P0678 – Cylinder 8 Glow Plug Circuit (OBD2)
P0678 means "Cylinder 8 Glow Plug Circuit". The control unit has detected a fault in the electrical circuit of the glow plug system: the signal or control current is missing or outside the expected window. The code does not say whether the value is too high or too low, so the wiring and the component itself both have to be checked.
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P0678 quick summary
| Fault code | P0678 |
|---|---|
| Meaning | Cylinder 8 Glow Plug Circuit |
| System | Powertrain (Engine & Transmission) |
| Vehicles | Generic OBD2 |
| Severity | Low |
| Can I drive? | Yes. In mild weather the engine usually starts normally; in cold weather expect hard starting and white smoke. |
| Repair cost | €20 – €800 |
How diesel glow plugs work
Glow plugs heat the combustion chambers of a diesel engine so it starts reliably and runs smoothly when cold; these codes cover individual glow plugs, the control module and its circuits.
Each cylinder has a glow plug that heats up to around 1,000 °C within seconds. A glow plug control module (or relay on older engines) supplies them with current before and after starting. Many modern systems also keep the plugs on for several minutes after a cold start (after-glow) to reduce smoke and noise.
The control module monitors each plug's current. It reports faults for individual cylinders (P0671–P0682), the overall circuit (P0380, P0382), the indicator lamp and the communication between module and engine control unit (P0683, P0684).
One failed glow plug often goes unnoticed in mild weather but causes hard starting, white smoke and rough running in winter. Glow plugs can seize in the cylinder head, so removal needs care.
The code refers to cylinder 8. Cylinder numbering differs between manufacturers (and between banks on V engines), so check the numbering for your engine in the service information before replacing parts.
On VAG vehicles (VW, Audi, Škoda, Seat, Cupra), VCDS and older VAG testers can show this code as the five-digit VAG code 17062 (P0678). Both numbers mean the same fault.
Symptoms of P0678
- Hard starting in cold weather
- White smoke and rough running after a cold start
- Glow plug lamp flashing or Check Engine light
- Knocking noise when cold
- Sometimes no symptoms in mild weather
Most common causes of P0678 (most likely first)
Mechanic's Pro Tip
Never force a seized glow plug out of a cold engine. Warm the engine, soak the thread, and loosen it slowly back and forth. A snapped glow plug often means removing the cylinder head.
1.Damaged wire or loose, corroded connector at the glow plug system
Most likelyGreen or white corrosion, pushed-back pins and chafed insulation near hot or moving parts are the most common findings.
2.Faulty glow plug system
An internal break or short in the component itself.
3.Blown fuse or missing supply voltage
If several codes appeared at the same time, check the shared fuse, relay or ground point first.
4.Burnt-out glow plug(s)
5.Communication fault between module and engine control unit
Step-by-step diagnostic guide
- 1
Read codes
Read the engine and glow plug module codes to find which cylinder or circuit is affected.
- 2
Check the circuit with a multimeter
With the ignition off, unplug the glow plug system and the control unit connector (or back-probe), then check each wire between them for continuity and for shorts to ground and to battery voltage. Compare the component's resistance or supply voltage with the value in the repair manual.
Multimeter: Wire continuity: below about 1 Ω. Wire to ground with both connectors unplugged: no continuity. Supply/reference with ignition on: usually 5 V for sensors, battery voltage for actuators.
- 3
Measure the glow plug
Disconnect the plug's terminal and measure its resistance to ground or its current draw.
Multimeter: Most glow plugs read below about 1–2 Ω; 'OL' means burnt out. Current draw: often around 8–15 A per plug when hot.
- 4
Check supply from the module
Check that the module supplies voltage to each plug during the glow phase.
- 5
Check fuse and module
Check the main fuse and the module's supply, ground and control signal.
- 6
Replace carefully
Warm the engine, use penetrating oil and the correct torque. Replace all plugs if they are old.
Frequently asked questions
Can I keep driving with P0678?
Yes. In mild weather the engine usually starts normally; in cold weather expect hard starting and white smoke.
How much does it cost to fix P0678?
Typical repair costs are 20 € – 800 €, depending on the cause and the vehicle. The most common fixes are: damaged wire or loose, corroded connector at the glow plug system; faulty glow plug system; blown fuse or missing supply voltage. Always confirm the cause with the tests above before buying parts.
Should I replace all glow plugs at once?
If they are old, yes – the others usually fail soon after and the work is similar. Replace them only if they come out without excessive force.
Can a bad glow plug damage the engine?
Not directly, but very hard starting puts extra strain on the starter and battery, and a broken glow plug tip in the cylinder can cause serious damage.
What is the difference between P0678 and similar codes?
P0678 belongs to a group of codes for the same component that differ in the type or location of the fault: P0675 – Cylinder 5 Glow Plug Circuit; P0676 – Cylinder 6 Glow Plug Circuit; P0677 – Cylinder 7 Glow Plug Circuit; P0679 – Cylinder 9 Glow Plug Circuit; P0680 – Cylinder 10 Glow Plug Circuit; P0681 – Cylinder 11 Glow Plug Circuit. The tests on this page apply to the whole group; the type of fault tells you where to start.
Related fault codes
This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.