P0678 — Cylinder 8 Glow Plug Circuit

OBD2 code P0678 flags a problem with the diesel glow plug system (cylinder 8 glow plug circuit). The car is usually drivable while you plan the repair. Common culprits: damaged glow plug wiring or connector, corroded glow plug connectors, blown fuse in the glow plug circuit.

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 P0678 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 P0678 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P0678 (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 P0678 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 P0678 mean?

Will P0678 clear itself?

Sometimes P0678 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 P0678?

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 is the most common cause of P0678?

The most common causes are: damaged glow plug wiring or connector, corroded glow plug connectors, blown fuse in the glow plug circuit.

How much does it cost to fix P0678?

Replace the glow plug relay/module — $150–$450; Replace a full set of glow plugs — $250–$600; 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.

Diesel engines rely on glow plugs to fire up efficiently, especially when temperatures drop. P0678 is a powertrain diagnostic trouble code that specifically flags a problem with the heating element in cylinder 8. When the engine control unit runs its self-check routine, it watches the glow plug circuit closely. If the voltage, current, or resistance falls outside the factory-defined window, P0678 gets stored in memory alongside a freeze frame snapshot of engine conditions at that exact moment. The code tells you something is wrong in the cylinder 8 heating circuit—but it does not immediately say which part is the culprit.

Glow plugs are threaded into each cylinder's combustion chamber. They heat up red-hot in seconds, turning compression and fuel into a workable starting mixture. On inline engines, cylinder 8 sits at the far end from the belt drive. On V-configuration engines, numbering varies by manufacturer, so consulting your firing-order diagram is the right move before spending money on parts you cannot easily reach. The diagnostic challenge with P0678 is that a failed plug, a bad relay, corroded terminals, or a blown fuse can all trigger the same code. Reading the freeze frame data first narrows the field considerably.

Symptoms of P0678

P0678 announces itself most clearly on cold mornings. The engine may crank longer than usual, sputter through a rough idle, or produce noticeable white smoke from the exhaust as unburned fuel passes through the cylinder that cannot heat itself. The glow plug warning lamp on the dash often stays illuminated after the usual pre-heat cycle finishes. A persistent Check Engine Light accompanies the issue, and until the block reaches operating temperature the idle quality suffers noticeably. In milder climates the problem may appear only intermittently, which makes it easy to ignore—until winter arrives and the truck refuses to start in the driveway.

Causes of P0678

The most frequent offender is the glow plug relay or the control module that manages the pre-heat sequence. These components carry high current loads and eventually wear out or develop internal resistance. Corroded connectors at the glow plug harness are equally common, especially in regions where road salt is used during winter months. A blown fuse in the glow plug supply circuit will cut power entirely and set P0678 instantly. Wiring that has chafed through from heat, vibration, or animal damage creates open or shorted circuits. Finally, the glow plug itself can fail internally, reading infinite resistance at the meter and delivering no heat to cylinder 8.

How to Diagnose P0678

Before buying parts, follow a systematic path to confirm what P0678 is actually telling you.

Plug in an OBD2 scanner, retrieve P0678, and record the associated freeze frame data. Note the RPM, load percentage, coolant temperature, and vehicle speed recorded when the fault occurred—this guides your inspection toward the right conditions.
Scan for any related diagnostic trouble codes. Secondary codes often point directly to the failing component. Also look up factory technical service bulletins for your specific make and model; many glow plug failures have published software corrections or updated parts.
Physically examine the wiring harness and all connectors feeding the glow plug circuit. Pull back loom tape where necessary and look for broken strands, melted insulation, green corrosion buildup, or loose terminals that create high resistance.
With the engine cold, measure the resistance of each glow plug individually. A healthy plug reads between 0.5 and 2 ohms. Infinite resistance means an open heating element.
During the pre-glow cycle, use a multimeter to verify battery voltage arrives at the glow plug rail. No voltage indicates a relay or fuse problem; low voltage suggests corroded connectors or loose ground.
After repairs, clear P0678 and complete a full drive cycle that reproduces the conditions captured in the freeze frame. The code should stay absent. If it returns, the root cause has not been addressed.

Repair Options for P0678

Tackle the inexpensive possibilities before replacing major assemblies.

Replace the glow plug relay or control module — $150 to $450 for parts and labor
Replace the entire glow plug set — $250 to $600 for all plugs plus labor
Replace a single glow plug — $80 to $200 depending on plug brand and labor time

How Much Does It Cost to Fix P0678?

Parts alone represent a small fraction of the bill on many diesel models. Labor to access cylinder 8 varies dramatically depending on engine bay layout; some applications require dropping the intake manifold or removing the valve cover, adding hours to the job. At independent shop rates, the full range for P0678 resolution falls between $80 and $600. The most common scenario is a faulty relay or corroded connector—neither requires expensive components. Confirming the actual failure point with a multimeter before ordering parts prevents unnecessary spending on a full plug set when a $30 relay is the real fix.

Can You Drive with P0678?

Once the engine warms up, driving is generally safe. The glow plugs only matter during the starting sequence. However, a cylinder 8 plug that reads open means one chamber contributes no heat. On frigid mornings with multiple failing plugs, the engine may not start at all or will idle roughly until coolant temperature climbs. Addressing P0678 before cold weather returns is the practical call—waiting until the first hard freeze turns a simple relay replacement into a tow-truck bill.