P0678 Code: Here's How to Solve It Fast

The P0678 trouble code is a diagnostic trouble code (DTC) that indicates a problem with the glow plug circuit in cylinder 8 of a diesel engine. This code is part of the OBD-II system, which monitors the performance of various components in vehicles, particularly those related to emissions and engine performance. When the powertrain control module (PCM) detects that the voltage from the glow plug circuit in cylinder 8 is outside the manufacturer's specified range, it triggers the P0678 code. This can lead to difficulties in starting the engine, especially in colder conditions, and may result in increased emissions and reduced engine performance.

P0678 Code Meaning	P0678 Code Common Causes
Cylinder 8 Glow Plug Circuit Malfunction	Faulty glow plug
Open or short circuit in glow plug wiring	Defective glow plug relay
Glow plug control module failure	Blown fuses in the glow plug circuit
Corroded or damaged connectors	Faulty glow plug timer

Symptoms of P0678 Code

When dealing with a P0678 code, vehicle owners may notice several symptoms that can indicate an underlying issue with the diesel engine's glow plug system:

• Check Engine Light: The most immediate indication will be the illumination of the check engine light on the dashboard.
• Hard Starting: Difficulty starting the engine, especially in cold weather, as the glow plugs are responsible for preheating the combustion chamber.
• White Smoke: The vehicle may emit white smoke from the exhaust during startup, indicating unburned fuel due to insufficient heating.
• Reduced Power: A noticeable lack of power until the engine reaches optimal operating temperature.
• Engine Hesitation: The engine may hesitate or misfire during acceleration until it warms up sufficiently.

Technical Explanation of P0678 Code

The P0678 code is specific to diesel engines, which utilize glow plugs to preheat air in the cylinders for better fuel combustion. Each cylinder typically has its own glow plug, and when one fails—like in cylinder 8—it can lead to issues such as hard starting and increased emissions. The PCM monitors the voltage levels from each glow plug circuit. If it detects that the voltage for cylinder 8 is significantly lower or higher than expected (more than 10% off), it triggers the P0678 code.

Step-by-Step Diagnosis

Diagnosing a P0678 code involves several steps:

1. OBD-II Scanner Check: Use an OBD-II scanner to confirm that P0678 is present and check for any additional codes that might provide more context.
2. Visual Inspection: Inspect the wiring and connectors associated with cylinder 8’s glow plug circuit for any signs of damage, corrosion, or loose connections.
3. Resistance Testing: Disconnect the glow plug connector and use a digital volt-ohmmeter (DVOM) to test the resistance of the glow plug itself. A healthy glow plug should show resistance between 0.5 to 2.0 ohms.
4. Circuit Testing: Check for continuity and resistance in the wiring leading from the PCM to the glow plug relay and from there to the glow plugs. Any significant resistance or open circuits should be repaired.
5. Relay and Module Check: Test or replace any faulty relays or modules that control power to the glow plugs.
6. Clear Codes and Test Drive: After repairs, clear any trouble codes and take the vehicle for a test drive to see if the P0678 code returns.

Solution Methods for P0678 Code

To resolve a P0678 trouble code effectively, consider these methods:

Replace Faulty Glow Plug

If testing shows that cylinder 8's glow plug is defective:

• Disconnect battery power.
• Remove any components obstructing access to the glow plug.
• Unscrew and replace with a new glow plug.
• Reconnect everything and clear codes.

Repair Wiring Issues

If you find damaged wires:

• Cut out damaged sections and splice in new wire.
• Ensure all connectors are clean and free of corrosion.
• Use heat-shrink tubing or electrical tape to insulate connections properly.

Replace Glow Plug Relay

If tests indicate a faulty relay:

• Locate the relay based on your vehicle's service manual.
• Disconnect battery power before replacing.
• Swap out with a new relay and reconnect everything properly.

Check Glow Plug Control Module

If issues persist after replacing plugs and relays:

• Test or replace the glow plug control module as it regulates power supply to all glow plugs.

Cost Estimates

The cost for repairs related to P0678 can vary significantly based on labor rates and parts prices:

• Glow Plug Replacement: $100 - $300 depending on parts.
• Wiring Repairs: $50 - $150 depending on extent of damage.
• Relay Replacement: $50 - $120 for parts plus labor.
• Control Module Replacement: $200 - $600 depending on make/model.

Warnings and Recommendations

When diagnosing or repairing issues related to P0678:

• Always disconnect battery power before working on electrical components to avoid short circuits or shocks.
• If you're unsure about any step, consult your vehicle’s service manual or seek professional assistance.
• Ignoring this trouble code can lead to more severe engine problems over time, including potential damage from prolonged hard starting conditions.

Closing Paragraph

In summary, understanding and addressing a P0678 trouble code is crucial for maintaining your diesel vehicle's performance. By following systematic diagnostic steps and implementing appropriate solutions—such as replacing faulty components or repairing wiring—you can ensure your engine starts reliably and operates efficiently. If you encounter challenges during this process, don’t hesitate to consult with a professional mechanic who can provide expert guidance tailored to your specific vehicle needs.

Frequently Asked Questions About P0678

• What does error code P0678 mean?
  The P0678 error code indicates a malfunction in the glow plug circuit for cylinder 8 of a diesel engine.
• Can I drive my vehicle with a P0678 code?
  While you can drive your vehicle, it may experience hard starting issues, especially in cold weather.
• How serious is a P0678 code?
  This code can lead to starting difficulties and increased emissions if not addressed promptly.
• What tools do I need to diagnose a P0678 code?
  A digital volt-ohmmeter (DVOM) and an OBD-II scanner are essential tools for diagnosing this issue.
• How much does it cost to fix a P0678 code?
  The repair costs can range from $100 for simple fixes like replacing a glow plug to over $600 for more complex issues like replacing control modules.
• Is it safe to ignore a P0678 code?
  No, ignoring this code can lead to worsening performance issues over time.
• What should I do first if I see a P0678 code?
  The first step is to use an OBD-II scanner to confirm the code and check for additional trouble codes.
• Can I replace just one faulty glow plug?
  You can replace just one faulty glow plug; however, it's often recommended to replace all plugs if one has failed.