OBD-II trouble code
P0678: Cylinder 8 Glow Plug Circuit
A fault on the cylinder 8 glow plug channel. The part is a cheap consumable; on a V8 diesel the access is the entire bill. That imbalance is what makes the do-the-whole-set decision different here than it is on a small engine — here it is usually the cheaper choice, not the extravagant one.
Quick facts
- System
- Powertrain
- Category
- Auxiliary Emissions Controls
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $90 – $2,200
- DIY difficulty
- Advanced DIY
What does P0678 mean?
P0678 tells you the glow plug controller did not get the current it expected from the cylinder 8 channel. The electrical fault behind it is ordinary. The economics are not, and that is what this page is about.
Cylinder 8 lives on a V8, V10 or V12 diesel — light trucks, heavy-duty pickups, large SUVs and vans, and a handful of luxury saloons. On most of them, cylinder 8 is a **rearmost plug on one of the two heads**, which is to say it is at the back of a physically large engine, in the narrow space between the head and the bulkhead or the cab, frequently underneath the turbocharger mounting, an EGR crossover pipe, a heat shield, or a section of intake plumbing.
What that means in practice is worth stating plainly. **The glow plug itself is one of the cheapest parts on a diesel engine — often twenty to fifty dollars.** Getting to the one at the back of a V8 can take a professional several hours, and on some heavy-duty applications the recommended route involves removing enough of the top of the engine that the labour dwarfs every other line on the invoice. A repair that would be a coffee-break job on the front cylinder of a small four becomes a half-day or worse.
That imbalance flips a decision that is genuinely optional on smaller engines. On an easy-access engine, replacing the whole set when one plug fails is a nice-to-have. **Here it is normally the cheaper path over any reasonable ownership horizon**, because every plug on the engine has taken the same number of heat cycles as the one that just failed, and the labour to reach them is being paid for right now, once. Paying that teardown twice in eighteen months because you replaced a single plug is the expensive outcome, not buying eight plugs.
There is a caveat that keeps this honest: **prove the element is the problem before committing to the teardown.** A corroded terminal, a chafed branch wire or a failed controller output produces exactly the same code and none of them requires removing the plug. Measure resistance at the plug's own terminal, check the feed under load, and inspect the connector first. Once the element is confirmed open or badly drifted, the set argument applies. Before that, it is just an expensive guess.
Finally, a small point that matters disproportionately on a rear plug. Glow plugs are slender and thread into aluminium or cast iron with a modest torque specification, and the temptation when working blind, at arm's length, with a swivel and three extensions is to keep pulling until it feels right. **It will not feel right, because the extensions absorb the feedback.** Use a torque wrench even where it is awkward. Overtightening a rear plug is one of the more avoidable ways to turn a routine service item into a cylinder head repair.
Common causes
- Cylinder 8 glow plug element open or drifted high in resistance from accumulated heat cycles
- Corroded or loose terminal on a rear plug that sits in a poorly ventilated, heat-soaked pocket
- Branch wire chafed against a heat shield, turbo mounting or intake pipework at the back of the engine
- Connector damaged or contaminated during previous work in the same congested area
- Failed cylinder 8 output channel in the glow plug control module
- Bank harness branch or bus bar terminal fault affecting the last plug on that head
- Coolant or oil seepage from a nearby gasket wicking into the plug connector
- Weak main supply, failed relay or high-current fuse where several channels report together
- A plug damaged or over-torqued during an earlier repair attempted without a torque wrench
Symptoms
- Extended cranking on cold mornings, worsening as ambient temperature falls
- Rough idle and misfire for the first minute from a cold start
- White smoke on cold start that clears once the engine warms
- Check engine light with P0678 stored, often with other cylinder glow plug codes
- A repeat visit for the same complaint after a single plug was replaced previously
- Cold-start behaviour that is unremarkable in summer and poor in winter
- No difference in running, power or economy once the engine is at temperature
Diagnostic steps
- 1.Read and record all stored codes, and note how many channels are reporting before planning any dismantling.
- 2.Confirm which physical cylinder is number 8 on this specific engine. Numbering schemes differ between manufacturers, and on a V engine the wrong assumption means stripping the wrong bank.
- 3.Test electrically from the connector end first. Measure resistance at the cylinder 8 plug terminal and compare with every other plug on the engine.
- 4.Check the feed voltage at that terminal during an energised preheat cycle, so a high-resistance connection is not mistaken for a failed element.
- 5.Inspect the terminal, nut and connector for corrosion, heat damage and fluid contamination — rear plugs sit in the hottest, least ventilated part of the engine bay.
- 6.Trace the branch wire where it passes the turbo mounting, heat shielding and intake pipework, looking for chafe points.
- 7.Only after the element is confirmed faulty, plan the access route and get a realistic labour estimate for it, because that number drives the next decision.
- 8.Decide the set question at that point: with the teardown already paid for, replacing all the plugs is usually cheaper over any sensible horizon than repeating the job.
- 9.Bring the engine to operating temperature before removal, apply penetrating oil, and free a stubborn plug in small alternating increments rather than one continuous pull.
- 10.Torque every plug to specification with a torque wrench, even where the access makes it awkward, and verify the repair after an overnight cold soak.
Repair cost
$90 – $2,200
The part is the small number here. A glow plug is typically $20 to $60. Diagnosis is $100 to $200. On a V8 diesel with a front-bank plug and reasonable clearance, a single replacement is $180 to $450 fitted. A rear plug requiring removal of intake pipework, an EGR crossover, heat shielding or turbo hardware is commonly $500 to $1,200, and heavy-duty applications where substantial disassembly is specified can exceed that. Replacing all eight while the engine is open typically adds $150 to $450 in parts to the same labour, which is why it is usually the cheaper decision. A seized plug that breaks during extraction adds specialist removal at $300 to $900 per cylinder, and a failed extraction means head removal.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with glow plug / glow plug module replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.