The P0671 code specifically indicates “Cylinder 1 Glow Plug Circuit/Open” – meaning the Engine Control Module (ECM) has detected an abnormal voltage reading or resistance in the glow plug circuit for cylinder 1.

Glow Plug System Components

How the ECM Detects P0671

The ECM monitors the glow plug circuit by sending a small voltage signal and measuring the resistance. When the resistance falls outside the expected range (typically 0.1-6.0 ohms), the ECM interprets this as a circuit malfunction and stores the P0671 code.

When the P0671 code appears, you may experience one or more of these symptoms, ranging from mild to severe depending on ambient temperature and engine condition:

Primary Symptoms

• Hard Starting in Cold Weather – The engine may struggle to start, especially when temperatures drop below 40°F (4°C). This is the most common symptom.
• Rough Idling – The engine may run unevenly or roughly at idle, particularly during the warm-up phase.
• Excessive White Smoke – You might notice more white smoke from the exhaust than usual, especially during cold starts due to unburned fuel.

Secondary Symptoms

• Check Engine Light – The most obvious symptom – the check engine light will illuminate on your dashboard.
• Reduced Engine Performance – You may experience decreased power, especially during acceleration.
• Poor Fuel Economy – Inefficient combustion can lead to decreased miles per gallon.
• Engine Misfires – In severe cases, the engine may misfire, particularly during cold operation.

Proper diagnosis is crucial to correctly identify and fix the issue. Follow these steps systematically to pinpoint the exact cause of the P0671 code.

Required Tools for Diagnosis

• OBD-II Scanner – To read and clear diagnostic trouble codes
• Digital Multimeter – For measuring resistance and voltage in the circuit
• Inspection Mirror & Flashlight – For visual inspection of hard-to-see components
• Basic Hand Tools – Sockets, extensions, and wrenches for component removal

Step-by-Step Diagnostic Procedure

1. Code Verification and System Check
   • Use an OBD-II scanner to confirm the P0671 code and check for any additional codes that might provide more context
   • Check the freeze frame data to see under what conditions the code was set
   • Clear the codes and perform a test drive to see if the code returns
2. Comprehensive Visual Inspection
   • Inspect the glow plug harness and connector for cylinder 1 for any visible damage, corrosion, or loose connections
   • Check the glow plug control module for signs of damage, corrosion, or water intrusion
   • Look for any damaged, chafed, or burnt wiring in the entire glow plug circuit
   • Inspect all related fuses and relays in the power distribution center
3. Detailed Resistance Testing
   • Disconnect the electrical connector from the glow plug for cylinder 1
   • Using a digital multimeter set to ohms (Ω), measure the resistance between the glow plug terminal and engine ground
   • Compare the reading with manufacturer specifications (typically 0.1-6.0 ohms for most diesel engines)
   • A reading of infinite resistance (OL) indicates an open circuit – the glow plug needs replacement
   • A reading of zero or very low resistance indicates a shorted glow plug
4. Comprehensive Voltage Testing
   • With the key in the ON position (engine not running), check for battery voltage at the glow plug connector
   • If no voltage is present, the issue may be in the wiring harness or glow plug control module
   • Check for voltage drop across the circuit while the glow plugs are activated
   • Verify proper ground connections for the glow plug circuit
5. Glow Plug Control Module Testing
   • Test the control module according to manufacturer specifications using a professional scan tool
   • Check for proper ground connections to the control module
   • Verify the module is receiving proper power and signal from the ECM
   • Test the control module’s output to the glow plugs during activation

Advanced Diagnostic Options

• Oscilloscope Testing – Use an oscilloscope to monitor the voltage waveform at the glow plug during operation to identify intermittent issues that may not show up during static testing.
• ECM Communication Testing – Verify proper communication between the glow plug control module and the ECM using a professional scan tool with bidirectional controls.
• Compression Testing – If multiple glow plug codes appear, perform a compression test to rule out engine mechanical issues that could be affecting combustion.
• Thermal Imaging – Use a thermal camera to identify glow plugs that aren’t heating properly during operation.

The P0671 code can be triggered by several issues. Here are the most common causes ranked by frequency:

Vehicle-Specific Considerations

Certain vehicle models have known issues with glow plug systems:

• Ford Powerstroke 6.0L – Known for glow plug control module failures due to location near hot engine components.
• Duramax Diesel – Common issues with glow plug harness connectors becoming brittle and breaking.
• Mercedes-Benz Diesel – Often experiences glow plug breakage during removal due to carbon buildup.

Once you’ve identified the cause, follow these detailed repair procedures to fix the P0671 code permanently.

Glow Plug Replacement Procedure

1. Safety Precautions
   • Disconnect the negative battery cable to prevent electrical shorts
   • Allow the engine to cool completely if it was recently running
   • Wear safety glasses to protect your eyes from debris
2. Access Preparation
   • Remove any components obstructing access to the glow plugs (intake manifold, injector lines, etc.)
   • Use an air compressor to clean around the glow plugs to prevent debris from entering the cylinder
   • Take photos or label connections if needed for reassembly
3. Glow Plug Removal
   • Disconnect the electrical connector from the faulty glow plug
   • Use a deep socket and extension to carefully loosen the glow plug
   • If the glow plug is stuck, apply penetrating oil and allow it to soak for 15-30 minutes
   • Turn the glow plug counterclockwise to remove it from the cylinder head
4. New Glow Plug Installation
   • Apply a small amount of anti-seize compound to the threads of the new glow plug
   • Carefully thread the new glow plug into the cylinder head by hand to prevent cross-threading
   • Use a torque wrench to tighten the glow plug to manufacturer specifications (typically 10-20 Nm)
   • Reconnect the electrical connector to the glow plug
5. Reassembly and Testing
   • Reinstall any components that were removed for access
   • Reconnect the battery cable
   • Clear the codes with an OBD-II scanner
   • Start the engine and verify proper operation
   • Perform a test drive to ensure the code doesn’t return

Wiring Repair Procedure

1. Locate the damaged section of wiring in the glow plug circuit
2. Cut out the damaged portion using wire cutters
3. Strip approximately 1/4 inch of insulation from both ends of the wire
4. Use butt connectors or solder to join new wire of the same gauge
5. Protect the connection with heat shrink tubing for a waterproof seal
6. Secure the repaired wiring away from heat sources and moving parts
7. Test the repair for proper voltage and resistance

The cost to fix a P0671 code varies significantly depending on the specific cause, your vehicle model, and whether you choose DIY or professional repair. Here’s a comprehensive breakdown of potential costs:

Cost Factors by Vehicle Type

• Passenger Cars – Typically lower costs due to better accessibility ($100-$400 total)
• Light Trucks & SUVs – Moderate costs with some accessibility challenges ($150-$600 total)
• Heavy-Duty Trucks – Higher costs due to larger components and more complex systems ($200-$900+ total)

Preventing glow plug issues can save you time and money. Follow these maintenance tips to extend the life of your glow plug system:

Regular Maintenance Schedule

• Regular Inspection – Have your glow plugs inspected during routine maintenance, especially before winter. Most manufacturers recommend inspection every 30,000-60,000 miles.
• Maintain Battery Health – A weak battery puts extra strain on glow plugs during cold starts. Test your battery regularly and replace when necessary.
• Use Quality Fuel – Poor quality diesel can lead to carbon buildup on glow plugs. Use reputable fuel sources and consider fuel additives for cleaning.
• Address Oil Leaks Promptly – Oil contamination can shorten glow plug lifespan. Fix any engine oil leaks as soon as they’re detected.

Cold Weather Precautions

• Use Engine Block Heaters – In extreme cold, use engine block heaters to reduce strain on glow plugs and improve starting reliability.
• Allow Proper Glow Time – In cold weather, allow the glow plug system to complete its cycle before attempting to start the engine.
• Battery Blankets – Consider using a battery blanket in extremely cold climates to maintain battery efficiency.

Can I drive with a P0671 code?

In warm weather conditions, you may be able to drive with a P0671 code, though it’s not recommended. The engine may start harder when cold and run less efficiently. In cold weather, the vehicle may not start at all or run very poorly until warm.

How urgent is P0671 repair?

P0671 repair should be addressed within a few weeks, or before cold weather arrives. While not an immediate safety concern, it can lead to being stranded if the vehicle won’t start in cold conditions.

Can I replace just one glow plug?

While you can replace just the faulty glow plug, it’s often recommended to replace all glow plugs as a set, especially in higher mileage vehicles. This ensures even heating and prevents future failures of other aging glow plugs.

What causes glow plugs to fail?

Common causes of glow plug failure include normal wear over time, carbon buildup, overheating, electrical issues, oil contamination, and physical damage during installation or removal.

Can a bad glow plug damage the engine?

While a single bad glow plug won’t typically cause serious engine damage, it can lead to incomplete combustion, increased emissions, hard starting, and potential damage to the catalytic converter over time.