The P0678 trouble code is a diagnostic trouble code (DTC) that indicates an issue with the glow plug circuit in cylinder number 8 of a diesel engine. Glow plugs play a critical role in diesel engines by preheating the combustion chamber to facilitate ignition, especially in cold conditions. 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 starting difficulties, increased emissions, and poor engine performance if not addressed promptly.
| P0678 Code Meaning | P0678 Code Common Causes |
|---|---|
| Cylinder 8 Glow Plug Circuit Malfunction | Faulty glow plug in cylinder 8 |
| 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 | Electrical issues within the glow plug circuit |
Symptoms of P0678 Code
The symptoms associated with the P0678 trouble code can vary, but common indicators include:
- Check Engine Light: The most immediate sign is the illumination of the check engine light on the dashboard.
- Hard Starting: Difficulty starting the engine, particularly in cold weather, is a prevalent symptom due to inadequate preheating.
- Engine Hesitation: The engine may hesitate or lack power during acceleration until it reaches optimal operating temperature.
- Excessive Smoke: White smoke may be observed from the exhaust during startup as unburned fuel is expelled.
- Poor Fuel Economy: Increased fuel consumption can occur due to inefficient combustion.
Technical Explanation
The P0678 code specifically relates to the glow plug circuit responsible for heating cylinder 8. Diesel engines rely on high compression and heat generated by glow plugs to ignite fuel. If there is a malfunction in this circuit, it can lead to insufficient heating of the cylinder, resulting in starting difficulties and poor performance.
Step-by-Step Diagnosis
Diagnosing a P0678 code involves several steps:
- Scan for Codes: Use an OBD-II scanner to confirm that P0678 is present and check for any additional codes that may provide more context.
- Visual Inspection: Inspect wiring and connectors related to the glow plug circuit for signs of damage, corrosion, or loose connections.
- Test Glow Plug Resistance: Disconnect the glow plug connector and use a digital volt/ohmmeter (DVOM) to measure resistance. The resistance should typically be between 0.5 and 2.0 ohms. If it falls outside this range, replace the glow plug.
- Check Glow Plug Relay: Test the relay for proper operation. A faulty relay can prevent voltage from reaching the glow plugs.
- Inspect Control Module: Check the glow plug control module for faults. If other components are functioning correctly but issues persist, this module may need replacement.
- Clear Codes and Test Drive: After repairs, clear any stored codes and take the vehicle for a test drive to see if the P0678 code reappears.
Solution Methods
To resolve a P0678 trouble code, several repair methods may be necessary depending on the root cause identified during diagnosis:
- Replace Faulty Glow Plug: If testing reveals that cylinder 8’s glow plug is defective, replace it with a new one.
- Repair Wiring Issues: Any damaged wires or connectors should be repaired or replaced to ensure proper electrical flow.
- Replace Glow Plug Relay: If tests indicate that the relay is malfunctioning, replacing it can restore functionality to the circuit.
- Fix Control Module Problems: If all other components are functioning correctly but issues persist, consider replacing or reprogramming the glow plug control module.
- Check and Replace Fuses: Ensure that all fuses related to the glow plug circuit are intact; replace any that are blown.
Cost Estimates
The cost of repairs associated with a P0678 trouble code can vary widely based on labor rates and specific vehicle models:
- Glow Plug Replacement: $100 – $300 per plug (including parts and labor).
- Glow Plug Relay Replacement: $50 – $150 depending on vehicle type and labor costs.
- Wiring Repairs: Costs can range from $50 – $200 based on complexity and parts needed.
- Control Module Replacement: This can be more expensive, ranging from $200 – $600 depending on vehicle make and model.
Warnings and Recommendations
When dealing with a P0678 trouble code:
- Do Not Ignore Symptoms: While it may be possible to drive with this code present temporarily, doing so can lead to further engine damage or increased emissions.
- Seek Professional Help When Necessary: If you are not comfortable diagnosing or repairing electrical issues yourself, consult with a qualified mechanic.
- Use Quality Parts: Always opt for OEM (Original Equipment Manufacturer) parts when replacing components like glow plugs or relays to ensure compatibility and reliability.
Frequently Asked Questions About P0678
- What does error code P0678 mean?
P0678 indicates a malfunction in the glow plug circuit for cylinder 8 in diesel engines.