How do I know when my glow plugs need replacing?

Glow plugs generally need replacement when your diesel engine won't start reliably in cold weather, starts slowly or misfires, or you see glow plug fault codes.

Glow plugs heat the air in the combustion chamber to aid ignition, especially when the engine is cold. Over time they wear out from heat cycles and can fail individually or as a group. This article walks you through common symptoms, how to diagnose the problem, replacement considerations, and maintenance tips.

Common symptoms of glow plug failure

These signs appear most often when starting in cold conditions, but some can show up any time the engine is cold or during warm-up. They help you decide whether a diagnostic check is warranted.

• Hard starting in cold weather or prolonged cranking before the engine fires.


• Rough idle or misfires during the initial warm-up phase.


• White smoke or a strong diesel smell on starting.


• Increased exhaust smoke and reduced engine performance until the engine warms.


• Warning lights such as a glow plug indicator or check-engine light, or OBD-II codes related to glow plug circuits.


• Uneven power delivery or misfires on particular cylinders if a cylinder-specific code is present.

These symptoms can overlap with other issues (fuel system, injectors, compression). A formal check is needed to confirm glow plug faults.

Diagnostics and testing

Use these steps to confirm a glow plug problem and identify faulty units. A combination of codes, visual inspection, and electrical testing provides the most reliable result.

• Check for diagnostic trouble codes with an OBD-II scanner. Look for general glow plug circuit faults (e.g., P0380) or cylinder-specific codes (e.g., P0671–P0676, depending on the engine).


• Inspect the glow plug harness and connectors for corrosion, heat damage, or loose connections.


• Visually inspect accessible glow plugs for discoloration, cracks, or glazing on the ceramic element.


• Perform an electrical resistance test on each glow plug with a multimeter. A healthy glow plug typically measures a low resistance (roughly 0.5–2 ohms depending on design); a reading that is significantly higher or an open circuit indicates a faulty plug.


• Optionally perform a current draw test or consult service documentation to compare each plug’s current draw with the manufacturer’s spec.

Concluding: If tests show multiple faulty plugs, it is common practice to replace them all on the same bank to restore uniform performance and avoid future immediate failures.

Testing glow plugs with a multimeter

Before testing, ensure the engine is cool and disconnect the electrical connector from each glow plug. Use a calibrated multimeter to measure resistance across the plug’s terminals or between the plug terminal and engine ground, following the vehicle’s service manual guidelines.

• Remove the glow plug connector and, if required, the glow plug itself to access the terminals.


• Set the multimeter to ohms and measure resistance. A healthy glow plug usually shows a low resistance (approximately 0.5–2 ohms; refer to your manual for exact specs).


• Compare readings across all plugs. One high or open-circuit reading indicates a bad plug; several high readings suggest replacing the entire set on the affected bank.

Concluding: A resistant plug indicates a faulty heating element, while consistent low readings across the plugs suggest the harness or wiring may be the issue rather than individual plugs.

Replacement considerations and procedure

When you confirm failing glow plugs, decide whether to replace a single unit or an entire bank. Replacing all on the same bank is common to ensure even heating and reliable starts.

• Replace all glow plugs on the same bank if one is failing or if the engine is older and has high mileage. This reduces repeated labor and future failures.


• Use the correct type and heat rating for your engine (coiled vs. non-coiled, OEM or equivalent). Mismatched plugs can cause performance issues.


• Install with clean, dry threads and use the manufacturer’s recommended fastener torque. Do not over-tighten to avoid thread damage in aluminum heads.


• Inspect or replace the glow plug harness/connectors if they show wear or damage. Damaged wiring can cause intermittent starts even with good plugs.


• Adhere to your vehicle’s service manual for any special procedures, such as pre-heating relay checks or unique mounting procedures.

Concluding: If you are unsure about the process or lack the proper tools, have a qualified technician handle removal and installation to avoid fuel-air mis-timing or thread damage.

Maintenance tips to extend glow plug life

Good maintenance can extend the life of glow plugs and improve starting reliability, particularly in cold weather.

• Drive on a route that warms the engine adequately; frequent short trips can prevent proper warm-up and increase wear on glow plugs and related systems.


• Keep the fuel system clean and use quality diesel fuel; contaminated fuel can affect combustion and plug performance.


• Ensure the cooling system is functioning properly; overheating can accelerate glow plug degradation.


• Address related ignition or fuel delivery issues promptly, as problems in injectors or sensors can place extra load on glow plugs.

Concluding: Regular inspections at routine service intervals (as recommended in your owner’s manual) help catch issues before they cause starting difficulties.

When to seek professional help

Glow plug work involves high temperatures and, on many engines, delicate threads in the cylinder head. If you’re unsure about diagnosing or replacing glow plugs, or if you encounter stubborn starting issues, consult a qualified technician.

• Persistent hard starting in cold weather despite a recent glow plug replacement.


• Repeated diagnostic trouble codes related to glow plug circuits that don’t clear after inspection and replacement.


• Signs of damaged wiring or a faulty glow plug relay that requires specialized testing equipment.

Concluding: A professional mechanic can confirm the fault, perform tests that may require specialized tools, and ensure proper installation to protect your engine head and electrical system.

Summary

Glow plugs are essential for reliable diesel starting in cold conditions. If your vehicle shows hard starting, rough idle, unusual exhaust smoke on startup, or glow plug-related diagnostic trouble codes, you likely need replacement. Use a combination of symptom evaluation, OBD-II codes, and direct electrical testing to confirm which plugs are faulty, then decide whether to replace a single unit or all plugs on the affected bank. Replacement is best done with attention to torque specs, wiring integrity, and seals, and professional help is advised if you’re not comfortable performing the work yourself. Regular maintenance and proper cold-weather driving habits can help extend glow plug life and keep starts smooth.

How often should glow plugs be changed?

Glow plugs should be replaced roughly every 100,000 miles, but it is more important to change them when they show signs of failure. Signs of failing glow plugs include hard starting in cold weather, rough idling, white or grey smoke from the exhaust, and a warning light on the dashboard. 
When to replace

• At a specific mileage: Replace them proactively around every 100,000 miles to prevent future problems. 
• When symptoms appear: Replace them immediately if you notice any of the following issues, as it is the most reliable indicator of a problem: 
  • Difficulty starting, especially in cold weather 
  • Engine misfiring or rough idling 
  • White or grey smoke from the exhaust 
  • Increased fuel consumption 
  • Dashboard warning light 

This video demonstrates how to remove glow plugs without breaking them and discusses the recommended replacement mileage: 1mGeorge AustersYouTube · Nov 25, 2019
Tips to extend glow plug life

• Avoid prolonged idling: Avoid letting the engine idle for too long after starting. 
• Drive in various conditions: Drive your vehicle regularly to help keep the engine running at its optimal temperature. 
• Keep RPMs up: Maintain higher engine RPMs when driving, especially during highway driving. 
• Ensure proper maintenance: Make sure the fuel system, glow plug control module, and the rest of the engine are in good condition to ensure they are working together correctly. 

How to tell if glow plugs need changing?

There are a few signs that indicate your glow plugs might need replacing:

1. The engine struggles to start when it's cold.
2. There's raw, uncombusted fuel in the exhaust soon after you start the engine.
3. Your vehicle is idling rough.
4. There are substantial amounts of smoke when you first start the engine.

Is replacing glow plugs a big job?

Cars may have become more complex over the years, yet changing your vehicle's glow plugs is, in most cases, still a job that you can handle yourself. However, before jumping into the job, you want to make sure you have a Manual Torque Wrench, and the right glow plugs for your vehicle.

How to test if your glow plugs are bad?

Symptoms of bad glow plugs include difficulty starting, especially in cold weather, rough idling, engine misfires, and a loss of power or acceleration. Other common signs are white or black smoke from the exhaust during startup and a glowing coil or check engine light on the dashboard. Decreased fuel efficiency can also occur due to incomplete combustion.
 
Common symptoms of bad glow plugs

• Hard starting: The engine cranks for a long time, especially in cold weather, before starting. 
• Engine misfires and rough idling: The engine may shake, vibrate, or sound rough, particularly after starting, because some cylinders aren't firing correctly. 
• White smoke: You may see white smoke coming from the exhaust during startup because unburnt fuel is being expelled. 
• Black smoke: This can indicate a problem with the air-fuel mixture, which can be caused by bad glow plugs. 
• Loss of power and acceleration: The engine may feel sluggish and not accelerate as quickly as it should. 
• Decreased fuel efficiency: You might notice lower miles per gallon because the engine has to work harder. 
• Dashboard warning lights: A glow plug warning light (often shaped like a coil) or the general check engine light will illuminate to signal a problem.