P0679 — Cylinder 9 Glow Plug Circuit/Open

OBD-II trouble code P0679 indicates a problem with the glow plug circuit for cylinder 9 in a vehicle equipped with a diesel engine. In simple terms, this code points to an electrical issue where the control module has detected a fault in the glow plug’s circuit or its wiring, preventing proper heating of the cylinder for cold starts.

Note: Various vehicle manufacturers may label this code differently, and the specific terminology can vary. Always refer to the manufacturer’s diagnostic guide for exact details.

Fast Facts

System affected: Glow plug circuit for cylinder 9
Severity: Moderate
Main symptoms: Hard starting, rough idle, increased emissions
Driveability: Limited (may affect cold start performance but not necessarily prevent driving)

Manufacturer variations for trouble code P0679

Different vehicle manufacturers may assign different labels to the same fault code based on their coding systems or technical terminologies. Below are some known variations for P0679:

Ford / Mazda: P0679 — Cylinder 9 Glow Plug Circuit/Open
Volkswagen / Audi / Seat / Skoda: Usually integrated under generic glow plug circuit codes or specific to cylinder numbers.
Mercedes-Benz: Codes related to glow plug circuit issues, often with specific suffixes.
BMW: Similar glow plug circuit fault codes, sometimes with different code numbers but referencing cylinder 9.

What does trouble code P0679 mean?

The Engine Control Unit (ECU), or more broadly the vehicle’s engine control system, constantly monitors the **glow plug circuits**—a set of heating elements that aid in starting diesel engines, especially when cold. P0679 is triggered when the ECU detects an abnormality in the electrical circuit of the **glow plug associated with cylinder 9**.

Specifically, the code is set off when the ECU senses an **open circuit** (no electrical continuity) or a **short circuit** (abnormal electrical connection) in the wiring, relay, or glow plug itself for that cylinder. This could mean the glow plug isn’t functioning, the wiring is damaged, or the relay controlling the circuit is faulty.

This fault often becomes noticeable during cold starts, as the vehicle struggles to start or requires more cranking time. Since the glow plug’s role is critical in ensuring a smooth cold start for diesel engines, issues here can affect starting reliability and emissions.

Severity and risks of trouble code P0679

The presence of P0679 is a **moderate risk**. While it doesn’t typically cause immediate engine failure, ignoring it can lead to **difficult starting**, increased emissions, and potential damage to the glow plug or wiring if the fault persists. Extended difficulty in cold starts could stress the starter motor or affect overall engine performance.

**Can the vehicle be driven?**—Yes, but with caution. The vehicle can usually still be operated, but the driver might experience starting issues, and emissions may be higher until the problem is addressed. If the vehicle exhibits persistent starting problems or the check engine light is on, it is advisable to seek diagnosis and repair promptly.

Symptoms of trouble code P0679

The primary signs of this trouble code are related to **cold start difficulties**. The vehicle may exhibit:

Hard starting or extended cranking time during cold conditions
Rough idle immediately after starting
Engine misfires in some cases
Increased exhaust emissions during cold start phases
Illumination of the check engine light on the dashboard

In many cases, a driver might notice that the vehicle starts more readily when warm, but struggles to start after overnight parking or in cold weather.

Most likely causes of trouble code P0679

Understanding the probable causes helps in diagnosing this fault efficiently. The causes are typically related to electrical or component failures in the glow plug circuit, with the most common being:

Faulty glow plug for cylinder 9 — The glow plug itself has failed and needs replacement.
Damaged or corroded wiring or connectors — The wiring leading to the glow plug may be broken, frayed, or corroded, disrupting the circuit.
Blown relay or faulty fuse — The relay controlling the glow plug circuit or associated fuse may be defective, preventing power from reaching the glow plug.
ECU or PCM malfunction — In rare cases, a fault within the engine control module can trigger false readings or fail to command the glow plug circuit correctly.
Incorrect wiring installation — After repairs or modifications, wiring may have been connected improperly, causing the circuit fault.

How to diagnose trouble code P0679

Effective diagnosis involves a systematic approach. Here are the generic steps to identify the root cause:

Visual inspection: Check all wiring, connectors, and relays related to the glow plug circuit for physical damage, corrosion, or disconnection.
Test the glow plug: Using a multimeter, verify the resistance of the glow plug (typically a few ohms). Replace if faulty.
Check the wiring continuity: Perform continuity tests on wiring harnesses leading to the glow plug to ensure there are no open circuits.
Test the relay: Swap the relay controlling the glow plug circuit with a known-good one or test it directly to verify operation.
Inspect the fuse: Ensure the fuse associated with the glow plug circuit is intact.
Monitor live data: Using a diagnostic scanner, observe the glow plug circuit status while the ignition is on and see if the ECU commands the glow plug to energize for cylinder 9.
Scan for additional codes: Check if other related faults are present that could affect the circuit or indicate broader issues.

Possible repairs for trouble code P0679

Repairs depend on the diagnosed cause. Below are common solutions, which may require a professional mechanic for proper execution:

Replace the faulty glow plug: If the glow plug for cylinder 9 tests as defective, install a new one—this is a relatively straightforward replacement task.
Repair or replace damaged wiring or connectors: Correct any issues found during the visual and continuity inspections, such as repairing broken wires or cleaning corroded connectors.
Replace the relay or fuse: If the relay or fuse is faulty, replace them with OEM (Original Equipment Manufacturer) parts.
Update or reprogram the ECU: In rare cases, a software update or reset of the control module may be necessary if a software glitch is suspected.
Ensure proper wiring installation: Correct any wiring errors after repairs to prevent recurrence of the fault.

Vehicles commonly associated with trouble code P0679 in Europe

This code is commonly found in diesel-powered vehicles, especially those with electronically controlled glow plug systems. Typical models include:

Many European brands and models with Common Rail diesel systems are susceptible, but specific data is not always publicly available. General examples include:

Volkswagen Group (Volkswagen, Audi, Skoda, Seat) diesel engines with CDI or TDI systems
Ford Transit and Ford passenger vehicles with diesel engines
Mercedes-Benz diesel models with electronically controlled glow plugs
BMW diesel engines with advanced glow plug management systems

Frequent mistakes with trouble code P0679

Diagnosing and repairing this fault requires care. Common mistakes include:

Assuming the glow plug is always at fault—Always perform electrical tests before replacing components.
Overlooking wiring and relay issues—Electrical problems often stem from wiring or relays rather than the glow plug itself.
Ignoring the importance of proper diagnosis—Jumping straight to component replacement without verifying the actual cause can lead to unnecessary expenses.
Working on the circuit without disconnecting the battery—This can cause further electrical issues or damage.
Not clearing the codes after repairs—Failure to clear the fault memory can lead to persistent warning lights even after fixing the issue.

FAQ — trouble code P0679

Can this code disappear on its own?

While some fault codes may clear after a system reset or minor repair, P0679 usually persists until the faulty component or wiring issue is addressed. If the problem caused a fault, it’s unlikely to resolve spontaneously without intervention.

Can I keep driving?

Generally, yes, but with caution. The vehicle might still start and run, but starting could be more difficult in cold weather, and emissions may increase. It’s advisable to diagnose and repair the issue promptly to prevent further risks.

Why does the code return after clearing?

The code returns if the root cause isn’t fixed, such as a damaged glow plug, wiring, or relay. The ECU continuously monitors the circuit, so unless the underlying fault is corrected, the code will reappear.