Glow Plug Light: What Is It and Can You Drive With It On?

The symbols on your vehicle’s dashboard light up to tell you the status of different parts and systems.

This includes the coil light – also known as a glow plug light.

Why is your glow plug light so important? What should you do if it’s on?    

Keep reading to find out more about glow plug lights and how they affect your driving performance!

Key Summary Points

• Glow plugs are essential for diesel engines, especially in cold temperatures, as they heat the air and fuel mixture in the combustion chamber to facilitate engine starting. They work by raising the internal temperature of the cylinders, which pressurizes and detonates the diesel fuel.
• The glow plug light symbol on the dashboard, lights up during the startup of a diesel engine to indicate that the glow plugs are in operation. This light should turn off once the engine is sufficiently warmed up, which can take between five seconds and one minute depending on the vehicle and ambient temperature.
• A blinking glow plug light indicates a malfunction within the glow plug system, which could range from issues with the glow plugs themselves to problems with the timer regulating their operation. Symptoms of faulty glow plugs include loss of acceleration, overall power reduction, decreased fuel economy and engine misfiring.
• It is generally safe to drive with a solid glow plug light, as it indicates the glow plugs are warming up the engine for startup. However, if the glow plug light is flashing, it suggests a malfunction, and while driving is possible, it's recommended to do so cautiously and at reduced speeds until the issue can be diagnosed and resolved.
• Glow plugs should be replaced every 100,000 miles as part of diesel preventive maintenance. It's also advisable to replace all glow plugs at once if one is found to be faulty. Diagnosing and fixing glow plug issues can be done with the help of a multimeter and an experienced diesel technician.

What Is a Glow Plug?

Sometimes, diesel engines need help starting in cold temperatures. That’s where glow plugs come in. Glow plugs are heating elements that heat the air and fuel mixture present in the combustion chamber to help your engine start efficiently.

The glow plugs also raise the cylinders’ internal temperature during the process, causing the diesel fuel to pressurize and detonate. Typically, this requires a driver to wait a short period of time before the engine is fully warmed up and ready to start.

Read: Diesel Terminology Explained

What does a glow plug do?

When you turn the ignition, the glow plug light turns on. This means that the glow plug system is heating the air and fuel mixture inside the combustion chamber, preparing your engine to come to life.

Once the engine reaches a warm enough temperature, the light should switch off – meaning that your vehicle is ready to go. However, if the glow plug light stays on longer than expected or starts to blink, it can mean that something’s wrong with your glow plugs or the system regulating them.

Ignoring these warning signs might mean finding yourself stranded, so treat your dashboard lights kindly and take action when needed!

Read: Diesel Careers

H2: Illuminated Glow Plug Light and Cold Starts

As covered in our gasoline vs. diesel engine post, diesel engines do not use spark plugs. They rely on the heat generated by very high compression to ignite the air/fuel mixture in the cylinders.

When diesel engines are cold, they can be difficult to start. To help overcome this challenge, diesel engines use glow plugs during startup to warm the air in the cylinder, which causes the coil light symbol on the dashboard to light up. Once the engine is running, the glow plug no longer plays a role in the combustion process.

The total warmup time needed varies depending on the vehicle and the ambient temperature. Usually, glow plugs take between five seconds and one minute to sufficiently heat the engine. At that point, the dashboard coil light symbol should turn off, and the driver should be able to start the vehicle.

In addition to glow plugs, diesel engines may also use other starting aids, including:

• Coolant heater
• Ether injection kit
• Oil pan heater
• Block heater
• Air intake heater

Read: What Skills Can I Learn at Diesel Mechanic School?

What Does a Blinking Glow Plug Light Symbol Mean?

A blinking glow plug light or coil light symbol on the dashboard indicates an issue. While there isn’t a quick fix to determine the specific problem, a skilled diesel technician should be able to diagnose the problem.

Issues with the diesel glow plug itself, including a loss of heating ability, can cause a blinking glow plug light. There could also be an issue with the timer, which is what regulates how long the glow plugs operate. If the timer is off, it can cause the glow plugs to wear out.

Symptoms of faulty glow plugs include a loss of acceleration or a loss of overall power in the engine. You might also notice a drop in fuel economy or even engine misfiring. Be sure and take your car for diagnostic testing if a blinking coil light comes on, as a trained technician should be able to identify and fix the issue.

Read: Becoming a Diesel Fleet Mechanic

Signs of malfunctioning glow plugs

Identifying the signs of malfunctioning glow plugs is important for maintaining optimal engine performance and ensuring reliable starts, especially in colder conditions. Here are the key technical indicators to monitor:

• Extended crank time: If the engine requires significantly more cranking time to start combustion, particularly in low-temperature environments, this may indicate compromised glow plug functionality.
• Acceleration deficits: A significant increase in throttle response time or a lack of acceleration could signal that the glow plugs are failing to heat the air-fuel mixture adequately, leading to incomplete combustion and power lags.
• Power loss: A persistent decline in power output, especially noticeable during acceleration or when climbing inclines, may suggest that one or more glow plugs are not reaching the required thermal operation level.
• Reduced fuel efficiency: An uptick in fuel consumption is often a symptom of subpar combustion efficiency, which can be attributed to faulty glow plugs – resulting in lower energy conversion from the fuel.
• Engine misfire events: If the engine exhibits irregular firing patterns or misfires during operation, this could be tied to ineffective heat generation from the glow plugs, interfering with the overall combustion cycle.

Addressing these symptoms promptly is essential to prevent further complications and ensure your diesel engine runs smoothly.

Read: Everything You Need to Know about a Diesel Technology Degree

Glow Plug Troubleshooting Tips

Here are ways to assist you in diagnosing potential glow plug light issues before consulting a mechanic:

• Check the dashboard lights: A steady glow plug light signifies that your glow plugs are in the warming phase. However, if this light remains lit up after starting the engine, further investigation is warranted.
• Observe the engine performance: A noticeable decrease in acceleration or the presence of engine misfires may indicate that your glow plugs require attention. Be mindful of any unusual sounds. Your engine should not sound as if it is struggling!
• Examine the battery and starter: If you find yourself repeatedly attempting to start the engine, ensure that your battery is in optimal condition. A depleted battery can worsen glow plug-related issues.
• Conduct a multimeter test: Use a multimeter to test the functionality of the glow plugs, ensuring they are heating as intended. Should you discover one that is underperforming, consider replacing it.
• Look for signs of wear: Inspect the glow plugs and their wiring for any physical damage or corrosion. If you see any signs of deterioration, you might need to replace them.
• Seek professional assistance: If you feel unsure about diagnosing the problem, do not hesitate to reach out to a qualified diesel technician for an accurate assessment. Or become a professional mechanic yourself.

Stick to these guidelines and you’ll be well-prepared to address glow plug indicator light issues and maintain your vehicle's performance.    

Read: 3 Ways UTI’s Diesel Mechanic Trade School Can Prepare You for a Career

Can You Drive When the Glow Plug Light is Flashing?

If your glow plug light has come on and you’re wondering if you should drive your vehicle, the answer depends on whether the light is blinking or not. If the glow plug light is solid, most cars can’t fully start until the light has turned off and the engine is fully warmed up.

If you need to keep driving but the glow plug light is flashing, take it easy and avoid high speeds as much as possible. Many times, an engine will go into “safety mode,” which could result in a drop in performance.

The purpose of a blinking glow plug light is to alert you as a driver to a potential problem, so it’s important to take it seriously and get it checked out when you can. This also goes for a glow light that comes on and doesn’t turn off, even once your engine has fully warmed up.

Read: Why UTI Avondale Grad Brandon Davis Believes His Education Was Important

Glow Plug Light FAQs

1. Can a diesel engine run without a functioning glow plug?

Glow plugs help initiate engine startup, so as long as you can get your engine to start, the vehicle is drivable. However, continuing to start the engine with a bad glow plug will place stress on other components like the battery and starter, creating the potential for additional issues.

2. How often do glow plugs need replacing?

The climate you live in and how often you drive can affect how often you need to replace glow plugs. As a general rule of thumb, replace glow plugs every 100,000 miles as part of diesel preventive maintenance.

3. Should I replace all my glow plugs at the same time?

Even if just one glow plug is faulty, it’s best to replace them all at once. Doing so will save you the trouble of replacing one every few months.

4. How can I diagnose and fix a glow plug issue?

If your glow plug dashboard light is blinking, you can test which glow plug is bad with a multimeter. However, as previously mentioned, it’s best to replace them all at once. An experienced diesel technician can help you do this – or you can help yourself as a trained pro with industry training.

5. Are glow plugs universal?

No, glow plugs fall into three categories: hot, medium or cold. Fuel type, engine type, engine size and acceleration will determine which is best. The vehicle manufacturer or a diesel technician can help you choose the right kind.

Read: Train for a Career That’s In-Demand: Advice From a Diesel Employer

Diagnose Glow Plug Light Issues With Diesel Training From UTI!

You can learn the ins and outs of diesel systems, including glow plug light systems, and diagnose problems after receiving training from Universal Technical Institute. The 45-week Diesel Technology program helps prepare students with foundational knowledge they can apply to a career in the industry.^1,7

The course covers engine repair and operation. Electrical diagnostics are also covered, giving you the knowledge you need for glow plug light inspection and maintenance.

Expand your knowledge and prepare for a career doing something you love. Find out more about the program by requesting more information online or by calling 1-800-834-7308 to contact an Admissions Representative.

UTI Campuses That Offer Diesel Mechanic Training

• Avondale, Arizona
• Long Beach, California
• Rancho Cucamonga, California
• Sacramento, California
• Miramar, Florida
• Orlando, Florida
• Lisle, Illinois
• Bloomfield, New Jersey
• Exton, Pennsylvania
• Austin, Texas
• Dallas/Fort Worth, Texas
• Houston, Texas