P068F — Cylinder 8 Glow Plug Control Circuit High

OBD2 code P068F flags a problem with the diesel glow plug system (cylinder 8 glow plug control circuit high). The car is usually drivable while you plan the repair. Common culprits: damaged glow plug wiring or connector, burned-out glow plug, faulty glow plug relay or control module.

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 P068F 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 With the cylinder 8 glow plug control disconnected, measure signal voltage at the harness connector: a high reading with the component unplugged indicates a short to power in the harness; if it drops to normal, the component itself is faulty.
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 P068F does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P068F (parts and labor, USD, independent shop rates): Replace the glow plug relay/module: $150–$450; Replace a full set of glow plugs: $250–$600; Replace one glow plug: $80–$200. Overall, fixing P068F 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

What does OBD2 code P068F mean?

How do I diagnose P068F myself?

Start with a scan: confirm P068F, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the cylinder 8 glow plug control, and verify the suspect component against live data before buying parts.

Is it safe to drive with P068F?

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 are the symptoms of P068F?

Drivers most often notice: glow plug warning light staying on, rough idle until warmed up, white smoke after a cold start, hard starting in cold weather.

Will P068F clear itself?

Sometimes P068F 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.

When your vehicle's engine control module detects an abnormal voltage condition in cylinder 8's glow plug control circuit, it logs trouble code P068F. This specific fault belongs to the powertrain category, which covers issues affecting the engine and transmission systems. In simple terms, P068F tells you that the voltage on the control wire for cylinder 8's glow plug is reading higher than it should—typically hovering near battery voltage instead of the lower switching voltages the module expects.

Diesel engines rely on glow plugs to heat the combustion chamber during cold starts. The powertrain control module manages a pre-glow sequence, energizing the plugs for a few seconds before cranking. Each cylinder has its own control circuit; when the module commands cylinder 8's plug on but sees an unexpectedly high voltage, it flags P068F. This isn't a one-shot detection—the module must observe the fault across multiple drive cycles under specific conditions before illuminating the Check Engine Light.

Understanding cylinder numbering matters for diagnosis. On inline engines, cylinder 8 sits at the flywheel end, farthest from the belt. V-type engines vary by manufacturer, so consulting your specific firing-order diagram prevents misdiagnosis. The "circuit high" designation points toward three general possibilities: the signal wire is shorted to a constant power source, the ground return path has opened, or the glow plug control circuit itself has failed in a way that pulls the line high. The freeze frame data stored alongside P068F records engine load, RPM, coolant temperature, and vehicle speed at the moment of failure—use those conditions as your testing baseline.

Common Symptoms of P068F

Check Engine Light remains illuminated after engine start
Glow plug warning indicator stays on the dashboard
Engine runs roughly during warm-up, smoothing out once operating temperature is reached
Visible white smoke billowing from the tailpipe during cold starts
Difficulty starting the engine, especially when ambient temperatures drop below freezing
Cylinder 8 contributes to misfire symptoms until the combustion chamber reaches working temperature

What Causes P068F

The root cause typically falls into one of several categories, ranging from simple connector issues to component failure:

The cylinder 8 signal wire has shorted directly to battery positive voltage
The ground return path for the cylinder 8 glow plug control circuit is open or broken
The cylinder 8 glow plug control circuit has failed internally, forcing the signal high
Corrosion or moisture intrusion has compromised the glow plug electrical connectors
The glow plug itself has burned out, creating an open circuit condition
Wiring insulation has chafed through, exposing the copper conductors to shorting
The glow plug relay or control module has developed an internal fault
A fuse protecting the glow plug circuit has blown due to overcurrent conditions

How to Diagnose P068F

A systematic approach prevents unnecessary part replacements and ensures the actual fault is identified:

Connect an OBD2 scan tool and verify P068F is stored as an active or pending code. Record the freeze frame data completely—this snapshot reveals the exact engine conditions present when the module first detected the problem.
Pull all stored codes from the module. Related codes often point toward the primary fault; for example, multiple cylinder glow plug codes might indicate a system-wide issue rather than a single component failure. Search for technical service bulletins specific to your vehicle make, model, and model year—manufacturers publish software updates and known fixes for recurring code patterns.
Perform a thorough visual inspection of the glow plug wiring harness for cylinder 8. Look for damaged insulation, melted wire jackets from heat exposure, corroded pins in the electrical connectors, and any signs of previous repairs that may have been completed incorrectly.
With the cylinder 8 glow plug control connector unplugged from the component, measure voltage at the harness side of the connector. A reading near battery voltage confirms a short-to-power condition in the wiring itself. If the voltage drops to normal levels with the connector separated, the glow plug or its internal circuit is the culprit.
Test individual glow plug resistance using a digital multimeter. A healthy glow plug shows a resistance between 0.5 and 2 ohms depending on design. Infinite resistance indicates an open filament—confirm this by testing an identical known-good plug for comparison.
During the pre-glow phase (key on, engine off), verify voltage present at the glow plug rail feed. This confirms the relay or control module is actually energizing the circuit. No voltage here suggests a failed relay, blown fuse, or power supply issue upstream.
After addressing the suspected cause, clear the code and complete a full drive cycle that reproduces the freeze frame conditions—typically a cold start followed by normal driving. Only clear the code after repairs are complete, and confirm the Check Engine Light does not return.

Repair Options and Costs

Component-level diagnosis determines which parts require replacement. The following price ranges reflect typical parts and labor costs at independent shops:

Glow plug relay or control module replacement: $150–$450
Complete set of glow plugs (typically 4–8 per engine depending on cylinder count): $250–$600
Single cylinder 8 glow plug replacement: $80–$200

Start with the least expensive possibilities first. Inspect connectors and wiring before committing to part replacement. On many diesel engines, accessing cylinder 8 requires removing the intake manifold or valve cover, which adds significant labor time—factor this into your repair estimate.

Overall, resolving P068F typically costs between $80 and $600 depending on which component fails. An accurate diagnosis before purchasing parts prevents spending $600 on a new glow plug set when only a $15 fuse needed replacement.

Driving Safety Verdict

Once the engine reaches operating temperature, a vehicle with P068F stored is generally safe to operate. The glow plug system only functions during cold starts, so driving characteristics remain normal after warm-up. The practical concern centers on cold-weather reliability: if multiple glow plugs have failed, the engine may struggle to start or fail to crank entirely on frosty mornings. Addressing this code before winter eliminates the risk of being stranded. Until repairs are completed, keep jumper cables accessible and consider garage parking in cold climates.