P0678 — Cylinder 8 Glow Plug Circuit/Open

The OBD-II trouble code P0678 indicates an issue with the glow plug circuit for cylinder 8. Specifically, it suggests that the electrical circuit controlling the glow plug for this cylinder is either open, or experiencing a failure, preventing proper operation. This code is primarily related to diesel engines that employ glow plugs to aid cold starting and emissions control. Keep in mind that labels for this code may vary between different vehicle manufacturers, which could influence how the fault is displayed in the diagnostic system.

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Manufacturer variations for trouble code P0678

Some vehicle manufacturers may identify this fault with different labels or code descriptions, but the core issue remains the same: an open circuit or failure in the glow plug circuit for cylinder 8. Variations can sometimes include different alphanumeric codes or additional fault codes linked to related glow plug circuits. Recognizing these variations can be essential for accurate diagnosis and repairs.

• Volkswagen / Audi / Skoda / Seat – P0678
• Mercedes-Benz – MSC codes or P0320 series
• Ford – may use similar codes with different prefix
• Other OEMs – often retain P0678 or similar, but always consult vehicle-specific documentation

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What does trouble code P0678 mean?

This code signals that the Engine Control Unit (ECU) has detected a problem with the glow plug circuit for cylinder 8. In diesel engines, glow plugs are used to preheat the combustion chamber during cold starts to ensure reliable ignition. Sensors monitor the voltage and current in the glow plug circuit. When the ECU detects an open circuit—meaning the circuit is broken or not completing properly—it triggers this fault code.

Typically, this fault is triggered during engine start-up when the glow plug for cylinder 8 either fails to heat properly or its circuit is disrupted. Environmental factors such as cold weather and engine age can increase the likelihood of this fault appearing.

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Severity and risks of trouble code P0678

This fault is considered moderate to high severity because it can impair the engine’s cold start performance. The most immediate risk involves difficulty starting the vehicle, especially in cold weather, which may lead to prolonged cranking times or failed starts. Over time, ignoring the issue could lead to increased emissions and potential damage to the glow plug driver circuit if the component is left unrepaired.

While the vehicle can often still be driven with this fault present, doing so regularly is not advisable. The code does not usually result in severe engine damage, but it can compromise emission controls and starting reliability.

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Symptoms of trouble code P0678

When this code is active, drivers may notice certain symptoms reflecting starting and engine performance difficulties. Such symptoms can include:

• Hard starting in cold weather — prolonged cranking or failure to start on first attempt
• Rough idling or rough running immediately after startup
• Increased emissions during cold start cycles
• Malfunction Indicator Lamp (MIL) illuminated on the dashboard
• Glow plug warning light may be active in some vehicle configurations

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Most likely causes of trouble code P0678

Diagnosing P0678 involves understanding the underlying causes that trigger this fault. The most common reasons for this fault occurring include:

1. Open circuit or damaged wiring in the glow plug harness leading to cylinder 8
2. Faulty glow plug itself, which may be broken or damaged internally
3. Failure of the glow plug relay or driver module in the ECU or fusebox
4. Corrosion or poor electrical contact at the connector or terminal points
5. ECU software or calibration issues, less common but possible

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How to diagnose trouble code P0678

Diagnosing this fault thoroughly requires a systematic approach. Here are general steps suitable for most technicians, irrespective of vehicle manufacturer:

1. Visual inspection of the glow plug wiring harness, connectors, and associated components for damage, corrosion, or disconnection.
2. Check fuses and relays related to glow plug circuits, ensuring they are intact and functioning properly.
3. Use a scan tool to observe live data — monitor the glow plug circuit voltage and current for cylinder 8 during engine start.
4. Perform continuity tests on wiring harnesses and connectors between the ECU and the glow plug to identify open circuits or shorts.
5. Test the glow plug directly by removing it and measuring its resistance to verify if it’s within manufacturer specifications.
6. Check the glow plug driver/module for proper operation, especially if wiring and plugs test good.
7. Clear the fault codes and witness whether the code returns after testing or driving further.

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Possible repairs for trouble code P0678

The solutions will depend on the identified cause, but common repairs include:

• Replacing a faulty glow plug — a relatively basic repair requiring mechanical skill, typically performed by a professional.
• Repairing or replacing damaged wiring or connectors — often involves soldering or connector replacement, best done by a qualified technician.
• Replacing faulty glow plug relays or driver modules — may require ECU or fusebox access, best performed by professionals.
• Addressing corrosion or poor connections — cleaning terminals and securing connections can often resolve intermittent faults.
• Updating ECU software — in rare cases, reprogramming can correct anomalies leading to false fault detection.

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Vehicles commonly associated with trouble code P0678 in Europe

This fault is common among diesel-powered vehicles equipped with glow plug systems for cylinder 8. In Europe, some make and model groups often affected include:

Volkswagen Group (Volkswagen, Audi, Skoda, Seat) — especially their diesel engines with individual glow plug circuits.

Specific engine families, such as common rail diesel engines, are more prone due to their complex glow plug circuitry. Always consult the vehicle-specific technical data for exact information.

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Frequent mistakes with trouble code P0678

When diagnosing and repairing this fault, some common errors can hinder proper resolution. To avoid these pitfalls, be aware of the following:

• Assuming the glow plug is always the fault without checking wiring or the driver circuit first.
• Neglecting to inspect related fuses and relays that could be faulty or blown.
• Overlooking connector corrosion or poor grounding which can cause intermittent faults.
• Replacing parts prematurely without a proper testing process, leading to unnecessary costs.
• Failing to clear the fault codes and confirm the repair before concluding the diagnosis.

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FAQ — trouble code P0678

Can this code disappear on its own?

It’s unlikely that P0678 will disappear without repair unless the underlying cause is intermittent or environmental conditions change. Usually, the fault persists until addressed.

Can I keep driving?

While many vehicles can operate with this fault, it is not advisable to drive long-term without repairs. It may lead to starting problems and increased emissions.

Why does the code return after clearing?

This indicates that the underlying fault—be it wiring, glow plug, or driver circuitry—is still present. The code will often come back until the root cause is fixed.