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P0684 – Glow Plug Control Module to PCM Communication Circuit Range/Performance (OBD2)

P0684 means "Glow Plug Control Module to PCM Communication Circuit Range/Performance". The glow plug system signal is electrically present but implausible: it does not match what the control unit expects for the current operating conditions or from other sensors. This is a performance fault, not a broken wire.

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P0684 quick summary

Fault codeP0684
MeaningGlow Plug Control Module to PCM Communication Circuit Range/Performance
SystemPowertrain (Engine & Transmission)
VehiclesGeneric OBD2
SeverityLow
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.

On VAG vehicles (VW, Audi, Škoda, Seat, Cupra), VCDS and older VAG testers can show this code as the five-digit VAG code 17068 (P0684). Both numbers mean the same fault.

Symptoms of P0684

  • 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 P0684 (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. 1.Glow plug system worn, contaminated or out of calibration

    Most likely

    The signal drifts slowly, so it is still within the electrical limits but wrong.

  2. 2.Mechanical problem in the system the glow plug system measures or controls

    For example a leak, a sticking part or a restriction that really changes the measured value.

  3. 3.High resistance in a connector or ground

    Corrosion causes a voltage drop that shifts the reading.

  4. 4.Burnt-out glow plug(s)

  5. 5.Faulty glow plug control module or relay

  6. 6.Damaged wiring or loose terminal nut on the glow plug

  7. 7.Blown fuse or fusible link

Step-by-step diagnostic guide

  1. 1

    Read codes

    Read the engine and glow plug module codes to find which cylinder or circuit is affected.

  2. 2

    Compare live data with expected values

    Read the glow plug system values in live data and compare them with related readings (engine speed, load, temperature or pressure) and with the specification. A value that is electrically fine but wrong for the conditions confirms a performance fault – then check the mechanical side before replacing the part.

  3. 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. 4

    Check supply from the module

    Check that the module supplies voltage to each plug during the glow phase.

  5. 5

    Check fuse and module

    Check the main fuse and the module's supply, ground and control signal.

  6. 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 P0684?

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 P0684?

Typical repair costs are 20 € – 800 €, depending on the cause and the vehicle. The most common fixes are: glow plug system worn, contaminated or out of calibration; mechanical problem in the system the glow plug system measures or controls; high resistance in a connector or ground. 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 P0684 and similar codes?

P0684 belongs to a group of codes for the same component that differ in the type or location of the fault: P0678 – Cylinder 8 Glow Plug Circuit; P0679 – Cylinder 9 Glow Plug Circuit; P0680 – Cylinder 10 Glow Plug Circuit; P0681 – Cylinder 11 Glow Plug Circuit; P0682 – Cylinder 12 Glow Plug Circuit; P0683 – Glow Plug Control Module to PCM Communication 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.