Can you drive with a bad glow plug?
In most diesel engines, you can still start and drive with a single faulty glow plug, but it’s not ideal and can lead to hard starting, rough idle, and higher emissions, especially in cold weather. If several glow plugs are failing, starting may become difficult or impossible.
Glow plugs heat the combustion chamber to aid starting in cold conditions. When one or more fail, starting performance and combustion can be uneven, which may affect drivability and long-term engine health. This article explains how glow plugs work, what happens if one goes bad, and what you should do about it.
What glow plugs do and how they fail
Glow plugs heat the air inside each cylinder to help ignite diesel fuel during cold starts. Failures can stem from a worn element, damaged wiring, a faulty glow plug relay, or a controller issue. Understanding these basics helps explain why driving with a bad plug can feel different.
Common signs your glow plugs may be failing include the following:
- Hard starting or long cranking on cold mornings
- Rough idle and misfires during the first minutes after starting
- Increased exhaust smoke, especially white smoke during startup
- Poor cold-weather performance and reduced engine power until warmed up
- Check Engine Light with codes related to glow plugs or misfire (e.g., generic misfire or P0380-type codes)
- Uneven engine performance or reduced fuel efficiency
If you notice these symptoms, a diagnostic check is warranted to confirm whether a glow plug or related wiring is at fault.
Can you drive with a bad glow plug? Immediate implications
Driving with a single faulty glow plug can be feasible for short trips, particularly in mild weather, but it is not recommended as a long-term solution. The main risks involve hard starting in cold conditions, rough running, and potential catalyst or exhaust-related issues if unburned fuel is emitted during startup.
Before driving further, consider these points:
- Starting reliability: a colder engine may require more cranking or may fail to start if other plugs are also weak
- Engine performance: rough idle or misfires can occur until the engine reaches operating temperature
- Emissions and fuel consumption: unburned fuel during cold startup can temporarily increase emissions and reduce efficiency
- Potential damage risk: continuous misfires or improper combustion can stress the catalytic converter and exhaust system over time
- Safety and reliability: in heavy traffic or remote areas, a starting failure can leave you stranded
If you rely on a vehicle with a suspected glow plug issue, limit driving to short, planned trips and arrange service promptly to avoid longer downtime or more extensive repairs.
Diagnosing and fixing glow plugs
Diagnosing glow plugs typically begins with a read of on-board diagnostics and tests of electrical circuits, followed by individual glow plug testing. Replacements are common in sets, since the plugs share related wiring and controllers, and replacing all at once can prevent future failures.
How professionals diagnose and fix glow plugs usually involves:
- Retrieving and analyzing engine trouble codes to identify glow plug-related fault codes
- Inspecting the glow plug harness, relay, and controller for damage or corrosion
- Testing individual glow plugs with a resistance ohmmeter to identify open or shorted elements
- Using a cylinder power/ignition test method or a glow plug tester tool to verify operation
- Replacing faulty plugs (and often all glow plugs in the same engine) and updating or replacing associated wiring if needed
- Clearing codes and conducting a cold-start test to confirm reliable operation
After replacement, your technician will typically advise on drivability recovery and any required post-repair checks, such as re-checking for exhaust smoke or idle behavior.
When it’s safer to tow rather than drive
If multiple glow plugs are failing or you experience repeated hard starts, misfires, or an illuminated check engine light that affects starting, it’s safer to avoid driving the vehicle. Towing to a repair shop reduces the risk of a breakdown in cold weather or at an inconvenient moment and minimizes the chance of further engine or exhaust damage.
In cold climates or if you rely on the vehicle for daily use, addressing glow plug issues promptly is wise to prevent worsening performance and to maintain fuel efficiency and emissions compliance.
Cost considerations and replacement timing
Replacement costs vary by engine type and region. Expect to pay for parts and labor, with the number of glow plugs in your engine driving the price. In many six-cylinder engines, replacing all glow plugs can be more cost-effective than replacing a single unit over time, especially if one failed plug indicates aging wiring or controller components.
Typical considerations include:
- Part price per glow plug, which can range depending on brand and engine design
- Labor time, often influenced by accessibility of plugs under the intake or valve cover
- Possibility of replacing related components (harness, relay, controller) if they show wear or fault codes
- Post-repair tests to confirm reliable starting and proper combustion
Consult your vehicle’s manual or a trusted diesel technician for a quoted estimate and the best replacement strategy for your specific engine model.
Summary
Glow plugs play a critical role in cold-start performance for diesel engines. A single bad glow plug may not prevent you from driving immediately, but it can cause starting difficulties, rough idle, and higher emissions. If several plugs are failing, you may experience significant starting problems or no start at all. Diagnosis typically involves code checks and electrical testing, and most mechanics recommend replacing faulty plugs—often all in a set—to avoid future failures. For safety and reliability, address glow plug issues promptly and consider towing if multiple components appear compromised or the vehicle struggles to start.
In short, you can drive with a bad glow plug in some cases, but it’s a temporary workaround rather than a fix. Prompt diagnosis and replacement help protect engine health and ensure dependable performance in all driving conditions.
For readers considering service, contact a qualified diesel technician or dealership with your engine’s exact make, model, and VIN to obtain a precise diagnosis and estimate.
What happens if I don't change my glow plugs?
If one or more glow plugs have burned out, are carbon fouled or damaged, the engine will become progressively harder to start as temperatures drop and will idle roughly and produce white smoke from the exhaust for several minutes once it finally starts.
Can you drive with a broken glow plug in diesel?
While you can still drive with one broken glow plug, you'll find it extremely difficult to start your car as soon as two or more glow plugs malfunction even if the weather is warm. It's important to note that a problem starting your car can also be a symptom of a problem in the fuel system or battery.
Can I still drive with the glow plug light on?
Is it safe to drive when my glow plug warning light is on? We would strongly advise against driving with a glow plug warning light, as there could be an issue with your glow plugs, their sensor, or even the engine. By driving with a glow plug warning light, you run the risk of developing serious engine problems.
What happens if a glow plug stops working while?
If your glow plug isn't doing its job, the fuel will not be hot enough, and fuel injection will fail. As a result, your engine will misfire.
