6 Symptoms of a Bad Glow Plug (and Replacement Cost)

Drive a diesel car or truck? Glow plugs preheat the engine so it starts easily in cold weather. As you can imagine, they’re pretty important, but they don’t last forever.

Let’s take a look at exactly how glow plugs differ from spark plugs, symptoms of a bad glow plug, and how much it will cost to replace it.

bad glow plugs

What Are Glow Plugs?

A glow plug works by generating heat to warm up the engine’s cylinder and ignite the fuel. Without it, your diesel engine won’t start.

Aside from warming and igniting diesel engines, glow plugs also keep your engine running smoothly. In most vehicles, you’ll find the glow plug installed within the combustion chamber, where it can efficiently draw heat to the diesel engine.

In ideal weather conditions, a faulty glow plug will struggle to generate enough heat to warm a cylinder and ignite the fuel. In frigid weather, a defective glow plug won’t work at all.

Related: Common Bad Spark Plug Symptoms

Glow Plug vs. Spark Plug

glow plug vs spark plug

While glow plugs and spark plugs perform the same function of igniting engines, they serve different purposes beyond that. Aside from the way they look, other significant differences between the two plug types include:

  • Fuel: Spark plugs work in gasoline-powered engines while you’ll find glow plugs in diesel-powered engines. In gas engines, spark plugs generate sparks to ignite the mixture of air and petrol in the combustion chamber. Glow plugs do the same thing but in diesel engines.
  • Heat Difference: Because gasoline is so volatile, all it takes is a spark to trigger its fumes. Because diesel is less volatile and a diesel engine has more intense cylinder compression, it requires higher levels of heat to ignite it. Only glow plugs can generate these levels of heat to ignite the air and diesel mixture within a diesel engine’s combustion chamber.
  • Durability: Glow plugs last longer than spark plugs. That’s because spark plugs work nonstop while a vehicle is in motion. Glow plugs, on the other hand, only work during the ignition process.
  • Ignition Process: Spark plugs use electrical energy to generate sparks that ignite the fuel and air mixture in the engine. Instead of sparks, glow plugs use electricity to heat a heating element, which creates enough prolonged heat to ignite diesel and air mixtures.

See Also: 4 Types of Spark Plugs

Bad Glow Plug Symptoms

If your diesel-powered vehicle shows at least one of the following symptoms, there’s a possibility you have one or more faulty glow plugs.

#1 – Difficulty Starting the Vehicle

car won't start

One of the most common symptoms of bad glow plugs is difficulty starting the vehicle. This is especially true in cold weather.

Glow plugs are responsible for heating the combustion chamber to the optimal temperature for efficient fuel ignition. If one or more glow plugs aren’t functioning properly, they may not generate enough heat to warm the cylinder and ignite the fuel effectively.

As a result, you may experience hard starting, requiring multiple attempts to get the engine running. In rare cases, particularly when glow plugs are near the end of their life and temperatures are freezing, the vehicle may not start at all.

#2 – Poor Acceleration

While you can start a diesel engine with a bad glow plug, you may notice a decrease in overall performance. This is particularly true when you press the accelerator pedal and the vehicle fails to respond with the normal power and acceleration you’re used to. The engine may feel sluggish, and you may struggle to reach higher speeds.

While poor acceleration can be caused by various engine issues, if you notice this symptom in conjunction with other bad glow plug signs, it’s likely that the glow plugs are the root cause.

#3 – Misfiring

Duramax diesel MPG

A backfiring exhaust can cause plenty of issues in your vehicle. It occurs when fuel fails to ignite as it should within the cylinder. Since the glow plug plays a crucial role in igniting fuel, you can assume a misfire in a diesel engine may have something to do with your glow plugs.

Engine misfires are another common sign of a faulty glow plug, and they can cause a range of problems for your vehicle. Misfires occur when the fuel in the cylinder fails to ignite properly, leading to unburned fuel being expelled through the exhaust.

With misfires, you may experience rough idling, hesitation during acceleration, and even engine stalling. If simply ignored, misfires can lead to increased emissions, reduced fuel efficiency, and potential damage to other engine components.

#4 – Dark Exhaust Smoke

black smoke from exhaust

Several factors can lead to dark gray or black exhaust smoke. If the fault lies with the combustion process, then you may have a problem with the glow plugs.

With diesel engines, dark smoke when accelerating is more common with diesels but if this symptom occurs with others on this list, it’s possible a faulty glow plug is to blame.

If you notice an unusual amount of dark gray or black smoke coming from your exhaust, it could be a sign of faulty glow plugs. Several factors can contribute to abnormal exhaust smoke, but if the issue lies within the combustion process, glow plugs may be to blame.

When the plugs don’t sufficiently heat the combustion chamber, the fuel may not burn completely, resulting in unburned fuel being expelled through the exhaust system. This can cause dark, thick smoke, especially under hard acceleration.

While some dark exhaust smoke is common in diesel engines, excessive smoke combined with other symptoms on this list strongly suggests a glow plug problem.

#5 – Check Engine Light

check engine light on

Modern diesel vehicles (1996 and newer) are equipped with sophisticated engine management systems that monitor various components, including glow plugs. If one or more glow plugs malfunction, the Check Engine light in your dash may be illuminated.

When scanned with an OBD2 scanner, you’ll get a glow-plug related error code such as P0380, which translates to “Glow Plug/Heater Circuit ‘A’ Malfunction.”

Additional glow plug related diagnostic trouble codes include: P0381, P0382, P0383, P0384, P0670, P0671, P0672, P0673, P0674, P0675, P0676, P0677, P0678, P0679, P0680, P0681, P0682, P0683, and P0684.

#6 – Reduced Fuel Efficiency

As already mentioned, faulty glow plugs can lead to incomplete combustion, which means that not all of the fuel is being burned efficiently. As a result, there may be a noticeable decrease in your vehicle’s fuel economy.

If you find yourself filling up your tank more often than usual along with other bad glow plug symptoms, it’s highly likely that your glow plugs are to blame. Make it a priority to get your glow plugs replaced or you’re literally going to be burning money (fuel).

Is It OK to Drive With a Bad Glow Plug?

Over time, glow plugs will wear out and negatively affect the performance of your vehicle. The vehicle might continue to start, and you can still drive it, but not as efficiently as when it had sound glow plugs.

As the glow plugs become weaker, starting the vehicle will take multiple tries, and reaching higher speeds will require flooring the gas pedal. If you live in colder regions, the vehicle won’t start at all, since the glow plugs can’t generate enough heat to trigger combustion.

Aside from the inconvenience of needing multiple attempts to ignite the engine and driving at reduced speeds, faulty glow plugs will compromise your vehicle’s fuel-efficiency. They will also increase the vehicle’s carbon footprint. Avoid these costs and inconvenience by replacing your plugs as soon as you notice any bad glow plug symptoms.

Glow Plug Replacement Cost

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glow plug replacement cost

Replacing glow plugs is a relatively affordable repair if just replacing one or you may approach $1k if you have to replace six hard-to-reach plugs on certain diesel engines. It all depends on your vehicle’s make and model, the quality of the replacement plugs, and whether you do the work yourself or hire a mechanic.

On average, glow plugs cost between $20 and $40 each, with higher-end name brands being slightly more expensive. Most modern diesels are 4 or 6-cylinder varieties (each cylinder has a glow plug). If you have the necessary tools and experience, you can save on labor costs by replacing the plugs yourself (the process is similar to replacing spark plugs).

However, if you’re unsure about the process, it’s best to leave the job to a professional as there’s always a chance you’ll come across a seized glow plug or even one having one break off in the head.

Labor costs for glow plug replacement is where it can get expensive, especially when all plugs are getting replaced. Considering it can take 2-6 hours to typically change a set of glow plugs at a $100 to $150 per hour labor rate, you’re looking at $200 to $900 just in labor plus the cost of the glow plugs.

But on average, expect to pay anywhere from $300 to $1,000 to have all the glow plugs in your engine replaced professionally. There may be cases where it can be under $300 or easily over $1,000.

To get an accurate estimate, call a few local mechanics or dealerships and provide your vehicle’s details. While it may be tempting to choose the cheapest option, consider the shop’s reputation and quality to make sure that the job is done correctly.

Common Myths About Glow Plugs

Unfortunately, there are several known misconceptions that can lead to confusion and improper maintenance. These mistaken beliefs can cause diesel vehicle owners to make incorrect assumptions about how glow plugs function

Myth #1 – Glow plugs need to be replaced every time the vehicle is serviced.

This is simply not true. While regular maintenance is important for your diesel engine’s overall health, glow plugs do not need to be replaced at every service interval. Glow plugs are designed to last longer than spark plugs and can typically last between 60,000 to 100,000 miles, depending on the vehicle and driving conditions.

Myth #2 – Glow plugs are only important in cold weather.

Sure, glow plugs are crucial for starting a diesel engine in cold temperatures, but they’re not just a winter component. Glow plugs are responsible for heating the combustion chamber to the optimal temperature for efficient fuel ignition, no matter what the outside temperature is.

Myth #3 – A faulty glow plug will prevent the engine from starting.

While a malfunctioning glow plug can make starting the engine more difficult, especially in cold weather, it may not necessarily prevent the engine from starting entirely. A diesel engine can still start with one or more faulty glow plugs, but it will most likely experience poor performance, greater emissions, and a high chance of misfires.

Myth #4 – Glow plugs are only found in older diesel engines.

Some people believe that modern diesel engines like Duramax, Cummins, and Powerstroke no longer use glow plugs, but this simply isn’t true. While newer diesel engines have advanced technologies like high-pressure common rail fuel injection and piezoelectric injectors, they still rely on glow plugs to help with starting and providing optimal combustion.

Myth #5 – Glow plugs and spark plugs are interchangeable.

Out of all the misconceptions listed, this one is actually dangerous. Glow plugs and spark plugs are designed for completely different types of engines and serve different purposes. Attempting to use a spark plug in a diesel engine or a glow plug in a gasoline engine can cause severe damage to the engine and should never be attempted.

Mark Stevens

3 Comments

  1. We have a yellow U.K. Vauxhall Corsa C 2005 SXi 1.3 CDTi 3dr with a full main dealer service throughout its history.
    We dropped the car off at the main dealer this afternoon ready for all four glow plugs (No.3 is faulty) to be replaced tomorrow.
    Originally the main dealer wanted £227.86 and when I mentioned a local independent service centre had quoted £156.95 (with Bosch glow plugs and not the genuine Vauxhall part) they matched it.
    I am aware of the possibility of No. 3 glow plug breaking because of the limited amount of space due to the turbo unit, we’ll find out soon enough tomorrow 😬.

    1. All was good, after we dropped it off yesterday afternoon, the dealership soaked the glow plugs with penetration fluid overnight and 9.30am this morning the car was ready for collection, there was no drama, even better, the invoice was only £148 😎.

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