How do I know if my glow plugs are bad in Duramax?

Short answer: If your Duramax struggles to start in cold weather, runs rough on startup, or emits white smoke from the exhaust when starting, your glow plugs could be failing and should be tested or replaced.

What glow plugs do in a Duramax and why they fail

In a Duramax diesel, each cylinder has a glow plug that preheats the air in the combustion chamber to facilitate cold starts. Over time, glow plugs can wear out, the heating coil can fail, or the wiring harness and relay can deteriorate, leading to misfires or no heat at the plug. Extreme cold, frequent short trips, and high-mileage engines increase the likelihood of failure.

Diagnostics: symptoms, tests, and how to interpret results

Common symptoms to watch for

Look for signs that may indicate glow plug problems:

• Hard starting in cold weather or after overnight sits


• Extended cranking before the engine starts


• Rough idle or stumble on startup


• White or blue smoke from the exhaust during startup


• Check Engine Light with glow-plug related codes (P0380, P0671–P0678 on many Duramax models)


• Uneven power delivery or misfires on cold starts

Note: These symptoms can also be caused by other issues such as fuel system problems, compression concerns, or battery faults. A proper diagnostic is required to confirm glow plug problems.

Electrical testing and code-based checks

Assess glow plug condition using the following tests and code checks:

• Use an OBD-II scanner to look for glow plug-related codes: P0380 and cylinder-specific P0671–P0678 on applicable models.


• Inspect the glow plug relay, fuse(s), and wiring harness for signs of damage or overheating.


• During cold-start, measure voltage at each glow plug connector if accessible; you should see near-12V when the ECM powers the plugs.


• Perform resistance testing on individual glow plugs with a multimeter. Healthy glow plugs typically show a low resistance (roughly under 2 ohms, depending on model); an open circuit or very high resistance usually indicates a bad plug.


• Check for corrosion, frayed insulation, or loose connections at the harness and connectors.

Interpreting results: a single faulty plug often triggers cylinder-specific misfire codes; multiple codes or no codes may point to a relay or wiring issue or a failing ECU module.

Mechanical considerations and safety

Because glow plugs are threaded into the cylinder heads, removing a seized or damaged plug can risk thread damage and engine damage if not done carefully. If you suspect multiple bad plugs or you notice coolant intrusion or severe corrosion, consider professional service.

Concluding paragraph for diagnostics: A systematic approach—start with diagnostic trouble codes and the glow plug relay, then test each plug’s resistance and the wiring—helps pinpoint whether you need replacement plugs, a new relay, or harness work.

Replacement and maintenance considerations

Replacement procedures and costs vary by Duramax model year and engine variant. Some glow plugs are accessed from the top via the valve cover, while others require more extensive disassembly. Replacing the glow plug(s) often includes inspecting or replacing the glow plug relay and wiring harness if they show damage or heat damage.

• Replacement intervals: Glow plugs commonly last 60,000–100,000 miles, but harsh climates and heavy cold starts can shorten this life.


• Parts to consider: Glow plugs, relay, and harness. In many cases, replacing all glow plugs at once prevents future failure due to worn components in the same circuit.


• Labor and downtime: Access complexity varies by model; expect a shop visit to range from about 2 to 6 hours depending on engine variant and technician experience.

Bottom line: If diagnostics show multiple bad plugs or a faulty relay/harness, replacement in a single service cycle is typically more cost-effective than tackling them piecemeal.

Safety, tips, and best practices

Always disconnect the battery before working on glow plugs. Let the engine cool before attempting removal. Use the proper sockets and torque specifications, and avoid forcing seized plugs. Follow your GM service manual’s model-specific guidance to prevent damage and ensure proper installation.

Summary

Glow plug failure in Duramax engines presents as hard starting in cold weather, rough startup, and exhaust smoke, often accompanied by specific diagnostic codes. Start with a code check and relay inspection, then test individual plugs with resistance measurements and inspect the wiring harness. Replace defective plugs and, if needed, the relay and harness. Keep in mind model-specific variations, and consult the service manual or a qualified diesel technician for precise procedures and torque specs. Regular diagnostics can prevent extended cranking and costly repairs.

How to test glow plugs on a Duramax?

Okay take your voltmeter. And set to DC volts. Attach one lead to good. Ground. Now put the other lead onto any contact in either of these connectors.

Can you drive a Duramax with a bad glow plug?

Yes, you can drive a Duramax with a bad glow plug, but it is not recommended, especially in cold weather. While the truck may start and run, you risk difficult cold starts, rough idling, poor fuel economy, white smoke from the exhaust, and potential long-term engine damage due to incomplete combustion. You should get the faulty glow plug(s) replaced as soon as possible to prevent further issues. 
What to expect when driving with a bad glow plug

• Difficult starting: You may experience long cranking times or the engine may not start at all, particularly in cold weather. 
• Rough idle: The engine may run roughly after starting. 
• White smoke: You may see white smoke, especially on cold starts, as fuel may not be combusting completely. 
• Reduced fuel economy: Overall efficiency can decrease. 
• Long-term engine damage: Ignoring the problem can lead to more significant issues over time, such as increased carbon buildup, which can damage engine components. 

What to do

• Drive with caution: If you must drive, be mindful of the potential for difficult starts, especially during cold weather. 
• Avoid prolonged idling: This can exacerbate the problem and increase emissions. 
• Get it fixed promptly: Schedule an appointment with a qualified mechanic to have the glow plug(s) replaced. Ignoring the problem can lead to more expensive repairs down the line. 

How do I know if my Duramax glow plugs are bad?

You can tell your Duramax glow plugs are bad by symptoms like hard starting (especially in cold weather), rough idling, a check engine light, and white smoke from the exhaust. A more precise diagnosis can be made by checking for specific trouble codes with an OBD-II scanner or by testing individual glow plugs with a multimeter to check their resistance or continuity. 
Common symptoms

• Hard starting: The engine takes longer than usual to crank and start, particularly in colder temperatures. 
• Rough idle: The engine may shake or run unevenly for a few minutes after starting. 
• Check engine light: A warning light on the dashboard indicates a fault in the glow plug system. 
• White smoke: Unburnt fuel can cause white smoke to come from the exhaust, especially on a cold start, until the engine warms up. 
• Loss of power: You may experience a noticeable decrease in engine power. 
• Decreased fuel economy: A malfunctioning glow plug system can negatively impact fuel efficiency. 

Diagnostic methods

• Scan tool: Connect an OBD-II scanner to check for glow plug related trouble codes, such as those in the P06xx range. 
• Multimeter test:
  • Disconnect the power to the glow plug. 
  • Set your multimeter to measure resistance (ohms) or continuity. 
  • Connect one probe to the glow plug's connector terminal and the other to a good ground point. 
  • A healthy glow plug will show very low resistance, close to 000 ohms. 
  • A high or infinite resistance reading indicates a bad glow plug that needs replacing. 
• Glow plug controller feedback: Some Duramax models use the glow plug controller to detect issues. A fault code can be triggered if the feedback voltage from the controller is outside the normal operating range (e.g., below 5−65 minus 65−6 volts). 

How to tell if diesel glow plugs are bad?

Common symptoms of a failing diesel glow plug include hard starting (especially in cold weather), rough idling, and white smoke from the exhaust upon startup. Other signs are a glowing coil warning light on the dashboard, engine misfires, poor acceleration, black smoke under acceleration, and decreased fuel efficiency.
 
Starting and running issues

• Hard starting: The engine may be difficult to start, especially when cold, sometimes requiring multiple attempts to crank. 
• Rough idle: The engine may shake or run unevenly immediately after starting due to incomplete combustion. 
• Engine misfires: You may experience sputtering or misfiring, particularly while the engine is cold. 
• Poor acceleration: The vehicle may hesitate or feel sluggish during acceleration. 

Exhaust and fuel problems 

• White smoke: Unburnt fuel can be expelled as white smoke from the exhaust, especially when starting from cold, as the cylinders don't reach the proper temperature. 
• Black smoke: Black or dark smoke can appear under heavy acceleration. 
• Decreased fuel efficiency: The engine may use more fuel to compensate for the lack of heat. 

Warning lights and electrical issues

• Glow plug warning light: A flashing or solid light on the dashboard shaped like a coiled spring is often the first sign of a fault. 
• Check engine light: The glow plug issue may trigger the general check engine light.