P0679 — Cylinder 9 Glow Plug Circuit/Open #

Note: The wording of this DTC may vary depending on your vehicle’s manufacturer. Always refer to your OEM (Original Equipment Manufacturer) service information for exact definitions.

• Cylinder 9 Glow Plug Circuit/Open (General)
• Glow Plug Circuit Malfunction, Cylinder 9 (Ford, GM)
• Cylinder 9 Heater Plug Circuit Fault (Mercedes-Benz)
• No. 9 Glow Plug Control Circuit Open (Dodge/RAM)

Summary of DTC P0679 #

The P0679 code indicates that the engine control module (ECM) or powertrain control module (PCM) has detected an open circuit or malfunction in the glow plug circuit for cylinder 9. Glow plugs are heating elements found in diesel engines, designed to preheat air in the combustion chamber for easier cold starts. When the ECM commands the glow plug relay to activate, it monitors the circuit of each individual glow plug. If it senses that the electrical resistance or response from cylinder 9’s glow plug is abnormal—typically indicating an open circuit—P0679 is set. This code is specific to diesel engines equipped with at least nine cylinders, often found in heavy-duty trucks or commercial vehicles.

Common Symptoms for DTC P0679 #

A vehicle with a P0679 code may show few symptoms in warm weather but can become frustratingly stubborn to start in cold conditions. The glow plug system is crucial for efficient cold starts; when one cylinder’s glow plug fails, you may experience rough running or extended cranking. In severe cases, the engine may misfire or emit white smoke at startup.

• Hard starting, especially in cold weather
• Rough idle on cold starts
• White exhaust smoke during initial startup
• Engine misfire during warm-up
• Illuminated check engine light
• Persistent glow plug warning light (on some vehicles)

Probable Causes for DTC P0679 #

Several issues can trigger the P0679 code, but some are more common than others. The most frequent offenders relate to the glow plug itself or its electrical circuit, but faults can also stem from control modules or wiring damage. Here’s a prioritized list based on frequency:

1. Failed glow plug in cylinder 9 (burned out or open circuit)
2. Damaged or corroded wiring to cylinder 9’s glow plug
3. Faulty glow plug relay or control module
4. Poor electrical connection at the glow plug harness
5. ECM/PCM issue (rare)

Step-by-Step Diagnostic for P0679 #

Diagnosing a P0679 code involves careful testing of the affected cylinder’s glow plug circuit. Since diesel engines rely on a precise balance of temperature and timing for smooth starts, even a minor issue can cause significant symptoms. Follow these steps for a systematic approach:

• 1. Verify the code with a professional scan tool and check for related codes (such as other glow plug DTCs).
• 2. Inspect the glow plug fuse and relay for proper operation.
• 3. Visually check the wiring harness leading to cylinder 9’s glow plug for signs of damage, melting, or corrosion.
• 4. Test the glow plug’s resistance at cylinder 9 using a digital multimeter. (Most OEM glow plugs should measure between 0.5 and 2 ohms; check your manufacturer’s specification.)
• 5. Check for voltage at the glow plug connector when the system is activated (usually during pre-glow or right after key-on, engine off).
• 6. Test the control module or relay output for correct operation, if all wiring and the glow plug itself check out.
• 7. Clear the code after any repair and retest the system under similar conditions (preferably cold start).

Possible Repairs for P0679 #

Once you’ve identified the source of the P0679 fault, focus on restoring the proper function of cylinder 9’s glow plug circuit. Repairs should always address the root cause to avoid repeat failures. Solutions may include:

• Replacing the failed glow plug in cylinder 9
• Repairing or replacing damaged wiring or connectors
• Cleaning corroded terminals at the glow plug or harness
• Replacing a faulty glow plug relay or control module
• In rare cases, reprogramming or replacing the ECM/PCM

Is It Safe to Drive with P0679? #

Limited— You can usually drive with a P0679 code, especially in warm weather, but cold starts will be rough and may become impossible in freezing conditions. Prolonged driving with a faulty glow plug may lead to increased emissions, rough running, and potential engine wear due to poor combustion. For commercial or work vehicles, prompt repair is strongly advised to avoid downtime and potential no-starts.

Vehicles Most Commonly Affected by P0679 #

P0679 is most often found on large diesel engines with nine or more cylinders, although some modular V8 or V10 diesel platforms may also use this numbering. It is especially prevalent in certain heavy-duty trucks and commercial vehicles.

• Ford F-Series Super Duty (F-650, F-750 with V10 diesel engines)
• Mercedes-Benz Sprinter (V6/V8/V12 diesels, rare)
• RAM 4500/5500 (with optional high-cylinder diesels)
• Freightliner, International, or GMC trucks with large-displacement diesels

Common Mistakes & Best Practices for P0679 #

Misdiagnosis is surprisingly common with glow plug circuit codes, often due to overlooked wiring or improper testing methods. To avoid unnecessary parts replacement or repeat failures, keep these tips in mind:

• Don’t replace all glow plugs unless they all test faulty—only address cylinder 9 unless others are also out of spec.
• Always check wiring and connectors for corrosion or damage before installing new glow plugs.
• Use a trusted digital multimeter and compare your readings to OEM specifications.
• Clear codes and confirm repair after a cold start cycle, not just at idle.
• If multiple glow plug codes are present, suspect a relay or module rather than multiple simultaneous plug failures.

FAQ Express: P0679 #

Here are quick answers to the most common questions about P0679:

• Q: Can I ignore a glow plug code in summer?
  A: You might not notice symptoms in warm weather, but the problem will return when temperatures drop. Fixing it now prevents future no-starts.
• Q: Will P0679 cause long-term engine damage?
  A: Persistent misfires or rough running can increase emissions and cause wear over time. While not immediately catastrophic, it’s best to repair promptly.