OBD-II trouble code
P0680: Cylinder 10 Glow Plug Circuit
A fault on the cylinder 10 glow plug channel. On a modern diesel, glow plugs do not stop working once the engine fires — they carry on during afterglow to clean up the warm-up. That is why this code can appear after a start that felt completely normal, and why "it starts fine" is a poor reason to ignore it.
Quick facts
- System
- Powertrain
- Category
- Auxiliary Emissions Controls
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $80 – $1,600
- DIY difficulty
- Intermediate DIY
What does P0680 mean?
P0680 says the glow plug controller measured a fault on the cylinder 10 channel. The parts involved are the same short list as every other cylinder in the series. What deserves attention here is a change in what glow plugs are actually for, because it is why this family of codes shows up on vehicles whose owners insist nothing is wrong.
On an older diesel the job was simple: heat the chamber before cranking, then switch off. Preheat, start, done. **On a modern emissions-controlled diesel the plugs stay energised well after the engine has started.** This is usually called afterglow or post-glow, and depending on the system and the ambient temperature it can continue for a couple of minutes into the warm-up.
It is not there for the driver's benefit. A cold diesel cylinder burns its fuel badly — the flame is unstable, combustion is incomplete, and the result is white smoke, extra noise, and a large slug of unburned hydrocarbons and particulates going out of the exhaust before the aftertreatment system is warm enough to do much about them. Keeping the plugs hot through those first minutes stabilises combustion, and on vehicles fitted with a particulate filter it means less soot loaded into the filter that later has to be burned back out.
That has three practical consequences that this code makes concrete.
**First, the fault can be logged on a start that felt perfectly normal.** The engine caught, it idled, nothing sounded wrong. The plug was still being commanded during afterglow, the module was still watching the current, and it recorded the fault then. "But it started fine" is not evidence against the code — it is a description of a different part of the cycle.
**Second, the symptom may not be a cold-start symptom at all.** Where the plug fails during afterglow rather than preheat, the complaint tends to be a rough, smoky, noisy warm-up over the first minute or two, and it can show up in mild weather where a preheat-only fault would go unnoticed until winter.
**Third, this connects the glow system to your aftertreatment.** Consistently poor combustion during warm-up means more soot into the particulate filter, more frequent regeneration, and on vehicles used mainly for short trips a filter that is loading faster than it can clear. If a diesel is complaining about regeneration frequency and also carrying glow plug codes, those two facts are worth considering together rather than as separate jobs.
None of this changes the diagnosis itself — measure the plug against its neighbours, check the terminal and the feed under load, work outward to the harness and the controller. What it changes is the urgency. A glow plug fault on a vehicle with a particulate filter is not purely a winter inconvenience, and leaving it for six months because the engine starts is a false economy.
Common causes
- Cylinder 10 glow plug element open or drifted high in resistance
- Element that still heats but no longer holds its current profile through the afterglow period
- Corroded, loose or heat-damaged terminal at the cylinder 10 plug
- Damaged branch wire or bank harness section serving cylinder 10
- Failed cylinder 10 output channel in the glow plug control module
- Controller shutting the channel down on a thermal or overcurrent protection during extended afterglow
- Marginal battery or charging system, since afterglow draws current while the engine is running
- Shared relay, high-current fuse, supply or ground fault where several channels report together
- Mixed or wrong-specification plugs after a previous partial replacement
Symptoms
- A rough, noisy or smoky warm-up over the first minute or two, even when the start itself was normal
- Grey or white smoke that persists briefly after the engine is running rather than only during cranking
- Check engine light appearing after an unremarkable start
- Extended cranking on genuinely cold mornings
- More frequent particulate filter regeneration on vehicles used for short journeys
- Glow plug or wait-to-start indicator behaving oddly after start-up on some vehicles
- Completely normal running, power and economy once at operating temperature
Diagnostic steps
- 1.Record every stored code, and note whether the vehicle also carries particulate filter, soot load or regeneration codes — on a modern diesel those belong in the same conversation.
- 2.Ask when the fault light appeared relative to starting. A code set after a normal start points at the afterglow phase rather than preheat.
- 3.Confirm the cylinder numbering for this engine and locate cylinder 10 physically before removing anything.
- 4.Measure the resistance of all the glow plugs at their own terminals and compare. A single outlier against a matched set identifies the failed element.
- 5.Watch glow plug commanded state and, if the tool supports it, channel current on live data through a full cold start including the afterglow period rather than only the preheat window.
- 6.Measure voltage at the cylinder 10 terminal while the circuit is energised, so a resistive joint is not mistaken for a failed plug.
- 7.Check battery condition and charging output, because afterglow loads the electrical system while the engine is running and a weak system distorts the module's current measurements.
- 8.Inspect the terminal, connector and branch wire for corrosion, chafe and heat damage.
- 9.If several channels report together, test the shared relay, fuse, supply and ground before condemning individual plugs.
- 10.Verify the repair over a full cold start and warm-up cycle after an overnight soak, not just by clearing the code and restarting.
Repair cost
$80 – $1,600
Diagnosis is $100 to $220. A single glow plug with reasonable access is $180 to $500 fitted; on a large V-configuration diesel with a poorly accessible cylinder, $500 to $1,200. Terminal or branch wire repair is $120 to $400. A glow plug control module is $300 to $900 fitted. Plugs for engines of this size are often higher-specification types at $50 to $180 each rather than the $25 part fitted to small diesels. The figure that does not appear on this invoice but should be in the decision is the aftertreatment one: repeatedly poor cold combustion loads the particulate filter faster, and a filter replacement is an order of magnitude more expensive than any glow plug repair.
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 intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.