control unit, to understand how to reach optimal performance standards of the glow plugs to satisfy exhaust gas limits.

Moreover, a glow plug control module adjusts the electric current size, duration, and timings of each glow plug to optimize engine operation. All of these elements are important.

A glow plug relay contrasts values measured by the control module to data gathered from sensors to adjust and regulate the current supplied to the glow plugs.

If the glow plug isn’t tuned to a high enough temperature or turned on long enough, it will not ignite the fuel mixture efficiently. Keep it on for long and the glow plug will sustain wear and tear at an accelerated pace.

Glow Plugs Vs Spark Plugs

Although spark plugs and glow plugs are quite similar, there are some notable differences between the two components. Do not mix up these parts because this is how they vary:

1. Fuel Type

Firstly, glow plugs are installed in diesel-powered engines whereas gasoline engines feature spark plugs. Spark plugs are required to generate a “spark” that combusts the fuel and air mixture inside the combustion chamber of gas engines. However, we have already talked about what a glow plug does in diesel engines so there is no comparison. Diesel engines do not require “sparks” for combustion; they only need heat.

2. Heat

Gas is a volatile compound. It only needs a little spark to start combustion. On the other hand, diesel fuel needs both heat and compression to ignite. The cylinders in a diesel engine must be packed with heat to start combustion, a job that is done by glow plugs.

Glow plugs generate a high level of heat that assists the engine to combust the air-fuel mixture in the ignition chamber.

3. Durability

Glow plugs generally last far longer than their spark counterparts, mostly because their job is simpler. They only have to warm the cylinders and that concludes their duty. Spark plugs, in contrast, have to work continuously to maintain engine operation. Cheap spark plugs may only last up to 30,000 miles.

4. Ignition Difference

Spark plugs require electrical energy to generate a spark which then ignites the fuel and air blend in the ignition chamber. Sparks are not used by diesel glow plugs. They turn the electricity to heat which ignites the air and fuel blend.

This video should help you understand better.

Glow Plug Light

Unlike gasoline engines, diesel-powered engines do not feature spark plugs. They depend on the heat produced by high compression to combust the fuel-air blend in the cylinders. When cold, diesel engines can be difficult to start.

This is why glow plugs are added to diesel engines – to warm up the air in the cylinder, causing the glow plug symbol to go off on the dashboard. The glow plug symbol is a coil light. The glow plug is relieved from duty when the engine runs as the combustion process has been initiated.

The total time required for a glow plug to heat differs based on the ambient temperature and vehicle temperature, as stated above. Typically, glow plugs require between 5 seconds to a minute to adequately heat the engine. By this stage, the dashboard glow plug symbol will turn off and you should be able to start your vehicle.

Apart from glow plugs, diesel-powered engines also use a variety of different starting aids, such as:

  • An ether injection kit
  • A coolant heater
  • An oil pan heater
  • An air intake heater
  • A block heater

Bad Diesel Glow Plug Symptoms

If it wasn’t clear already, glow plugs are a necessity for diesel-powered engines. Suffice to say, any problem with the diesel glow plug will negatively affect your vehicle and its overall performance. Thankfully, the signs of glow plug decay are rather visible. If you see any of the eight signs mentioned below in your car, know that the glow plug system needs your help.

1. Trouble Starting The Vehicle

It may be difficult to start your diesel engine if the glow plugs are damaged. The malfunctioning plugs do not produce sufficient heat to warm the cylinders. You may have to make several attempts at starting the motor if the plugs do not create adequate heat quickly.

Furthermore, if the temperatures are exceedingly low and glow plugs practically, the engine may not start at all.

2. Poor Acceleration

Bad glow plugs can negatively impact the engine’s performance, particularly in colder climates. If you see no reaction from your vehicle despite pressing down on the accelerator, you may have to change the glow plugs. That being said, poor acceleration can be indicative of many other problems in your vehicle. To be completely sure, make sure to do a thorough investigation of your vehicle and its symptoms.

3. Decreased Fuel Efficiency

Fuel and temperature mostly decide fuel efficiency but not glow plug behavior. But, in a few cases, a glow plug might have a role to play. Depending on local conditions, diesel might be mixed with numerous additives in cold months to decrease its freezing point. Blended fuel of this nature will most likely offer decreased fuel efficiency and feature lower energy content.

The operating temperatures of the engine should be regulated to ensure that all of the fuel injected is thoroughly burned. Glow plugs contribute to extra heat generation in the ignition chamber.

If the proper temperature is not achieved owing to malfunctioning glow plugs, the performance of the engine suffers and it must work harder to satisfy the road’s demand. The car’s fuel economy will suffer as a result.

4. Rough Idling

Rough idling is a common symptom of glow plug failure in cars where these components have burnt out, damaged, or fouled (which you can try fixing by figuring out how to clean a spark plug and how to clean spark plugs). The engine is significantly harder to start as the temperature falls. This also generates white smoke from the exhaust (we will get into that shortly) for a brief moment before the car finally starts.

Inspect the fuel injection timing if most, if not all the glow plugs have been burnt at their tips.

5. Engine Misfiring

Engine misfires happen when the fuel cannot properly combust in the cylinder. Make no mistake, any kind of misfire should worry you. But, you can consider yourself lucky if it is due to a bad glow plug as these problems are easier to fix.

Glow Plug

The glow plug must heat the cylinder appropriately so it can combust the fuel. Although misfiring can be the result of other problems, a faulty glow plug sits at the top of the list.

6. Black Exhaust Smoke

Is your vehicle emitting a huge volume of dark gray or black smoke from the exhaust? If yes, there is certainly an issue. These colors mean that the combustion system is suffering. Check the glow plugs to make sure they are the reason behind the exhaust smoke.

7. White Smoke

White smoke is typically produced when there is insufficient heat to combust the fuel. Unburned gasoline particles exit the exhaust as white smoke and often emit a strong fuel odor. During chilly weather, it’s not uncommon to see smoke coming from the exhaust till the engine gets warmed up.

A damaged glow plug can cause white smoke in engine start-up. However, there are several additional potential reasons for excessive white smoke, such as poor compression, the air inside the fuel system, and decreased engine cranking speed.

If white smoke persists despite the engine warming up, the engine may have a damaged injection pump.

8. Glow Plug Light

The Check Engine Light is triggered whenever something is wrong with a vehicle’s interior organs. When the glow plugs are faulty, you can use an OBD-II diagnostic tool to scan the system and get error codes. In many cases, code P0380 which stands for “Glow Plug/Heater Circuit A Malfunction” will pop up. However, a handful of other codes like P0673, P0684, or something else could appear.

If the vehicle exhibits any of the other symptoms indicated above, as well as decreased fuel economy, it should be sent to an authorized workshop as soon as possible for thorough diagnosis and servicing.

Is It Okay To Drive With A Bad Glow Plug?

Naturally, like all other parts of a vehicle, glow plugs wear out over time and this affects the performance of the var. Your vehicle may still start and be drivable but not as efficient as when the glow plugs were sound.

You will notice how it takes more attempts to start your car when the glow plugs get weaker and reaching greater speeds will take excessive gas pedal usage; unless you live in colder climates then the car will not start at all.

Apart from the inconveniences mentioned above, poor glow plugs will decrease vehicle fuel efficiency while increasing carbon footprint. Replace the plugs when you notice symptoms of decay displayed by these parts for smooth operations.

DIY Glow Plug Replacement

Glow Plug

You can replace the glow plugs yourself with the right technical know-how and tools. Just make sure to follow this guide to the tee.

Materials Needed

  1. Replacement glow plugs
  2. Ratchet and deep socket set
  3. Socket set
  4. Glow plug removal tool/pliers
  5. Valve cover gaskets (if needed to be replaced)

Steps To Replace Glow Plug

Often when changing components like a glow plug, you have to dig around for a certain time to reach the part. The time and effort required to access such a part depend on the model, make, and year of the car. Make a mental note of everything that has been removed or set aside. We find bagging and labeling bolts and screws make the job easier.

Step 1

Detach the battery cable. Find and detach the black negative battery cable from its socket. Whenever you are working on your vehicle’s electrical system, make it a point to first disconnect the battery cable.

Step 2

Take off the valve cover. If valve covers protect the glow plugs underneath, remove the covers first. If there are no valve covers to remove, move on to Step 3. For Step 2, unscrew the screws by the cover’s perimeter. Look out for the valve cover gaskets amidst the covers and the engine.

Pro tip: Consider replacing the valve cover gaskets at this time.

Step 3

Find the glow plugs – they will be screwed in the cylinder head. The tops will be sticking out so find those.

Step 4

Unfasten the glow plugs’ power supply. On top, you will see either a plastic terminal, bolt, nut, or similar connection device to take out the wire from the diesel glow plug fast. Disconnect the terminal and keep the wires aside.

Newer vehicle models feature a terminal like that of spark plugs and can be pulled off without using pliers. You can use a tool but be careful.

Step 5

Take out the glow plug with a ratchet and socket. Put the ratchet on top of the plug and turn it counterclockwise. You may need a deep socket for this. These should stay clean in most cases. The shape and length of the plug will tell how damaged it is.

Note: Diesel glow plugs can disintegrate within their component. Remove all broken bits from inside the engine if you are taking out a broken plug.

Step 6

Time to install the new plug. Take some precautionary measures and try to improve the components associated with the plugs. Brush off any dirt near the opening of the glow plug hole. If there is dirt inside the supply terminal, clean it with a wire brush.

Step 7

Install the new plugs. Secure them in position using just your hand and then use the ratchet and socket to fasten them properly. Be careful and do not over-tighten the glow plugs. It will damage the plug, hindering optimal function or even making the plug snap in certain cases.

Step 8

Connect the power supply. Reconnect the electrical wires that deliver power to the glow plugs. These should clip or screw into position in the same way as you took them off in Step 4.

Step 9

Change the valve covers. If you took off the valve covers, you may want to replace the gasket at this time. Insert all the screws you have removed after the gasket has been changed. Hand-tighten them first and then snug them in place using a ratchet.

Note: Many manufacturers will mention the torque to which the valve screws have to be secured. Look for manufacturer-specific instructions if you are doing valve cover removal.

Step 10

Attach the negative battery cable. Reconnect the battery and the negative battery cable. This reinstates power to your car. Make sure to scan the vehicle with a scan tool to ensure everything is working fine.

Glow plugs are tasked with converting freezing conditions within an engine to working operating temperatures. Plugs are also expected to turn off and go back to their original temperature just as quickly. Dealing with drastic temperature changes is hard work and takes a toll on these parts. It is common to replace them or other components associated with the process.

We only recommend this DIY process to someone who has some form of automotive repair experience. Otherwise, a certified mechanic is the best fit for the job.

Glow Plug Replacement Cost

Typically, glow plug replacement costs between $95 to $210. Replacement of individual glow plugs is around $15 and $50 and you have to shell out about $80 to $160 for labor charges. Remember that every cylinder contains its plug so replace all the plugs even if only one is known to have gone bad. Labor cost does not change too much even if you change multiple.

Spark Plugs and Glow Plugs: Facts You Need to Know

  1. Both spark plugs and glow plugs serve as ignition sources in combustion engines.
  2. Spark plugs are found in gasoline engines, while glow plugs are in diesel ones.
  3. Spark plugs receive a high-voltage charge from the ignition coil and create a spark that ignites the fuel-air mixture in gasoline engines.
  4. Spark plugs come in different types, sizes, sealing types, and spark gaps.
  5. Glow plugs are electrical heating devices located in each cylinder of a diesel engine.
  6. Glow plugs are used to ensure proper ignition of the injected fuel in all weather conditions, which reduces exhaust emissions.
  7. Unlike spark plugs, glow plugs are only needed during the ignition process of a diesel engine.
  8. Glow plugs work by heating up and emitting visible light, which helps to ignite the fuel when it is injected onto the hot tip of the glow plug.
  9. Spark plugs work continuously to ensure the engine runs properly, while glow plugs are used only during the ignition process.
  10. Spark plugs and glow plugs fulfill different roles in the starting process of combustion engines, depending on the type of engine they’re found in.


Despite its small stature, a diesel glow plug plays a crucial role in a diesel engine. These small bits make sure your vehicle can start on days that are colder than usual. Fortunately, glow plugs do not usually break down without something abnormal tampering with the regular functionalities of a car. If your vehicle’s glow plugs have gone bad, make sure to replace them as soon as possible.

Frequently Asked Questions (FAQs)

Here are some popular FAQs:

What Is A Glow Plug

A glow plug is a heating device that assists in starting diesel-powered engines.

What Does A Glow Plug Do

Diesel engines may face difficulties starting in cold temperatures. A glow plug’s job is to aid the process by providing a heating boost.

How To Test A Glow Plug

A digital multimeter is required to test a glow plug. An OBD-II scan tool will read all error codes in a vehicle and find anything wrong with the plugs.

How Does A Glow Plug Work

A glow plug heats incoming air and fuel to promote efficient fuel burning in diesel engines.

What Is A Glow Plug Igniter

A diesel glow plug delivers ignition inside the engine by heating the small wire coil inside the plug. A 1.5 volts battery inside a glow igniter accomplishes this task.

You may also like

Leave a Comment