Glow Plug


Glow Plug

Unlike gasoline engines, their diesel counterparts do not have an ignition system. Their air/fuel mixture blows up due to huge compression that causes self-ignition. However, when the engine is cold, the air/fuel mixture is harder to compress. For this reason, modern diesel engines feature electrically heated glow plugs. They warm up the air/fuel mixture in the engine’s cylinders to simplify cold starts. 


By design, a glow plug is a heated device, which job is to help you start the engine at low temperatures. As a rule, each cylinder is equipped with its own glow plug powered by the vehicle’s electrical system and operated by the engine control unit.

A glow plug operates similar to an incandescent bulb or a toaster as it also has a filament that warms up. Needless to say that a glow plug consumes a lot of electricity, which is why it is activated by the engine control unit based on special cold start procedures and is turned off once the normal combustion process is running. 

Signs of a bad glow plug

Here are the most common signs of a faulty glow plug:

You can’t start the engine when it’s cold

The job of a glow plug is to warm up air/fuel mixture inside a specific cylinder, however, if it malfunctions, the mixture will not warm enough to ignite, preventing the combustion process from running. The good news is that your engine may eventually start with just one poor glow plug, but anyway the problem should not be ignored. You should get in tough with your mechanic to have the problem fixed as soon as possible.

Your glow plug indicator light doesn’t turn on

The dashboard of your diesel-powered vehicle should feature a glow plug indicator light. It usually turns on and illuminates until the glow plugs stop working which means you can now start the vehicle. However, if one or more glow plugs fail, the light may flash or even not come on. If so, call your mechanic for a piece of advice and repairs.

Rough idling, poor performance and fuel efficiency

These symptoms can be caused by a poor glow plug. If it’s the root of the problem, it should go away once the engine warms up to its normal temperature and the engine control unit turns off the glow plugs.

Black or white smoke from the exhaust

A faulty glow plug leads to the incomplete combustion process in one of the cylinders. This means that all or almost all fuel sprayed into the cylinder will be forced out. As a result, you’ll see black smoke if the fuel was partially burned, or white smoke if no fuel was burned in the cylinder. Whichever the color is, you should stop the engine and have it tested and repaired immediately.

Your check engine light is on

Due to a malfunctioning glow plug, your engine will waste a lot of fuel, which leads to increased emissions. Since this is one of the points that are constantly checked by the engine control module, it may turn on the check engine light to alert you of the problem. 

Can I drive a car with a bad glow plug?

The short answer is you can. Since the only function of glow plugs is to help you start the engine and warm it up until it hits its normal operating temperature, you can technically drive the car if you were lucky to start it. Although, you should warm up the engine for about 5-10 minutes before driving the car.

Even if your car is easy to start with a bad glow plug, the problem should not be ignored. Driving a car with a poor glow plug for a long time will cause carbon buildup in the cylinder, which may lead to expensive repairs and a failed emissions test. 

Is it possible to fix a faulty glow plug?

The first step you should do is to determine which glow plug is bad. To reach this goal you may use a scan tool that reads error codes in the onboard computer. The other glow plugs along with the engine control module and related components should also be checked to prevent potential problems.

A faulty glow plug is not repairable, so it must be replaced. The final step is to wait until the engine gets cold, so you can test the glow plugs. 

Can I tackle the job myself?

In most cases, replacing a glow plug doesn’t require a lot of skills or experience. If you have ever changed your engine’s oil or spark plugs, you should manage the job. However, if the glow plugs are hard to access, making you remove any parts of the engine, hire a mechanic to avoid possible damage to the engine. Besides, keep in mind not to overtighten a new glow plug and be careful when removing the old one to prevent its breaking. 

Glow plugs are the kind of engine’s parts that need to be replaced from time to time. Follow the manufacturer’s recommendations and replace your glow plugs in accordance with recommended change intervals even if they seem to be in mint condition.

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