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Le faulty diesel glow plugs They can cause difficult cold starting, longer cranking times than normal, rough idling in the first few seconds, and, on some cars, the glow plug warning light or engine warning light coming on.
Unlike spark plugs in gasoline engines, glow plugs they don't produce a spark: they heat the combustion zone to make starting the diesel engine from cold easier in particular.
Before replacing them, it is a good idea to check which glow plug is not working and also check the power supply, wiring and preheating module.
What you will learn:
- What are the symptoms of bad diesel glow plugs.
- How to understand which glow plug is not working.
- Because the problem is especially noticeable when cold.
- How to distinguish it from a battery or other faults.
- If you can continue driving.
- When to change them and whether to replace one or all of them.
Index
Anyone switching from a petrol car to a diesel can easily confuse candles and glow plugs, but they are different components.
In a gasoline engine, the spark plug produces the spark needed to initiate combustion. In a diesel engine, combustion occurs through compression, while glow plugs help achieve favorable conditions, especially when the engine is cold.
For this reason, a faulty spark plug can be particularly noticeable when you first turn it on of the day or after several hours of inactivity.
What are diesel glow plugs for?
The glow plug contains a heating element that quickly reaches a high temperature.
During cold starts it helps create more favorable conditions for combustion in the cylinder.
On modern diesel engines, its work doesn't necessarily end as soon as the engine starts. Depending on the system used, the electronic management can continue to control the glow plugs even after the engine has started.
This is why they are not simply components that are needed “when it’s cold outside”.
Symptoms of failed diesel glow plugs
One or more faulty spark plugs can cause:
- cold starting difficulties;
- starter motor runs longer;
- rough idling immediately after ignition;
- vibrations in the first few seconds;
- possible abnormal smoke when starting;
- glow plug warning light or engine warning light;
- error codes stored in the control unit.
The presence of one of these symptoms It does not automatically prove that the glow plugs are bad.
Battery, fuel supply, injectors and other problems can produce similar behavior.
Because the problem is especially noticeable when cold
This is one of the most characteristic clues.
After several hours of inactivity, the engine may take longer to start, while when warm, starting apparently returns to normal.
At low outside temperatures the defect can become even more evident.
If the diesel struggles to start, however both cold and hot, it is advisable to immediately expand the controls and not focus exclusively on the glow plugs.
Why a glow plug fails
Glow plugs work under harsh conditions and can deteriorate over time.
Possible causes include:
- normal wear and tear;
- broken heating element;
- electrical connection problems;
- incorrect power supply or control;
- preheater module failure.
If multiple glow plugs are faulty at the same time, it is especially important to also check the system that supplies and controls them.
How to understand which glow plug is faulty
The diagnosis can start from the control unit and continue with electrical checks.
Read errors with OBD2 diagnosis
On many cars, a fault in the glow plug circuit can generate one or more error codes.
Electronic diagnosis can therefore indicate which circuit or cylinder to start the checks from.
But be careful: a DTC doesn't automatically mean the glow plug needs replacing. The problem could also be related to the connector, wiring, power supply, or control module.
To understand how to read the errors stored by the control unit, our help may be useful OBD2 car guide: what it is and how to read errors, where we explain how the diagnosis works and how to interpret the codes.
Electrical glow plug check
An electrical check allows you to compare the glow plugs and identify any broken or faulty elements.
However, the method must be suitable for the system installed on the car.
Especially on modern diesels, You should not directly power a glow plug without knowing its characteristics and operating voltage.
For simple electrical checks, a digital multimeter can also be useful for testing the battery and other car circuits. Before taking measurements on glow plugs, however, you need to understand the system used by the vehicle.
HELPFUL TIP
Digital multimeter for car checks
Useful for simple electrical measurements on car batteries and circuits. Choose a model with insulated probes and always check the vehicle's recommended procedure before taking measurements.
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Check power supply and preheat module
If the glow plugs appear to be working properly but the fault persists, you should also check:
- connectors;
- wiring;
- diet;
- electrical protections provided by the vehicle;
- preheating module or control unit.
This way you avoid replacing a perfectly working glow plug due to a problem elsewhere.
Bad glow plug or battery?
This is an important distinction because both problems can make it difficult to get started.
A weak battery can cause the starter motor to turn over more slowly and make it difficult to start a diesel, especially in cold temperatures.
With a faulty glow plug, however, the starter motor may turn normally but the engine may take longer to start or run erratically in the first few seconds.
However, this is not definitive proof.
If starting is difficult, it is also worth checking:
- battery status and voltage;
- starting speed;
- fuel supply;
- injectors;
- sensors;
- compression, when necessary.
If your doubts concern the battery, you can also find out more in the guide How long does a car battery last?, where you can find the main signs of a battery that is losing efficiency.
Can you drive with a bad glow plug?
If the engine starts and subsequently runs normally, a single faulty glow plug It does not necessarily mean that the car will stop while driving.
The problem can, however, make cold starts more difficult.
With several failed glow plugs, the situation can worsen, especially when the outside temperature drops.
If you also have an engine warning light, persistent rough running, or other symptoms, it's a good idea to get a diagnosis instead of blaming the glow plugs.
When to change diesel glow plugs
There is no universal mileage after which all glow plugs must be replaced.
Their duration depends on:
- type of glow plug;
- motor;
- number of starts;
- terms of use;
- preheating system;
- manufacturer's instructions.
For this reason, it is not a good idea to replace them automatically based on mileage alone.
It makes more sense to check them when symptoms or diagnostic errors appear, or when it is required by the vehicle's maintenance schedule.
Should I change one glow plug or replace them all?
It is not mandatory to replace all the glow plugs when only one is faulty.
However, if they are all the same age and have the same mileage, it is useful to check their overall condition before deciding.
The accessibility and condition of the glow plugs must also be considered.
Be careful when removing
A glow plug that has been installed for many years can be difficult to unscrew.
Forcing it too much can cause it to break in the cylinder head, turning a relatively simple replacement into a much more complex job.
For this reason, when a glow plug offers a lot of resistance, It's not a good idea to insist blindly.
What to check on removed glow plugs
Once removed, it can be useful to compare them with each other.
Check:
- tip conditions;
- any deformations;
- unusual deposits;
- obvious damage;
- marked differences between the glow plugs of the various cylinders.
The visual aspect can provide some indication, but does not replace electrical inspection and diagnosis.
A glow plug with unusual damage also deserves a more thorough inspection before simply fitting a new one.
In summary
Le faulty diesel glow plugs They are especially noticeable during cold starts.
If the diesel starts badly:
- check for any stored errors;
- check the glow plugs electrically;
- check power, wiring and module;
- also check the battery status;
- do not rule out injection and other starting problems;
- Replace components only after identifying the cause.
A faulty glow plug does not automatically mean a serious engine problem, but a correct diagnosis avoids unnecessary replacements.
FAQ - Frequently Asked Questions
How to tell if a diesel glow plug is bad?
Difficulty starting from cold, erratic operation in the first few seconds, and a warning light may be signs. Diagnosis and electrical checks are needed to confirm the problem.
Can a faulty glow plug cause a diesel engine to start poorly?
Yes, especially when the engine is cold. With just one faulty glow plug, the problem may be less obvious, while with multiple faulty components, starting can become more difficult.
If one glow plug is faulty, do you have to change all of them?
Not necessarily. You can replace the defective one, but if all of them are the same age and have the same mileage, it's a good idea to check their overall condition.
Do the glow plugs only work during start-up?
Not always. On several modern diesel engines, they can also be controlled after starting according to the strategy set by the engine management system.
Are spark plugs and glow plugs the same thing?
No. Spark plugs produce the spark in gasoline engines. Glow plugs are heating elements used in diesel engines.
Conclusion
When a diesel starts poorly when cold, the glow plugs are certainly among the components to check, but I'm not the only possible cause.
A correct diagnosis can distinguish a truly faulty glow plug from a battery, wiring, glow plug module, or power supply problem.
And before unscrewing an old glow plug, it's worth remembering one more thing: Finding the fault is often easier than removing a stuck glow plug without breaking it.
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