Symptoms of Bad Glow Plug – Potential Causes | Diagnosing | Fixes | Precautions

As potential car buyers exploring the world of diesel vehicles, there’s a vital component you should be well-acquainted with – the glow plugs. These inconspicuous yet indispensable elements play a crucial role in the starting and combustion process of diesel engines. In this guide, we’ll shed light on the symptoms that indicate bad glow plugs, their potential causes, methods for diagnosis, effective fixes, and precautions to ensure a smooth journey with your diesel-powered vehicle.


Symptoms of Bad Glow Plugs:

bad glow plug

As potential car buyers, understanding the inner workings of a diesel engine is essential. One crucial component you should be well-versed in is the glow plug. These seemingly insignificant devices play a pivotal role in the starting and combustion process of diesel engines. To ensure a smooth ride with your diesel vehicle, it’s crucial to recognize the symptoms that indicate the health of your glow plugs.

1. Extended Cranking Time

Imagine turning the key in the ignition, and your engine takes longer than usual to roar to life. If you find yourself experiencing extended cranking times before the engine finally starts, it could be a clear sign of bad glow plugs. These plugs are responsible for heating the combustion chamber in cold conditions, aiding in the initial ignition.

2. Engine Misfires

Bad glow plugs can lead to misfires, resulting in a variety of noticeable issues. You might experience a rough idle, poor performance, or an engine that just doesn’t seem to be firing on all cylinders. These misfires are often attributed to improper combustion caused by faulty glow plugs.

3. Excessive Smoke

Picture thick plumes of smoke billowing from your exhaust, especially during startup. Bad glow plugs can disrupt the combustion process, causing excessive smoke due to unburned fuel. This not only affects the environment but also indicates a problem that needs attention.

4. Glow Plug Warning Light

bad glow plug

Modern diesel vehicles are equipped with sophisticated onboard diagnostics. If you see the glow plug warning light illuminated on your dashboard, it’s a clear indicator that something is amiss with these vital components. Pay close attention to warning lights, as they can provide early indications of potential issues.

5. Cold Weather Starting Difficulty

Hard Steering Wheel

Diesel engines, especially older ones, often face challenges starting in cold weather. Bad glow plugs exacerbate this issue, making cold starts a strenuous affair. Difficulty starting in colder temperatures is a strong indication that your glow plugs may be failing or already failed.

6. Decreased Fuel Efficiency

If you notice that your diesel vehicle’s fuel efficiency has taken a nosedive, it could be due to bad glow plugs. These plugs play a critical role in the combustion process, and when they’re not functioning correctly, the engine may burn more fuel than necessary.

Recognizing these symptoms of bad glow plugs is the first step in ensuring the reliability and performance of your diesel engine. When considering a diesel vehicle, it’s essential to be vigilant and informed to make the best choice for your driving needs.


Potential Causes of Bad Glow Plugs:


When it comes to the intricate world of diesel engines, understanding the potential causes behind bad glow plugs is essential. Glow plugs are the unsung heroes responsible for heating the combustion chamber, ensuring a smooth start in cold conditions. However, like all components, they can sometimes fail. Let’s delve into the possible culprits that can lead to the demise of these critical engine elements.

1. Normal Wear and Tear

One of the most common causes of bad glow plugs is normal wear and tear. Just like any part of your vehicle, glow plugs have a finite lifespan. Over time and extensive use, they can wear out and lose their effectiveness. High-mileage vehicles are particularly susceptible to this issue.

2. Carbon Buildup

Carbon deposits can accumulate on the surface of glow plugs over time, negatively impacting their performance. These deposits can insulate the plugs, preventing them from reaching the necessary temperature for proper ignition. Carbon buildup can be exacerbated by poor-quality diesel fuel or irregular maintenance.

3. Faulty Wiring

bad glow plug

The electrical components that power the glow plugs are equally important. Faulty or damaged wiring can disrupt the electrical flow to the glow plugs, rendering them ineffective. Corroded or broken connections can lead to inconsistent heating and starting problems.

4. Temperature Sensor Malfunction

Temperature Sensor

Diesel engines rely on temperature sensors to determine when to activate the glow plugs. If these sensors malfunction or provide inaccurate readings, it can lead to starting problems. A malfunctioning sensor may fail to activate the glow plugs when they are needed, particularly in cold weather.

5. Poor Maintenance

Regular maintenance is crucial for the longevity and performance of glow plugs. Failure to follow the manufacturer’s recommended maintenance schedule can lead to premature glow plug failure. Routine inspections and replacements as needed should be a part of any diesel engine owner’s maintenance routine.


Diagnosing Bad Glow Plugs:

As a conscientious diesel engine owner, being able to diagnose the health of your glow plugs is essential for maintaining smooth starts and optimal performance. When these critical components start to falter, it can lead to a host of issues. Thankfully, there are methods to diagnose bad glow plugs, ensuring you catch and address any problems early on.

Visual Inspection

One of the first steps in diagnosing bad glow plugs is a visual inspection. Start by locating the glow plugs within your engine. They typically resemble small metal rods with wires connected to them. Look for visible signs of damage or wear, such as corrosion or broken connections. If you notice any irregularities, it’s a good indicator that your glow plugs may need attention.

Using a Multimeter


A multimeter is a valuable tool for testing the resistance of glow plugs. Here’s how you can use it for diagnosis:

  1. Disconnect the glow plug wires: Begin by disconnecting the wires from the glow plugs. Make sure the engine is cool before attempting this.
  2. Set the multimeter: Set your multimeter to measure resistance (ohms).
  3. Test each glow plug: Touch the multimeter probes to the terminals of each glow plug. A working glow plug should have low resistance, typically around 0.6 to 2.0 ohms, depending on the manufacturer’s specifications.
  4. Compare readings: Compare the resistance readings of each glow plug. If one or more glow plugs show significantly higher resistance or an open circuit (infinite resistance), they are likely faulty and should be replaced.

Consulting a Mechanic

For a precise diagnosis, especially if symptoms are unclear or if you lack experience with electrical testing, it’s wise to seek the expertise of a qualified mechanic or technician. They have specialized knowledge and equipment to diagnose glow plug problems accurately. They can perform more advanced tests, such as checking the current draw of each glow plug during operation, to confirm their condition.



When conducting diagnostic tests on glow plugs, exercise caution. Ensure the engine is cool before touching any components. Be mindful of electrical connections, and disconnect the battery to avoid accidental electrical shocks.


Fixes for Bad Glow Plugs:


When your diesel engine’s glow plugs start to show signs of wear or malfunction, it’s essential to address the issue promptly to ensure reliable starts and optimal engine performance. Fortunately, there are several effective fixes for bad glow plugs, ranging from simple maintenance tasks to component replacements. Let’s explore these illuminating solutions:

1. Replacing Glow Plugs

In many cases, the most effective solution for bad glow plugs is to replace them with new ones. Glow plugs have a limited lifespan, and if they’ve worn out or become damaged, new plugs are the best way to restore proper function. Consult your vehicle’s service manual or a mechanic for the correct replacement parts and specifications.

DIY Tip: If you’re a skilled DIYer with the right tools, you may be able to replace glow plugs yourself. However, if you’re unsure or inexperienced, it’s advisable to seek professional assistance to avoid damaging other engine components.

2. Cleaning Carbon Deposits

If carbon deposits have accumulated on the surface of the glow plugs, cleaning them can often improve their performance. To do this:

  1. Remove the glow plugs: Carefully remove the glow plugs from the engine using the appropriate tools.
  2. Clean the plugs: Use a suitable cleaning agent or a wire brush to gently remove the carbon deposits from the tips of the glow plugs.
  3. Inspect and reinstall: After cleaning, inspect the plugs for any visible damage. If they appear to be in good condition, reinstall them in the engine.

3. Repairing Wiring

Damaged or corroded wiring can disrupt the electrical flow to the glow plugs, rendering them ineffective. If you discover faulty wiring, repairing or replacing it is essential. Make sure to use high-quality wiring and connectors to ensure a reliable electrical connection.

4. Addressing Sensor Issues

If the temperature sensors responsible for activating the glow plugs are malfunctioning, it’s crucial to address and repair or replace them. Malfunctioning sensors can lead to starting problems, especially in cold weather.

5. Regular Maintenance

Preventing glow plug issues is often a matter of routine maintenance. Follow the manufacturer’s recommended maintenance schedule, which typically includes regular inspections and replacements of glow plugs as needed. By staying on top of maintenance, you can prolong the life of your glow plugs and ensure they function optimally.

Precautions for Diesel Engine Owners:

Owning a diesel engine is an exciting experience, but it comes with a responsibility to ensure its smooth operation and longevity. Glow plugs, essential for cold starts and efficient combustion, are an integral part of your diesel engine. To keep your diesel engine in top shape and prevent issues with glow plugs, consider these important precautions:

1. Use Quality Fuel

Quality diesel fuel is the lifeblood of your engine. Opt for reputable fuel sources that provide clean and well-filtered diesel. Low-quality or contaminated fuel can lead to carbon buildup on your glow plugs, affecting their performance. Investing in high-quality fuel ensures that your engine runs efficiently and reduces the risk of premature glow plug failure.

2. Follow Maintenance Schedule

Regular maintenance is the key to a healthy diesel engine. Follow the manufacturer’s recommended maintenance schedule diligently. This typically includes inspections and replacements of glow plugs at specified intervals. By staying on top of maintenance tasks, you can catch potential issues early and ensure that your glow plugs function optimally.

3. Keep Fuel System Clean

A clean fuel system is essential for preventing glow plug issues. Ensure that your fuel tank and fuel filters are regularly inspected and cleaned or replaced as needed. Clean fuel and well-maintained filters contribute to extended glow plug life and prevent contaminants from reaching critical engine components.

4. Cold Weather Precautions

Diesel engines can face challenges starting in cold weather, and glow plugs play a crucial role in this process. Take precautions during winter months:

  • Use block heaters: Block heaters can warm the engine block and aid in cold starts. Consider using one, especially in extreme cold.
  • Follow manufacturer-recommended cold-weather starting procedures: Consult your owner’s manual for guidance on starting your diesel engine in cold weather. Some engines have specific procedures to ensure successful starts.
  • Use additives: Diesel fuel additives designed for cold weather can help prevent fuel gelling and improve cold-weather performance.

5. Consult Experts

When you suspect glow plug-related issues or face starting problems, don’t hesitate to consult experts. Qualified mechanics or technicians have the knowledge and experience to diagnose and address glow plug problems accurately. Seeking professional assistance can prevent further damage to your engine and ensure effective repairs or replacements.



In the world of diesel engines, glow plugs are unsung heroes that play a vital role in starting and combustion. As potential car buyers, being aware of the symptoms, causes, diagnosis methods, fixes, and precautions related to bad glow plugs is essential. These insights will empower you to make informed decisions, ensure your diesel vehicle runs reliably, and embark on a smooth journey ahead.

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