What happens when a glow plug goes bad?

A glow plug going bad typically makes cold starts harder, can cause rough idle or misfires, and may trigger warning codes; in warm weather the issue can be less noticeable. In short, starting reliability and smooth operation are the primary casualties of a faulty glow plug.

Glow plugs heat the air inside diesel engine cylinders to help ignite the fuel-air mixture on startup. When one or more plugs fail, the preheating process is compromised, which cascades into starting difficulties, running irregularities, and potentially higher emissions. The impact varies by engine design and how many plugs are affected.

What glow plugs do and how they fail

Diesel engines rely on glow plugs to preheat the combustion chamber before ignition, easing starts in cold conditions. Over time, plugs can degrade from heat cycling, carbon buildup, and electrical wiring faults, leading to weak heating, intermittent operation, or an open circuit that prevents heat from reaching the cylinder.

Common symptoms of a failing glow plug

Symptoms cluster around starting and running performance. The following list highlights the most frequent indicators:

• Hard starting or extended cranking when the engine is cold


• Rough idle or misfires on one or more cylinders


• White or blue smoke on startup that clears after the engine warms up


• Increased fuel consumption or reduced power until the engine reaches operating temperature


• Engine warning light or glow plug warning codes in the vehicle’s computer


• Engine cranks longer than usual to start

Note: In some vehicles, a single failing glow plug may cause discernible symptoms in that cylinder, while others may show broader starting or idle issues depending on the design and load on the engine.

Diagnosing a bad glow plug

Reliable diagnosis generally requires a combination of visual checks and electrical tests. Technicians may use a diagnostic scan tool and resistance measurements to verify each plug’s condition. In many diesel applications, ECU fault codes related to the glow plug circuit are logged when a problem is detected, with codes such as P0380–P0384 indicating glow plug circuit faults depending on the make and model.

To diagnose, technicians typically perform these steps:

• Check for fault codes related to glow plug circuits in the on-board computer


• Visually inspect glow plugs and wiring for scorching, corrosion, or loose connections


• Measure the electrical resistance of each glow plug with a multimeter to identify open circuits or out-of-range readings


• Perform a relative compression test or cylinder drop test if starting symptoms point to a misfire in a specific cylinder


• Test the power supply and control circuitry (glow plug relay or ECU control) to rule out a faulty driver circuit

Experts note that while a single defective plug can cause localized issues, many technicians recommend replacing all plugs on the affected bank to ensure balanced heating and reliable startup after a failure.

If you suspect a glow plug problem, prompt diagnosis is important. Working on diesel glow plug systems involves high electrical resistance and hot components; seek a qualified technician to avoid burns or engine damage.

Replacement and maintenance considerations

When a glow plug fails, replacement is the standard remedy. Parts are not expensive individually, but labor can be significant because plugs are often located in tight, hot areas of the engine. Replacing plugs on the affected bank—and sometimes the entire set—helps ensure even heat distribution and prevents repeat failures.

Below are practical considerations that commonly guide replacement decisions:

• Replace all glow plugs on the same bank to maintain balanced heating


• Use manufacturer-approved parts with the correct heat rating for reliability


• Labor times vary by engine layout, typically ranging from 1 to 4 hours


• Expect total costs to include parts, labor, and potential diagnostic fees

Routine checks during service visits and careful driving in cold weather can help prevent late-stage failures. If you’re DIY-ing, follow the vehicle’s service manual and observe all safety precautions.

Summary

A glow plug that goes bad mostly affects cold starts, idle quality, and emissions. Diagnosis blends fault codes, electrical testing, and visual inspection, with replacement often performed on the affected bank or all plugs to ensure even heating. Addressing the fault promptly preserves starting reliability and engine longevity.

What happens if a glow plug burns out?

Glow plugs help to provide the additional heat in the combustion chamber. If the required temperature is not reached due to faulty glow plugs, the engine's performance will be weakened, and it will have to work extra hard to meet the demands of the road. The vehicle's fuel efficiency will be negatively affected.

Will a diesel still start with bad glow plugs?

A diesel engine may start with bad glow plugs, but it will be difficult, especially in cold weather. The engine will struggle to get going, may crank several times, and could run roughly for a while after starting. Symptoms of bad glow plugs include hard starting, white exhaust smoke, and rough idling.
 
How it works

• Diesel engines rely on high compression to heat the air in the cylinders enough to ignite the fuel, a process called compression ignition. 
• Glow plugs provide extra heat to the combustion chamber, which is crucial for starting in cold weather. 
• If one or more glow plugs are bad, the engine may not reach the necessary temperature and will have trouble starting on its own. 

What to expect if you start with bad glow plugs

• Hard starting: You may have to crank the engine for a longer period to get it to catch. 
• Rough idling: The engine may run unevenly for a short time after starting. 
• White smoke: Unburnt fuel can come out of the exhaust as white smoke, especially when the engine is cold. 
• Reduced performance: Over time, poor combustion can lead to reduced fuel economy and carbon buildup. 

This video explains the symptoms of a bad glow plug: 1mFixITYouTube · Apr 11, 2025
Long-term consequences

• Continuing to start a diesel with bad glow plugs can lead to more serious problems, such as oil contamination and damage to other engine components.
• It is best to get the glow plugs replaced to ensure reliable operation and prevent expensive damage down the line. 

You can watch this video to learn how to replace glow plugs: 30sMoparYouTube · Jul 24, 2017

What happens if you drive with a bad glow plug?

Driving with a bad glow plug can lead to poor engine performance, hard starting, rough idling, decreased fuel efficiency, and increased emissions. While you may still be able to drive, it's not recommended as it can strain engine components and lead to more significant, costly repairs over time.
 
Performance and starting issues

• Hard starting: The engine may struggle to start, requiring multiple cranks, especially in cold weather, because the combustion chamber isn't heating up sufficiently. 
• Rough idle/Misfiring: The engine may run roughly or misfire, particularly right after starting, as some cylinders fail to ignite the fuel properly. 
• White smoke: You may see white smoke from the exhaust when starting the engine, which is a sign of unburnt fuel due to a lack of heat. 
• Poor acceleration and power: The engine will feel less powerful and accelerating may be sluggish. 

This video explains the symptoms of bad glow plugs, including hard starting and white smoke: 53sAuto Info GuyYouTube · May 7, 2022
Long-term consequences

• Increased fuel consumption: The engine works harder to compensate for the poor combustion, leading to worse fuel economy. 
• Higher emissions: Incomplete combustion from failing glow plugs leads to increased exhaust emissions. 
• Potential for further engine damage: Over time, the strain on other components like the starter motor and fuel injectors can cause more serious and expensive damage. 
• Check engine light: A glow plug system fault will likely trigger the "check engine" or a specific glow plug warning light on the dashboard. 

You can watch this video to learn more about the symptoms of bad glow plugs and how to diagnose them: 46sHonest Mechanic ColoradoYouTube · Mar 15, 2024
What to do

• Get it checked: It is best to have the glow plugs inspected and replaced as soon as you notice any of these symptoms to prevent further problems. 
• Don't ignore it: Driving with bad glow plugs is not a good long-term solution and can lead to costly repairs. 

How to know if a glow plug is bad?

Combustion. This can cause the engine to shake right after startup due to incomplete combustion. The next symptom is a flashing glow plug.