Glow Plug Failure: Starting Problems and Diagnosis for VW and Audi Diesel

Glow plugs are essential for starting diesel engines, especially in cold weather. Unlike petrol engines that use spark plugs for ignition, diesel engines rely on compression heat to ignite fuel. Glow plugs preheat the combustion chambers, ensuring reliable cold starts.

Failed glow plugs are one of the most common causes of starting problems on Volkswagen and Audi TDI engines. Understanding glow plug symptoms, knowing how to diagnose problems, and recognizing when replacement is necessary can prevent being stranded on cold mornings and ensure reliable diesel engine operation.

This guide covers glow plug operation, failure symptoms, diagnostic procedures, and replacement considerations for VW and Audi diesel vehicles.

How Glow Plugs Work in Diesel Engines

Diesel ignition principle:

• Diesel fuel ignites from heat and pressure, not spark
• Compression heats air to 500–700°C (sufficient for fuel ignition)
• Cold engines need assistance reaching ignition temperature

Glow plug function:

• Preheating: Glow plugs heat to 800–1,000°C before starting
• Cold start aid: Ensures combustion chambers reach ignition temperature
• Post-start heating: Some systems continue heating briefly after starting for smooth running

Modern systems:

• Pre-glow: Heat before starting (2–30 seconds depending on temperature)
• After-glow: Continue heating for 1–3 minutes after starting (reduces emissions, improves idle)
• Regeneration glow: Assist DPF regeneration in some systems

Problems occur when:

• Glow plugs fail (won't heat)
• Glow plug control module fails
• Wiring or connector problems prevent operation
• Multiple plugs fail (causes starting problems)

5 Warning Signs Your Glow Plugs Are Failing

1. Hard Starting, Especially in Cold Weather

What to experience: Engine cranks longer than normal before starting, or requires multiple attempts to start. Problem is worse in cold weather (below 10°C).

Why it happens: Failed glow plugs can't preheat combustion chambers. Engine struggles to reach ignition temperature through compression alone, especially when cold.

Severity: One failed plug may cause slight delay; multiple failed plugs cause severe starting problems or no-start condition.

Test: If starting improves significantly in warm weather, glow plugs are likely the issue.

2. Rough Idle or Misfiring After Starting

What to feel: Engine runs rough immediately after starting, may misfire or feel unstable for the first 30–60 seconds.

Why it happens:

• Failed glow plugs in specific cylinders cause misfires in those cylinders
• Incomplete combustion in cold cylinders produces rough running
• Problem typically improves as engine warms (combustion chambers heat from compression)

Progression: Rough running may worsen as more glow plugs fail, or if after-glow system is also malfunctioning.

3. White or Blue Smoke from Exhaust on Startup

What to see: Excessive white or blue smoke from exhaust immediately after starting, especially when cold.

Why it happens:

• White smoke: Unburned fuel (diesel) exiting exhaust due to incomplete combustion in cold cylinders
• Blue smoke: Oil burning in cylinders without proper preheating
• Reduction: Smoke typically decreases as engine warms and combustion improves

Connection: While smoke can indicate other problems, glow plug failure is a common cause, especially if smoke reduces as engine warms.

4. Check Engine Light with Glow Plug Codes

What to see: Warning lights on dashboard with fault codes such as:

• P0670: Glow Plug Control Module Circuit Malfunction
• P0671: Cylinder 1 Glow Plug Circuit/Open
• P0672: Cylinder 2 Glow Plug Circuit/Open
• P0673: Cylinder 3 Glow Plug Circuit/Open
• P0674: Cylinder 4 Glow Plug Circuit/Open
• P0675: Cylinder 5 Glow Plug Circuit/Open (if applicable)
• P0676: Cylinder 6 Glow Plug Circuit/Open (if applicable)

Why it happens: Engine management systems monitor glow plug operation. Failed plugs or control system problems trigger diagnostic trouble codes.

Action required: Have codes read with professional diagnostic equipment. Glow plug codes indicate problems requiring replacement.

5. Glow Plug Indicator Light Problems

What to see:

• Glow plug indicator light doesn't illuminate when key is turned on
• Light stays on longer than normal (may indicate system problems)
• Light doesn't turn off after starting (control system failure)

Why it happens:

• No light: Control module failure or wiring problems
• Extended light: System problems or excessive heating time required
• Light stays on: Control module or relay failure

Normal operation: Light should illuminate for 2–30 seconds (depending on temperature), then turn off when starting is possible.

Why Glow Plugs Fail

Age and wear:

• Normal wear: Glow plugs typically last 100,000–150,000 km
• Heat cycling: Repeated heating and cooling causes wear over time
• Material degradation: Heating element wears with use

Electrical problems:

• Overvoltage: Excessive voltage can damage heating elements
• Control module failure: Faulty control system can damage plugs
• Wiring issues: Poor connections or damaged wiring affect operation

Physical damage:

• Carbon buildup: Excessive carbon can cause plugs to stick or fail
• Corrosion: Moisture or contamination can damage plugs
• Impact damage: Rare, but possible during engine service

Manufacturing defects:

• Premature failure: Some plugs fail before expected service life
• Quality issues: Lower-quality aftermarket plugs may fail early

Diagnosis: How to Test Your Glow Plugs

Professional Diagnosis

Diagnostic procedure:

1. Fault code reading: Check for glow plug-related diagnostic trouble codes
2. Resistance testing: Measure glow plug resistance (should be 0.1–2.0 ohms typically)
3. Voltage testing: Check voltage supply to glow plugs during pre-glow cycle
4. Control module testing: Verify glow plug control module operation
5. Visual inspection: Check plugs for physical damage or carbon buildup
6. Current draw testing: Measure current consumption during heating

Professional advantages:

• Accurate identification of failed plugs
• Proper testing of control system
• Identification of wiring or connector problems

DIY Testing (Limited)

Visual inspection:

• Check glow plug indicator light operation
• Inspect accessible plugs for physical damage (if visible)

Resistance testing (requires multimeter):

• Measure resistance between glow plug terminal and ground
• Compare readings between plugs (should be similar)
• Warning: Requires proper tools and knowledge—incorrect testing can damage plugs

Starting behavior:

• Monitor starting performance, especially in cold weather
• Note rough running after starting
• Check for diagnostic trouble codes with OBD scanner

Limitations: Professional diagnosis is recommended. Glow plug testing requires proper equipment and knowledge. Incorrect diagnosis can lead to unnecessary replacement or missed problems.

Solutions: Replacement vs. Repair

Glow Plug Replacement

When required:

• Glow plug failure confirmed through testing
• Starting problems in cold weather
• Rough running after starting
• Diagnostic trouble codes indicating glow plug failure

Components typically replaced:

• Failed glow plugs (individual or complete set)
• Glow plug control module (if faulty)
• Related wiring or connectors (if damaged)

Recommendation: Replace all glow plugs as a set, even if only one has failed. Others are likely near end of service life, and replacement labour is the same for individual or complete set.

Cost: €15–€50 per plug (parts) + €200–€400 (labour for complete set) Labour time: 1–2 hours for complete set replacement

Important: Always use genuine OEM glow plugs. Aftermarket plugs may have different characteristics causing starting problems or control system issues.

Search our catalog for glow plugs - genuine OEM parts with guaranteed fitment for your specific TDI engine.

Control Module Replacement

When applicable:

• Control module failure (no glow plug operation)
• Wiring problems affecting control module
• Diagnostic codes indicate control module problems

Cost: €150–€400 (parts) + €100–€200 (labour) Consideration: Verify glow plugs are functional before replacing control module—failed plugs can appear as control module problems.

Prevention: Extending Glow Plug Life

Regular maintenance:

• Service intervals: Follow manufacturer recommendations for glow plug inspection/replacement
• Quality components: Use genuine OEM glow plugs for reliability
• Proper installation: Correct installation prevents damage and ensures proper operation

Early intervention:

• Address symptoms early: Don't ignore starting problems or warning lights
• Replace as set: Replacing all plugs together prevents recurring problems
• Diagnostic attention: Have glow plug codes investigated promptly

Driving habits:

• Proper warm-up: Allow glow plug system to complete pre-glow cycle before starting
• Avoid excessive cranking: Can damage glow plugs and starter motor

Cold Weather Starting Tips

Proper procedure:

1. Turn key to "on" position (don't start immediately)
2. Wait for glow plug indicator light to turn off (typically 2–30 seconds)
3. Start engine immediately after light turns off
4. Avoid extended cranking—if engine doesn't start, wait 30 seconds and repeat

In very cold conditions:

• May require multiple pre-glow cycles
• Allow extra time for pre-heating
• Consider engine block heater for extremely cold climates

If starting problems persist:

• Have glow plugs tested—may need replacement
• Check battery condition (weak battery affects glow plug operation)
• Verify fuel system (waxed fuel in extreme cold)

Cost Comparison: Prevention vs. Repair

• Preventive replacement (as set): €200–€500 (parts and labour)
• Emergency replacement (no-start): €200–€500 (same cost, but inconvenience)
• Towing/stranded costs: €100–€300+ (if unable to start)
• Battery damage (from excessive cranking): €150–€400

Preventive replacement prevents inconvenience and potential additional costs. Replacing as a set ensures reliable operation.

Related Components That Fail Together

Common associated failures:

• Glow plug control module: Often replaced when multiple plugs fail
• Battery: Weak battery affects glow plug operation
• Starter motor: Excessive cranking can damage starter
• Fuel system: Starting problems can mask fuel system issues

During glow plug replacement, consider:

• Battery condition check
• Starter motor inspection (if excessive cranking occurred)
• Fuel system verification (if starting problems are severe)

Next Steps

If you're experiencing glow plug-related symptoms or starting problems, especially in cold weather, don't delay diagnosis. Failed glow plugs cause inconvenience and can leave you stranded.

Contact our technical advisors for expert guidance on glow plug diagnosis and replacement. We can verify your vehicle's glow plug part number using your VIN and provide genuine OEM components with rapid European dispatch.

Browse our engine electrical components for glow plugs, control modules, and related parts for Volkswagen and Audi TDI engines.