P0679 — Cylinder 9 Glow Plug Circuit

P0679 — Cylinder 9 Glow Plug Circuit. A fault in the diesel glow plug system. The car is usually drivable while you plan the repair. Typical causes: blown fuse in the glow plug circuit, damaged glow plug wiring or connector, corroded glow plug connectors.

powertrain (engine/transmission)

Is it safe to drive?

Short-term driving is OK Safe to drive once the engine is started and warmed up. The main risk is cold-weather starting: with several dead plugs the engine may not start on a cold morning. Fix before winter.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm P0679 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many P0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the diesel glow plug system for damage, corrosion, chafing or poor contact.
Subsystem check Measure resistance of each glow plug — an open plug reads infinite resistance.
Subsystem check Check voltage at the glow plug rail during the pre-glow phase.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm P0679 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P0679 (parts and labor, USD, independent shop rates): Replace a full set of glow plugs: $250–$600; Replace the glow plug relay/module: $150–$450; Replace one glow plug: $80–$200. Overall, fixing P0679 usually costs between $80 and $600. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

How much does it cost to fix P0679?

Replace a full set of glow plugs — $250–$600; Replace the glow plug relay/module — $150–$450; Replace one glow plug — $80–$200. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

Will P0679 clear itself?

Sometimes P0679 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.

Is it safe to drive with P0679?

Safe to drive once the engine is started and warmed up. The main risk is cold-weather starting: with several dead plugs the engine may not start on a cold morning. Fix before winter.

What does OBD2 code P0679 mean?

How do I diagnose P0679 myself?

Start with a scan: confirm P0679, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the diesel glow plug system, and verify the suspect component against live data before buying parts.

When your diesel engine stores code P0679, it is reporting a specific problem within the glow plug circuit dedicated to cylinder 9. This is not a generic engine issue — the powertrain control module has pinpointed an electrical fault in one precise location. The module runs multiple checks on this circuit during operation and only logs P0679 after confirming the abnormality across several drive cycles, which filters out random electrical noise or temporary sensor glitches.

Diesel engines rely on glow plugs to heat the combustion chamber before and during startup, especially in cold conditions. Each cylinder that requires pre-heating has its own individual plug threaded directly into the cylinder head. On inline engines, cylinder numbering begins at the front of the block (the belt end). On V-configuration engines, the firing order determines cylinder numbering, which varies between manufacturers. This matters for repairs because some cylinders sit deep within the engine bay or behind components that must be removed first, directly affecting labor time and final cost.

The control module continuously watches the glow plug system while the engine runs. If the electrical resistance or voltage reading for cylinder 9 falls outside the factory-defined acceptable range, the module saves P0679 and illuminates the check engine light. It also captures freeze frame data — a snapshot recording engine speed, load percentage, coolant temperature, and vehicle speed at the exact moment the fault occurred. This data is your best starting point when you connect a scanner, because it tells you under what conditions the problem appeared.

Symptoms of P0679

Cylinder 9 may fail to heat properly, and drivers typically notice these issues first:

Engine cranks slowly or struggles to start when temperatures drop below freezing
Check engine light remains illuminated after starting
Engine runs roughly or misfires during the warm-up period until operating temperature is reached
White smoke billows from the exhaust pipe following a cold start, indicating unburned fuel due to poor combustion
The glow plug indicator on the dashboard stays lit longer than normal or fails to turn off
Reduced fuel efficiency becomes noticeable over multiple tanks of fuel

Common Causes of P0679

The fault rarely appears without an underlying electrical issue. Here are the most probable culprits, ordered from most to least frequent:

A burned-out glow plug in cylinder 9, which reads as infinite resistance when tested
A damaged wire harness or loose connector feeding the cylinder 9 plug, often from heat cycling or road debris
A blown fuse protecting the glow plug circuit, which disables the entire bank
Corrosion or moisture intrusion at the electrical connector, creating high resistance
A failing glow plug control module or relay, which may affect multiple cylinders simultaneously
One or more additional bad glow plugs in the same cylinder bank

How to Diagnose P0679

Follow these steps in order to isolate the root cause before spending money on parts:

Connect an OBD2 scanner, retrieve code P0679, and record all freeze frame data. This snapshot tells you engine conditions when the fault triggered and helps reproduce the problem during your road test afterward.
Scan for any other stored trouble codes. Related codes — especially those in the P0600 range or other glow plug circuit codes — often point to the actual root cause rather than the single cylinder fault.
Visually inspect the wiring harness and connectors for cylinder 9. Look for melted insulation, broken wires, corrosion buildup, or connectors that are not fully seated. Press and pull gently to check for intermittent contact.
Test the resistance of the cylinder 9 glow plug using a digital multimeter. A functioning plug shows a low resistance reading typically between 0.5 and 2 ohms. Infinite resistance confirms a failed open plug.
Check for battery voltage at the glow plug feed wire during the pre-glow cycle with a multimeter or test light. No voltage indicates a blown fuse, bad relay, or wiring break upstream.
Clear the code, complete a drive cycle that reproduces the conditions from your freeze frame data, and verify that P0679 does not return. If the code reappears after clearing, the fault is confirmed and not intermittent.

Repair Options and Costs for P0679

Start with inexpensive fixes before replacing major components. Common repair scenarios for P0679, including parts and labor at independent shop rates in USD:

Replace one failed glow plug: $80–$200
Replace the glow plug control relay or module: $150–$450
Replace the entire glow plug set on the affected bank: $250–$600

The total cost to repair P0679 typically falls between $80 and $600 depending on which component actually failed. An accurate electrical diagnosis before purchasing parts prevents unnecessary spending — the priciest scenario is rarely the first place to look.

Can You Drive with P0679?

Yes, you can operate the vehicle once the engine reaches normal operating temperature. The glow plug system only functions during cold starts, so a warm engine runs on its remaining good cylinders. However, the practical concern is winter driving: if multiple glow plugs are failing, the engine may refuse to start on cold mornings. Address P0679 before the heating season arrives, particularly if you live in a cold climate or notice the hard-starting problem worsening.