Does my diesel have glow plugs?

The vast majority of diesel engines built in the last several decades use glow plugs to aid cold-starts. Most modern diesel cars and light trucks have them, but you can confirm by checking the dash preheat indicator, the owner’s manual, or inspecting the engine bay for the glow plug hardware.

Understanding glow plugs and why they matter

Glow plugs are electric heaters located in the cylinder head. When you turn the key to the “on” position, they heat the air in the combustion chamber to help diesel fuel ignite reliably, especially in cold weather. They are standard on most modern diesel engines, including many passenger cars and light trucks. Some engines may use alternative preheating methods or incorporate a separate block heater for extreme cold, but glow plugs remain the primary in-cylinder aid in most configurations.

How to tell if your diesel has glow plugs

There are several practical signs to verify whether your vehicle uses glow plugs. Use these checks as a quick guide before calling a mechanic.

• Dash indicator: A glow-plug or preheater symbol typically lights up when the ignition is turned on and goes out once the plugs reach operating temperature. In many vehicles you’ll hear a soft click or feel a brief vibration during preheating.


• Owner’s manual: The manual will state whether the engine uses glow plugs and describe the preheating procedure.


• Engine bay appearance: Glow plugs are small cylindrical heaters threaded into the cylinder head with wiring harness connections. You’ll generally see one unit per cylinder (for a 4-, 6-, or 8-cylinder engine).


• Diagnostic codes: If the glow-plug circuit has a fault, an OBD-II scanner may report codes such as a glow plug circuit malfunction.

Note that some modern diesel designs may use a different preheating approach or integrate the heating function with the engine control system. When in doubt, check the official manual or ask a technician.

Where glow plugs are located and what they look like

Knowing where to look can save time during maintenance or replacement. Glow plugs are installed in the cylinder head, with wiring harnesses running to each plug. The number of plugs matches the engine’s cylinder count.

• Location: In most inline-4, inline-6, and V-series diesel engines, glow plugs sit in or near the cylinder head, often toward the top of each cylinder’s combustion chamber.


• Appearance: Each glow plug resembles a small threaded rod with a ceramic or insulated tip that heats when powered.


• Access: Some designs place plugs beneath intake manifolds or other components, which can require partial disassembly to service. In other engines, plugs are more readily accessible from the top of the head.


• Block heater alternative: In very cold climates, some vehicles also use an external block heater (a cord that plugs into the engine block). This is separate from glow plugs but serves a similar purpose for easier starting.

If you’re planning maintenance or replacement, consult your service manual for exact access points and torque specs, or have a qualified technician perform the work.

Block heaters vs glow plugs: when you need both

Block heaters and glow plugs can coexist in some systems, particularly in regions with extreme cold. Understanding their roles helps with winter preparations and diagnostic work.

• Glow plugs: In-cylinder heating that aids ignition during starting, active for a brief period at startup.


• Block heater: External heating element embedded in the engine block or oil pan; plugged into mains to warm the engine before starting, reducing cold-start stress.


• Usage: Glow plugs are common in most diesel engines; block heaters are common in very cold climates and may be used in addition to glow plugs.

Remember that a vehicle can have glow plugs without a block heater, or both, depending on climate and design.

What to do if you’re having starting problems

If your diesel is hard to start in cold weather or you suspect a glow-plug problem, follow these steps to diagnose and address common issues.

• Check the dash preheat indicator and wait for it to cycle before attempting to start. If it doesn’t illuminate or stays on too long, the glow-plug system may be faulty.


• Inspect fuses, wiring, and connectors to the glow-plug harness for corrosion or breaks.


• Use an OBD-II scanner to check for glow-plug circuit codes if your vehicle supports it.


• If a plug or circuit is suspected faulty, have a mechanic test the resistance of each glow plug and verify the harness and ECU controls. Replacing failing plugs is usually straightforward but may require specialized tools for access.

In very cold environments, a functional block heater can also improve starting; follow manufacturer guidelines for usage and safety when plugging in outside.

Summary

Most diesel engines, especially in modern passenger cars and light trucks, use glow plugs to facilitate cold starts. You can confirm via the dash preheat indicator, the owner’s manual, or by inspecting the engine bay for the glow-plug hardware. If starting remains difficult in cold weather or you see related warning codes, have the system checked by a technician to determine if a plug, harness, or control issue is at fault. Also consider external block heaters in regions with severe winter temperatures, as these devices operate independently of the glow-plug system but serve a similar purpose.

What are the symptoms of failing glow plugs?

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. The engine warning light on the dashboard is typically the initial sign of a problem.
• Hard Starting.
• Engine Misfiring.
• Rough Idling.
• Decreased Fuel Effeciency.
• White Smoke.
• Black Smoke.

How often do diesel glow plugs need replacing?

You should change diesel glow plugs every 80,000 to 100,000 miles, or when they show signs of wear like hard starting or rough idling. While they can last this long, factors like frequent short trips, extreme cold weather, and poor quality replacements can shorten their lifespan. It's best to have them checked during routine maintenance to avoid future problems. 
When to replace glow plugs

• Mileage: A good rule of thumb is to replace them proactively around the 80,000 to 100,000-mile mark. 
• Symptoms: Replace them immediately if you experience issues such as: 
  • Difficulty starting the engine 
  • Engine misfiring or rough idling 
  • Visible smoke (white or black) when starting 
  • Decreased fuel economy 
  • A dashboard warning light related to the glow plug system 

Factors that affect glow plug lifespan

• Climate: Frequent use in cold weather can cause them to wear out faster. 
• Driving habits: Short trips that don't allow the engine to reach its full operating temperature can lead to premature wear. 
• Quality: Using lower-quality replacement glow plugs can reduce their lifespan. 
• Maintenance: Regular inspection during routine maintenance is a good way to catch problems early. 

Do all diesel engines have glow plugs?

No, not all diesel engines have glow plugs, as some engines use other heating methods like a grid heater or rely solely on high compression ratios to start, especially direct-injection engines. Glow plugs are primarily a starting aid, pre-heating the air inside the combustion chamber to help start the engine in cold weather. 
This video explains what glow plugs are and how they work: 55sSimmo's Home WorkshopYouTube · Apr 29, 2025
Engines that may not have glow plugs

• Engines with high compression ratios: Many modern diesel engines can start without glow plugs because their high compression generates enough heat to ignite the fuel, particularly direct-injection engines. 
• Engines with grid heaters: Some engines, particularly in colder climates, use a grid heater mounted in the intake system to preheat the air as it enters the cylinders, instead of using glow plugs. 
• Older engines: Some older diesel engines, particularly two-stroke models, did not use glow plugs and relied on other methods or simply wouldn't start in very cold conditions. 

What glow plugs do

• Glow plugs are used to pre-heat the combustion chamber to ensure efficient starting in cold weather. 
• When a diesel engine is cold, the air inside the cylinders may not get hot enough through compression alone for the fuel to ignite properly. 
• Glow plugs heat up before and during the starting process, providing the necessary heat for combustion. 
• Some advanced "smart" glow plugs can sense and measure pressure inside the cylinder to provide the correct amount of heat. 

Do 6.7 Cummins use glow plugs?

Yes, the 2025 model year and newer 6.7L Cummins engines have glow plugs, while previous models (2007-2024) used a grid heater system instead. The switch to glow plugs for the 2025 model year is a change for cold-weather starting. 
6.7L Cummins with glow plugs 

• Model years: 2025 and newer
• Function: Replaced the grid heater system for cold starts 

6.7L Cummins with grid heater 

• Model years: 2007 through 2024 
• Function: Used an intake heater (grid heater) for cold starts, a change from glow plugs