Symptoms of Bad Glow Plugs: Causes and Replacement Cost

Glow plugs sure sound familiar, right? Much more if your vehicle operates with a diesel engine. You must have seen those plugs that warm and preheats the engine’s cylinder. Glow plugs are pretty essential, as they allow easy diesel combustion. But, what are the symptoms of bad glow plugs?

Glow plugs have excellent longevity but gradually wear after some time of constant use. This engine management component can cause your car’s driveability problems when they start developing some issues.

Hence, it would help if you notice when your vehicle’s glow plugs need a swift replacement. Nevertheless, I would be revealing what causes glow plugs to fail and the common symptoms associated with bad glow plugs.

What are the Symptoms of Bad Glow Plugs?

As stated above, glow plugs help gather adequate heat that ignites the fuel and helps get the engine cylinder warmed up. Nevertheless, you can still find some vehicles operating with spark plugs. Kindly note that spark plugs only provide heat for engines that operate on gas.

Symptoms of Bad Glow Plugs

 Also, spark plugs use electricity in the generation of “sparks” that ignites the engine. Hence, there is no spark plug in diesel engines since they don’t generate sufficient heat for the combustion chamber’s mixture.

Can broken glow plugs damage an engine? With the reduced heat, the engine of your car may soon start facing some problems. Hence, you need to note important signs that tell you when you need to replace your glow plugs.

The symptoms of faulty glow plugs start slowly and are almost invisible till it results in a complete failure. There is a need for an immediate replacement for bad diesel glow plugs, as any delay may worsen the issue.

How to tell if diesel glow plugs are bad? If you find it tasking detecting when your glow plugs are faulty, you can look out for these symptoms.

Difficulties Starting the Engine

When starting a diesel engine with bad glow plugs, you may find it challenging. Although this can also occur when there are some issues with the car’s battery or fuel. Nevertheless, it can be due to bad glow plugs, much more in cold conditions.

Without functional glow plugs, the car’s combustion chamber may not get an adequate increase in temperature. Hence, they may be some issues igniting the fuel, which further leads to the engine not starting. So, you should check your glow plugs when you notice such a sign.

Engine Misfiring

An engine misfiring often results from the fuel failing to ignite right within the cylinder. The bad news is that an engine misfiring can cause some other issues for your car. It would interest vehicle owners to know that this is one of the most common symptoms of bad glow plugs for diesel engines.

There is a need for high compression rates for the combustion process that occurs in diesel engines. Also, the fuel should appear in an adequate amount, and the fuel injection should occur at the right time.

Nevertheless, your car’s combustion chamber may need temperatures around 450 degrees F. or 232 degrees C. to ignite. But, the engine chamber would not get such a temperature if you have faulty glow plugs.

Reduced Fuel Efficiency

The fuel efficiency of vehicles depends on the fuel and temperature. So, some may think that the glow plugs do not cause such issues. Still, glow plugs play great roles in providing additional heat for the combustion chamber.

Hence, when the glow plugs malfunction, the required temperature becomes unavailable. The low temperature would weaken the engine’s performance and barely reach the road’s demands. So, adverse fuel efficiency could be due to faulty glow plugs.

Low Acceleration

Various reasons may cause your vehicle to have poor acceleration. Still, this could be a sign that the glow plugs of your diesel engines are faulty. With the ignition issue resulting from faulty glow plugs, you may find that the car starts but doesn’t run for long.

Even when your car runs, you’ll notice low acceleration. It may seem pretty annoying with your vehicle not running optimally. A quick check on the glow plugs may help you detect and fix this issue.

Dark Smoke Emission

Ever seen cars emitting dark smoke from their exhaust? Such emissions can cause some great environmental pollution. It often results from an air-fuel ratio imbalance, where either the fuel is excess or insufficient oxygen.

The process of diesel combustion gets affected by faulty glow plugs. Hence, the car starts emitting black smoke from the tailpipe. Such a sign is not ignorable, and you should check the functionality of the diesel glow plugs.

White Smoke

Aside from black smoke emitting from your car’s exhaust, you can also notice white smoke. It occurs when the combustion chamber does not get sufficient heat for burning the fuel. Kindly note that white smoke is common when the weather condition is cold.

It is persistent until the engine starts warming up. Other possible causes of this white smoke include the fuel system having air and low compression. But, this is one of the symptoms of a bad glow plug control module.

The Engine Light Turns On

The dashboard of many cars comes with a light that turns on when your engine has some issues. Some cars also reveal a diagnostic code to help you determine the car’s faults and know the best way of fixing them.

You would often find your check engine light on when your diesel glow plug is faulty. With the scanner’s connection to the OBD2 (On-Board Diagnostics) port, you can easily tell that the glow plugs are malfunctioning.

So, you may find the diagnostic code for faulty glow plugs showing as “P0380.” Other common diagnostic codes include P0384, P0673, P0677. The car’s manual often holds such details, and you can scan through to learn more.

how to test glow plugs

What Causes Glow Plugs to Fail?

Car engines may have injectors that often drip on your glow plugs. With this, the plugs soon start eroding, and in some cases, the controller of the glow plug would “stick on.” The effects cause the glow plugs to fail.

Aside from this, the battery chargers with high amperage that helps start an engine can cause the glow plugs to fail over time. With too much voltage from these chargers, the tip of your vehicle’s glow plugs may explode.

How much to replace glow plugs? Well, you can get a glow plug from auto stores from as low as $15 up to $50. Nevertheless, you may need to pay $80 to $160 for labor if you need an expert to install the glow plugs. In total, you may spend up to $95 to $215 for replacing your bad diesel glow plug.

You can also decide to change the glow plugs yourself to cut costs. All you have to do is get the wire disconnected and remove the existing glow plug. Then, fix the new one; you may add some anti-seize on the threads to make them come out with much ease.

Also, you should try changing all the glow plugs at once. It’s fine to change the faulty glow plug for a cylinder, but there are greater chances of the others getting affected. Full-sized trucks having a V8 engine often have eight cylinders, and you should consider replacing all eight plugs.

Frequently Asked Questions (FAQs)

Q: Is it OK to drive with a bad glow plug?

You can drive with faulty glow plugs when the damages seem pretty low. But, as the functionality of the plugs reduces, there would be serious problems. Sooner or later, the engine would start misfiring, and the fuel efficiency reduces.

The symptoms of a faulty glow plug start “slowly” before you start noticing worse damages. Nevertheless, it is pretty crucial to get the faulty plugs changed earlier before your vehicle starts facing some serious damage. Only when the combustion chamber gets adequate heat would the engine work effectively.

Q: How do you check glow plugs without removing them?

One excellent solution on how to test glow plugs is employing a test light. Get a 12-volt equipped test light and get it connected to the positive battery terminal. Then, check out the wires of your glow plugs and get them disconnected. Get the test light attached to the glow plug’s terminal.

A bad glow plug would leave the test light not emitting any light. With this, you can easily tell that it’s time to replace the glow plugs. Another easier way of checking the plugs is with the aid of a professional tester device.

Q: How do you check glow plugs?

Some symptoms that accompany a faulty glow plug may often be due to your car’s underlying faults. In essence, it would help to check if really the glow plugs are bad and would need a replacement. You can decide to remove the glow plug in your car to see if there is an explosion due to high voltage.

Alternatively, you can get your test light connected to see if the light would come on; if it doesn’t, the plugs are bad. Again, a multimeter can tell you about your glow plugs’ current status, and you can start planning for a possible replacement.

Q: Will new glow plugs improve performance?

Undoubtedly, the replacement of a faulty glow plug will cause some breakthroughs in the engine’s performance. Glow plug relay problems when they are faulty, and till they get changed, the car won’t perform at its best. You may even start noting some decrease in its acceleration.

However, once you can get top-quality glow plugs from stores and install them, your car’s engine would work better. The combustion chamber can now get the right temperature level to warm the vehicle and ignite the engine. Those annoying signs you had noticed would be all gone.

Q: Are glow plugs expensive to replace?

You may find it pretty cheaper to replace spark plugs when compared to glow plugs. In total, you may spend up to $95 to $215 for replacing your bad diesel glow plug. This cost covers the cost of the individual plug and the cost of labor.

Nevertheless, you may need to pay $80 to $160 for labor if you need an expert to install the glow plugs. Then, you can get a glow plug from auto stores from as low as $15 up to $50. The cost varies based on the car type and the glow plug manufacturer.

Q: Will a diesel engine run without glow plugs?

You can even find some bigger diesel engines without glow plugs since they have increased volume and higher surface areas. Smaller diesel engines lose heat much quicker, and this increased temperature is needed for the diesel to compress the air.

In essence, an engine can start without the diesel glow plugs. However, you may have to deal with some time-consuming issues. Again, it may be quite challenging for your car’s starters and batteries.

Final Words

Can faulty glow plugs cause loss of power? Why should I feel concerned about faulty glow plugs? These are common questions for car owners with engines that operate with diesel. You should know that glow plugs work well in warming up the vehicle’s engine and causing an ignition.

However, bad or faulty glow plugs can decrease the heat supply, resulting in other serious problems. Nevertheless, you can look out for the symptoms of bad glow plugs and find excellent ways to fix such issues.


Hi There, I am R. Hasan Tito, a mechanic, and owner of this website. My friend and I created this website to share our knowledge, expertise, and experience with our fellow mechanics' community and car users. I am a specialist and certified automotive mechanic (Both Heavy Commercial and Private Cars). I have been working as a mechanic for over fifteen years. I worked for a long time at Global Rebound Automotive companies (Toyota, TATA, BMW, Nissan, TVs, and Others ) as a Mechanic and Mechanics Supervisor.

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