Testing Glow Plugs In 7 Steps

As a veteran in the diesel engine industry, particularly within the realms of heavy equipment, trucks, and trailers, I’ve come to appreciate the critical role of glow plugs in preserving reliability and efficiency. Yet, I’m acutely aware of how they can be the bane of vehicle owners when they malfunction.

Testing glow plugs is a skill I honed while working with Bay Diesel, a premier mobile diesel repair service in the Bay Area. We would often explain to our customers the importance of this process and how it could be done in seven easy steps. The procedure starts with the essentials: a clamp-meter, digital multimeters, or glow-plug testers are indispensable in accurately diagnosing issues.

Next, I would perform a preheat check to ensure the plugs adequately heat the air in the combustion chamber, a must for those cold engine starts. Any failure to do so could lead to trouble starting, accompanied by smoke from the exhaust, signaling potential glow plug problems.

Carrying out a visual inspection often reveals tell-tale signs of corrosion, overheating, or mechanical damage—a hint at metal fatigue or outright failure. Removing the plugs to test them came next, a step where delicacy was paramount to prevent unintentional damage.

The use of multimeters to check resistance, or glow-plug testers for more intricate diagnostics, could uncover a variety of problems. In moments of uncertainty, however, consulting with a seasoned mechanic could spare one from the DIY hazards. An expertly installed glow plug made all the difference in preventing dreaded cold starts.

Each of these steps reflected the easiest way I’d found to troubleshoot – a blend of methodical testing and a bit of know-how from years of hands-on experience.

How Do Glow Plugs Work?

Testing the glow plug system is a straightforward diagnostic process utilizing a multimeter to assess continuity, resistance, and voltage. This ensures that the appropriate electrical current flows through the electrical wiring to the plugs situated in the cylinder head.

 Regular checks can preempt potential failures, contributing to an engine’s seamless combustion process, critical for vehicles running on diesel, biodiesel, or operating in a demanding environment. By maintaining the integrity of the glow plugs, your engine remains robust and ready to tackle any starting challenges, particularly under harsh cold weather conditions.

What are the Symptoms of a Failing Glow Plug?

From personal experience, I can attest that starting a diesel engine on a cold morning can quickly indicate the health of your glow plugs. One of the hallmark symptoms of a failing plug is difficulty in startup, where the engine cranks longer than usual. This can often lead to uncombusted, raw fuel ending up in the exhaust system. 

Moreover, a distinct sign is a rough-running idle that smooths out as the engine reaches its warm operating temperature. You may also notice large amounts of white or black smoke during startup, which is another red flag. A diesel engine relies heavily on the glow plug’s efficiency to kick off flawlessly, irrespective of the weather conditions. These symptoms, although subtle at times, are the early whispers of your vehicle, hinting at a deeper dive into the glow plug system’s health.

How to Test Glow Plugs with a Clamp-Meter

From personal experience, the best tool for testing glow plugs efficiently and with great ease of use, is a DC clamp meter. This versatile instrument allows you to test the complete system at once, providing a quick and safe diagnosis without the risk of damage to the glow plugs. 

By simply clamping the meter around the glow plug power wire at the top, one can measure the DC current flowing through it when the engine is turned on. The clamp-meter closes around the wire, ensuring no direct contact and thus, a safe and secure testing environment.

As the engine stabilizes in its cold state, clamping the meter and watching the current display can tell you a lot about the energy consumption of the glow plugs. If the system exhibits a consistent pulse of power, it indicates that the plugs are effectively raising the temperature within the engine’s combustion chambers. 

These clamps can pick up on the slightest electrical parameters that a multimeter test might miss, helping to narrow down on a single test—the resistance in the connections—in a few simple steps. It’s particularly handy when tackling potential engine problems that may not be immediately apparent, thereby ensuring your engine’s readiness in cold starts.

The Process Of Testing Glow Plugs

Embarking on the process of testing glow plugs is akin to ensuring that a complex mechanism runs safely and smoothly. You wouldn’t dive off a cliff without a parachute, would you? Similarly, to avoid plummeting into the depths of engine troubles, a meticulous seven-step plan is required. We must follow this plan to uphold the integrity and performance of our vehicle’s diesel engine.

Diagnostic inspection

Let the journey begin with a thorough check-up of the engine. Just as a seasoned doctor reads symptoms before a diagnosis, a diagnostic inspection of the vehicle can reveal much about the health of the glow plugs. If the check engine light comes on, it’s like your car is crying for help. Grab the owner’s manual, identify the diagnostic trouble code, and look for signs of damage or dirty electrical wiring. It’s like detective work, but under the hood of your trusted steed.

Use an infrared thermometer

Now, imagine you’re a chef. Just like checking your oven’s temperature, you start with an infrared thermometer to gauge if your engine’s getting too feverish. After revving the engine a few times, give those glow plugs a good scan. If a plug reads a temperature more strangely high or low than its comrades, there’s a chance it’s not working right. But don’t be fooled by false positives—the trusty multimeter is the main tool for testing here.

Prepare your multimeter

Armed with your multimeter, it’s almost like preparing for battle. Adjust those leads properly to read the electrical current levels. A digital multimeter set to ohms tells you the resistance value. This ritual ensures your readings are as accurate as a marksman’s shot. We’re seeking the positive and negative signs of life within each unit of our glow plugs.

Test battery voltage

With your multimeter at the ready, it’s time to check the lifeblood of your vehicle—the car battery. Set it to read volts, take the positive lead, and introduce it to the positive terminal; the negative lead plays coy with the negative terminal. You’re like an energy maestro, ensuring the process yields a bold number around 13 volts; anything substantially less, and it’s a sign of a sorrowful battery unready to drive the glow plugs and your vehicle into motion.

Remove glow plug connectors

The glow plugs eagerly await beneath their connectors. Just like a gardener pruning plants, refer to your vehicle’s manual to meticulously remove the caps. They can accumulate grime, and just like testing one’s patience, testing each plug one at a time is advised. Doing all at the same time? That’s a bad idea. But, if you want to go ahead anyway, nobody’s stopping you.

Connect the multimeter to your glow plugs

Now set your multimeter to ohms and take the red positive lead to meet the plug, while the black negative lead should find solace in a good grounding source. The owner’s manual might suggest a nice bolt on the engine wall or the engine block as a grounding point. Some adventurous souls might choose the battery’s negative terminal as their dance floor.

Evaluate the reading when testing glow plugs

Different vehicles, depending on their size, model, and type, will whisper to you the ideal glow plug ratings. If in doubt, the trustworthy manual helps you find the true resistance value for each plug. A perfect reading measures a value in ohms that corresponds with the manual’s wisdom. If the test reveals an amount that could ruin a good plug, or if it simply reads ‘okay’ but looks like it’s seen better days, it might be time to bid it farewell and replace it.

How to Test Glow Plugs with a Digital Multimeter

When I faced starting issues with my change truck old diesel workhorse on chilly mornings, it didn’t take long to suspect the glow plugs. Using a Digital Multimeter (DMM), I decided to tackle the problem head-on. First things first, an accurate test lead is vital in ensuring efficient diagnostics. I connected the DMM in Ω mode to measure electrical resistance, which is critical in assessing the health of the glow plug.


In conclusion, mastering the art of testing glow plugs is not just a technical skill; it’s a testament to the dedication required to keep diesel engines running smoothly. Through the seven-step process outlined here, one can navigate the complexities of diagnosing glow plug issues with confidence and precision. 

From conducting thorough visual inspections to employing specialized tools like clamp-meters and digital multimeters, each step serves as a crucial checkpoint in ensuring the reliability and efficiency of diesel engines, particularly in the face of cold weather challenges. 

By understanding the symptoms of failing glow plugs and employing the appropriate testing methods, vehicle owners can proactively address potential issues before they escalate, thereby preserving the integrity of their engines and maintaining optimal performance over time.


What is the working principle of glow plug?

The glow plug works by electrifying the heating element so it heats up and emits visible light (hence the name).

How do you know glow plugs are bad?

In this article, we look at six common signs that indicate glow plugs could be faulty or damaged.

  • Something’s wrong: the engine warning light
  • Hard starting
  • Engine misfiring
  • Rough idling
  • Decreased fuel efficiency
  • White smoke
  • Black smoke

Can you test a glow plug with a battery?

Disconnect the glow plugs. Attach a test light to the positive battery terminal of the battery. Touch the glow plug terminal. If the test light lights up, it’s good.

How do glow plugs fail?

Although designed to operate between 850 to 1,100 degrees centigrade, when the electrical current to the glow plug is held too long, because of a faulty relay and/or controller, it can lead to overheating, which will cause the glow plug to swell, split or burst and result in immediate failure.

What is the glow plug symbol?

Look for that yellow or orange coil light on the dash, which quite literally looks like a double looped coil. That’s the glow plug symbol, which transforms into a glow plug warning light if: It takes the engine a long time to heat up and the light stays on for longer than 10 – 15 seconds.

What is the temperature of a glow plug?

A typical glow plug will have a tip temperature of just under 1,900 degrees F. The liquid fuel (atomized by the nozzle/injector) can then combust, and the engine starts when it comes in contact.