P0678 Cylinder 8 Glow Plug Circuit

The P0678 – Cylinder 8 Glow Plug Circuit diagnostic trouble code (DTC) indicates that the engine control system has detected an electrical problem in the glow plug circuit for cylinder 8.

This code is mainly associated with diesel engines equipped with eight or more cylinders. The glow plug system is responsible for heating the combustion chamber during cold starting and, on some diesel engines, for a short period after startup.

P0678 specifically points toward the cylinder 8 glow plug circuit, rather than identifying a general glow plug system failure.

The problem can be caused by a failed glow plug, damaged wiring, poor connector contact, excessive resistance, a faulty glow plug relay or control module, or a power-supply problem.

What Does P0678 Mean?

In simple terms:

The engine control module has detected abnormal electrical behavior in the glow plug circuit for cylinder 8.

A simplified circuit looks like:

Battery → Fuse → Glow Plug Control Module/Relay → Cylinder 8 Glow Plug

The ECM or PCM monitors the circuit and checks whether the expected electrical conditions are present.

If the circuit does not behave as expected, P0678 may be stored.

Depending on the vehicle, the detected problem can involve:

• Open circuit

• Short circuit

• Excessive resistance

• Incorrect current draw

• Poor electrical connection

• Failed glow plug

What Is a Glow Plug?

A glow plug is an electrically heated component used in diesel engines.

Unlike a gasoline spark plug, a glow plug does not normally ignite the fuel with a spark. Instead, it produces heat inside the combustion chamber to help the diesel engine start when the engine is cold.

Glow plugs are especially important during:

• Cold starts

• Low ambient temperatures

• Initial engine operation

Some modern diesel engines also use glow plugs after startup to improve combustion and reduce emissions.

What Does the Cylinder 8 Glow Plug Do?

The cylinder 8 glow plug provides additional heat inside cylinder 8.

When the engine is cold, the control system activates the glow plugs for a calculated period.

If the cylinder 8 circuit fails, that cylinder may not receive the required preheating.

The engine may still start normally in warm conditions, but cold-start performance can deteriorate.

What Causes P0678?

Possible causes include:

• Failed cylinder 8 glow plug

• Open glow plug circuit

• Shorted glow plug circuit

• Damaged wiring

• Corroded connector

• Loose electrical terminal

• Excessive circuit resistance

• Faulty glow plug relay

• Faulty glow plug control module

• Blown fuse

• Poor power connection

• Poor ground connection

• Damaged glow plug terminal

The exact cause depends on the vehicle's glow plug system.

Can a Bad Glow Plug Cause P0678?

Yes.

A failed cylinder 8 glow plug is one of the first things that should be tested.

The glow plug may have:

• An internal open circuit

• Excessive resistance

• Internal electrical damage

• Heat-related damage

• Age-related deterioration

However, the glow plug should be tested before replacement because the wiring or control circuit may be the actual cause.

Can Broken Wiring Cause P0678?

Yes.

A broken wire between the glow plug controller and cylinder 8 can interrupt current flow.

Inspect the wiring for:

• Broken conductors

• Chafed insulation

• Melted sections

• Corrosion

• Pinched wires

• Poor previous repairs

The engine compartment exposes this wiring to considerable heat and vibration.

Can a Bad Connector Cause P0678?

Yes.

A loose or corroded connector can create excessive resistance or interrupt the circuit completely.

Inspect the cylinder 8 terminal for:

• Corrosion

• Loose fit

• Burn marks

• Bent contacts

• Water contamination

• Poor terminal tension

Can a Blown Fuse Cause P0678?

Yes, depending on the vehicle.

Some diesel engines use dedicated glow plug fuses or high-current protection devices.

If the fuse is blown, the reason for the failure should be determined before replacing it.

Can a Bad Glow Plug Relay Cause P0678?

Yes, on vehicles equipped with a conventional glow plug relay.

Some systems use one relay for multiple glow plugs, while modern systems may use an electronic glow plug control module capable of monitoring individual circuits.

If several glow plug codes appear at the same time, the common relay or control module becomes more important to investigate.

Can a Bad Glow Plug Control Module Cause P0678?

Yes.

A faulty control module may fail to supply the correct electrical current to cylinder 8.

However, if the other glow plugs work normally and only P0678 is stored, inspect the cylinder 8 glow plug, wiring, and connector before condemning the control module.

Can Low Battery Voltage Cause P0678?

Yes, potentially.

Glow plugs consume significant electrical current.

Low battery voltage can interfere with glow plug operation and with the control module's ability to monitor the circuit correctly.

Check:

• Battery condition

• Battery voltage

• Cranking voltage

• Charging voltage

Can a Bad Alternator Cause P0678?

Potentially, but it is less likely when cylinder 8 is the only affected circuit.

A charging-system problem can cause abnormal voltage throughout the vehicle.

If several glow plug circuits and voltage-related codes appear together, inspect the charging system.

Symptoms of P0678

Possible symptoms include:

• Check Engine Light

• Glow plug warning light

• Difficult cold starting

• Extended cranking

• Rough cold idle

• Engine vibration after startup

• White smoke

• Increased emissions

• Poor cold-start performance

Symptoms can be minimal when the engine is already warm.

Can P0678 Cause a No-Start Condition?

Usually not by itself.

A single failed glow plug generally does not prevent a modern diesel engine from starting under moderate temperatures.

However, a no-start or severe hard-start condition becomes more likely when:

• Several glow plugs have failed

• Ambient temperatures are very low

• Battery voltage is low

• Engine compression is poor

• There are additional fuel-system problems

Can P0678 Cause Hard Starting?

Yes, especially in cold weather.

Without adequate heating in cylinder 8, initial combustion may be less efficient.

The engine may crank longer before starting and may run roughly for the first few seconds.

Can P0678 Cause White Smoke?

Yes.

Insufficient heat in a cold combustion chamber can result in incomplete combustion.

Unburned diesel fuel may appear as white or light-colored exhaust smoke during startup.

Can P0678 Cause Rough Idle?

Yes, particularly immediately after a cold start.

Cylinder 8 may contribute less effectively to combustion until the engine warms up.

This can cause:

• Rough idle

• Engine vibration

• Uneven combustion

• Temporary misfire-like symptoms

Can P0678 Affect Fuel Economy?

Potentially.

A single glow plug fault normally has little effect after the engine reaches operating temperature.

However, repeated difficult starts and incomplete cold combustion can increase fuel consumption and emissions.

Is P0678 Related to the Fuel Injector?

Not directly.

P0678 identifies the cylinder 8 glow plug circuit.

A fuel injector problem can cause similar symptoms, but it normally produces a different diagnostic code.

If the glow plug circuit tests correctly and the engine still runs poorly, the injector and other engine systems should be investigated separately.

How Is P0678 Diagnosed?

A systematic approach is recommended.

1. Scan for all DTCs

Look for:

• Other glow plug codes

• Injector codes

• Voltage-related codes

• Communication codes

• Engine performance codes

Multiple glow plug codes can indicate a common power or control-module problem.

2. Inspect the cylinder 8 glow plug

Check the physical condition and electrical connection.

3. Test glow plug resistance

Measure resistance according to the manufacturer's specifications.

There is no single resistance value that applies to every glow plug.

4. Check the wiring

Inspect the complete circuit between the glow plug and its controller.

5. Inspect the connector

Check for corrosion, loose terminals, heat damage, and poor terminal tension.

6. Check the fuse

Verify the relevant glow plug fuse or high-current protection device.

7. Check control-module power and ground

Make sure the glow plug controller receives the correct power and has a reliable ground.

8. Check the control signal

Verify that the controller is commanding cylinder 8 correctly.

9. Test the glow plug control module

If the glow plug and wiring are good, the control module may require further testing.

Can a Multimeter Diagnose P0678?

Yes.

A multimeter can be used to check:

• Glow plug resistance

• Circuit continuity

• Supply voltage

• Voltage drop

• Fuse condition

• Ground quality

However, some modern glow plug systems use PWM or electronically regulated current, so a basic voltage test may not tell the whole story.

Can a Clamp Meter Help?

Yes.

A suitable current clamp can measure the current flowing through the glow plug circuit.

This can help determine whether cylinder 8 is actually drawing current when commanded.

Can an Oscilloscope Help?

Yes.

An oscilloscope can be useful for advanced diagnosis of electronically controlled glow plug systems.

It can reveal:

• PWM signals

• Missing control signals

• Abnormal voltage patterns

• Irregular current behavior

What If P0678 Appears With Other Glow Plug Codes?

This is an important diagnostic distinction.

If only P0678 is stored, concentrate first on:

Cylinder 8 glow plug → Connector → Wiring → Individual control circuit

If several glow plug codes are stored, investigate:

• Glow plug control module

• Main power supply

• Ground

• Fuse

• Battery voltage

• Glow plug relay

A common electrical problem may be affecting multiple cylinders.

What If P0678 Appears With P0670?

P0670 generally concerns the glow plug control module/control circuit, while P0678 identifies the cylinder 8 glow plug circuit.

If both codes are present, check the control module's power, ground, and wiring before replacing individual glow plugs.

What If P0678 Appears Only in Cold Weather?

This is an important clue.

A marginal glow plug may operate adequately in mild temperatures but fail to provide sufficient heat during a very cold start.

Cold-weather symptoms can include:

• Longer cranking

• Rough startup

• White smoke

• Temporary engine vibration

What If P0678 Appears After Glow Plug Replacement?

Verify:

• Correct glow plug type

• Correct voltage rating

• Proper installation

• Correct connector seating

• Terminal condition

• Wiring condition

A glow plug that physically fits may still have incorrect electrical characteristics for the engine.

What If P0678 Returns Immediately After Clearing?

If the code returns immediately, the problem is probably still present.

Focus on:

• Open glow plug

• Shorted circuit

• Broken wire

• Poor connector

• Control-module output

If the fault is temperature-dependent, further testing during a cold start may be necessary.

What If P0678 Appears After Engine Repair?

Inspect the engine-bay wiring around cylinder 8.

Engine work can result in:

• Disconnected connectors

• Pinched wires

• Damaged harnesses

• Loose grounds

• Incorrectly routed wiring

Is P0678 a Serious Code?

Usually, P0678 is not an immediate engine-damage emergency.

However, it should be repaired because it can cause:

• Difficult cold starts

• Longer cranking

• Increased starter load

• Increased battery load

• Rough cold operation

• Increased emissions

• White smoke

The importance of the problem increases in cold climates.

Can You Drive With P0678?

Usually, yes, if the engine starts and runs normally.

A single glow plug circuit fault generally does not prevent normal driving.

However, the fault should not be ignored for an extended period, especially if:

• Cold starting is difficult

• Several glow plug codes are present

• The engine produces excessive smoke

• The engine runs very roughly after starting

How Is P0678 Repaired?

The repair depends on the confirmed cause.

Possible repairs include:

• Replacing the cylinder 8 glow plug

• Repairing damaged wiring

• Repairing the connector

• Replacing damaged terminals

• Repairing a fuse or power supply

• Replacing the glow plug relay

• Replacing the glow plug control module

• Repairing a poor ground

• Correcting an incorrect glow plug installation

Does P0678 Require Replacing All Glow Plugs?

Not necessarily.

If testing confirms that only cylinder 8 has a failed glow plug, replacing all glow plugs may not be required.

However, if the glow plugs are old and several are showing signs of deterioration, replacing them as a set may be considered according to the manufacturer's recommendations.

Should the Glow Plug Be Tested Before Replacement?

Yes.

The complete circuit should be tested before replacing components.

The goal is to distinguish between:

Failed cylinder 8 glow plug

and

Fault in the cylinder 8 glow plug circuit

A new glow plug will not fix a broken wire or faulty control module.

Common Diagnostic Mistakes

Replacing the glow plug without checking the wiring

A broken wire will remain faulty after installing a new plug.

Replacing all glow plugs immediately

P0678 specifically identifies cylinder 8.

Using the wrong glow plug

The replacement must match the engine's electrical and physical specifications.

Ignoring the connector

A loose or corroded terminal can create excessive resistance.

Relying only on resistance

Modern glow plug systems may require current or waveform testing for a complete diagnosis.

P0678 vs. Other Cylinder Glow Plug Codes

The primary difference is the cylinder number.

P0671 → Cylinder 1 Glow Plug Circuit

P0672 → Cylinder 2 Glow Plug Circuit

P0673 → Cylinder 3 Glow Plug Circuit

P0674 → Cylinder 4 Glow Plug Circuit

P0675 → Cylinder 5 Glow Plug Circuit

P0676 → Cylinder 6 Glow Plug Circuit

P0677 → Cylinder 7 Glow Plug Circuit

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

The basic diagnostic approach is similar for each cylinder-specific glow plug code.

How Do You Clear P0678?

The correct procedure is:

Scan the vehicle → Record all DTCs → Inspect cylinder 8 glow plug → Test resistance/current → Inspect connector → Check wiring → Check fuse → Verify control-module power and ground → Test control output → Repair the confirmed fault → Clear DTCs → Perform a cold-start test → Rescan

If P0678 returns, the cylinder 8 glow plug circuit still has an unresolved problem.

Final Thoughts on P0678

P0678 – Cylinder 8 Glow Plug Circuit indicates an electrical problem involving the glow plug circuit for cylinder 8.

Possible causes include a failed glow plug, damaged wiring, loose or corroded connector, excessive circuit resistance, faulty glow plug relay or control module, blown fuse, or poor electrical connection.

The important point is that P0678 does not automatically mean the glow plug itself has failed. The complete circuit should be tested before replacing parts.

The best diagnostic sequence is:

Test the cylinder 8 glow plug → Inspect connector → Check wiring → Check fuse and power supply → Verify ground → Test control signal → Evaluate the glow plug control module.

If the vehicle experiences hard cold starting, white smoke, rough idle, or increased cranking time, the glow plug circuit should be repaired before cold weather makes the symptoms more severe.