P0679 Cylinder 9 Glow Plug Circuit

The P0679 – Cylinder 9 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 9.

This code is primarily associated with diesel engines equipped with a cylinder 9 and an independently monitored glow plug circuit. Since most passenger vehicles do not have nine-cylinder engines, P0679 is relatively uncommon and is more likely to be encountered on larger diesel engines used in commercial vehicles, heavy-duty applications, and some specialized equipment.

The problem may be caused by the cylinder 9 glow plug, its wiring, connector, fuse, glow plug relay, or glow plug control module.

What Does P0679 Mean?

In simple terms:

The engine control system has detected abnormal electrical operation in the glow plug circuit for cylinder 9.

A simplified circuit looks like:

Battery → Fuse → Glow Plug Control Module/Relay → Cylinder 9 Glow Plug

The ECM or PCM monitors the glow plug system and can detect abnormal current or voltage behavior.

Depending on the vehicle, the detected problem may involve:

• Open circuit

• Excessive resistance

• Short circuit

• Abnormal current

• 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, it does not normally create a spark. Instead, it produces intense heat inside the combustion chamber to help diesel fuel ignite during cold starting.

The glow plug system is particularly important when the engine is cold.

On some diesel engines, glow plugs may also continue operating after the engine starts to stabilize combustion and reduce emissions.

What Does the Cylinder 9 Glow Plug Do?

The cylinder 9 glow plug heats the combustion chamber associated with cylinder 9.

When the engine control system determines that additional heat is necessary, it activates the glow plug.

If the circuit fails, cylinder 9 may not receive sufficient preheating during cold starting.

What Causes P0679?

Possible causes include:

• Failed cylinder 9 glow plug

• Open circuit

• Shorted circuit

• Damaged wiring

• Corroded connector

• Loose terminal

• Faulty glow plug relay

• Faulty glow plug control module

• Blown fuse

• Poor electrical connection

• Excessive resistance in the circuit

• Damaged glow plug terminal

The exact cause depends on the vehicle's electrical design.

Can a Bad Glow Plug Cause P0679?

Yes.

A failed cylinder 9 glow plug is one of the first things that should be tested.

A glow plug can fail internally because of:

• Open circuit

• Excessive resistance

• Internal damage

• Age

• Overheating

However, replacing the glow plug without testing the circuit can lead to unnecessary parts replacement.

Can Broken Wiring Cause P0679?

Yes.

A broken wire between the glow plug controller and cylinder 9 can prevent current from reaching the glow plug.

Inspect the wiring for:

• Broken conductors

• Chafing

• Melted insulation

• Corrosion

• Pinched sections

• Poor previous repairs

Engine vibration and high temperatures can damage glow plug wiring over time.

Can a Bad Connector Cause P0679?

Yes.

A loose or corroded terminal can increase circuit resistance or interrupt current completely.

Check the cylinder 9 connector for:

• Corrosion

• Loose terminal

• Burn marks

• Bent contacts

• Water contamination

• Poor terminal tension

Can a Blown Fuse Cause P0679?

Yes, depending on the vehicle.

Some glow plug systems use dedicated fuses or high-current protection.

A blown fuse can prevent the glow plug circuit from operating.

If the fuse is blown, determine the reason before replacing it.

Can a Bad Glow Plug Relay Cause P0679?

Yes, on systems that use a glow plug relay.

Some diesel engines use a conventional relay to control the glow plugs.

Other systems use an electronic glow plug control module that can monitor individual cylinders.

The correct diagnostic procedure depends on the vehicle.

Can a Bad Glow Plug Control Module Cause P0679?

Yes.

A faulty control module can fail to provide the correct power or control signal to cylinder 9.

However, if only cylinder 9 is affected and the other glow plug circuits operate correctly, the glow plug and its wiring should be checked first.

Can Low Battery Voltage Cause P0679?

Yes, potentially.

Glow plugs draw substantial electrical current.

Low battery voltage can affect the operation and monitoring of the glow plug system.

Check:

• Battery condition

• Battery voltage

• Cranking voltage

• Charging voltage

Can a Bad Alternator Cause P0679?

Potentially, but it is less likely when only cylinder 9 is affected.

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

If multiple glow plug codes or voltage-related DTCs are present, inspect the charging system.

Symptoms of P0679

Possible symptoms include:

• Check Engine Light

• Glow plug warning light

• Difficult cold starting

• Extended cranking

• Rough idle after cold startup

• Engine vibration

• White smoke

• Increased emissions

• Poor cold-start performance

When the engine is warm, the symptoms may become much less noticeable.

Can P0679 Cause a No-Start Condition?

Usually not by itself.

One failed glow plug normally does not prevent a modern diesel engine from starting in moderate temperatures.

However, starting can become difficult when:

• Temperatures are very low

• Several glow plugs have failed

• Battery voltage is low

• Engine compression is poor

• There are additional fuel-system problems

Can P0679 Cause Hard Starting?

Yes.

If cylinder 9 does not receive sufficient preheating, combustion in that cylinder may be weaker during startup.

This can increase cranking time and cause rough running immediately after the engine starts.

Can P0679 Cause White Smoke?

Yes.

Insufficient combustion temperature can cause incomplete combustion of diesel fuel.

This may produce white or light-colored smoke, particularly immediately after a cold start.

Can P0679 Cause Rough Idle?

Yes, especially when the engine is cold.

Cylinder 9 may contribute less effectively to combustion until the engine reaches a higher temperature.

The result can be:

• Rough idle

• Engine vibration

• Uneven combustion

• Temporary misfire-like symptoms

Can P0679 Affect Fuel Economy?

Potentially.

One failed glow plug usually has little effect once the engine is fully warm.

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

Is P0679 Related to the Fuel Injector?

Not directly.

P0679 identifies a problem with the cylinder 9 glow plug circuit, not the fuel injector.

A faulty injector can produce similar symptoms, but it normally generates different DTCs.

If the glow plug circuit tests correctly and the engine continues to run poorly, the injector and other engine systems should be investigated separately.

How Is P0679 Diagnosed?

A systematic diagnosis is recommended.

1. Scan for all DTCs

Look for:

• Other glow plug codes

• Injector codes

• Voltage-related codes

• Communication codes

• Engine performance codes

If multiple glow plug circuits are affected, investigate the common power supply or control module.

2. Inspect the cylinder 9 glow plug

Check the plug and electrical terminal.

3. Test glow plug resistance

Measure resistance according to the manufacturer's specifications.

Do not assume that one resistance value applies to every diesel glow plug.

4. Check the wiring

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

5. Inspect the connector

Look for corrosion, looseness, heat damage, and poor terminal tension.

6. Check the fuse

Verify the relevant glow plug fuse or power protection device.

7. Check the control-module power and ground

Make sure the glow plug controller has the correct electrical supply.

8. Check the control signal

Verify that the controller is commanding cylinder 9 correctly.

9. Test the glow plug control module

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

Can a Multimeter Diagnose P0679?

Yes.

A multimeter can be used to check:

• Glow plug resistance

• Circuit continuity

• Supply voltage

• Voltage drop

• Fuse condition

• Ground quality

However, modern glow plug systems may use PWM control, meaning a simple voltage measurement may not provide a complete diagnosis.

Can a Clamp Meter Help?

Yes.

A suitable current clamp can measure the current drawn by the glow plug.

Current testing can help determine whether the glow plug is actually operating 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 control signals

• Abnormal voltage patterns

• Missing control signals

• Irregular current behavior

What If P0679 Appears With Other Glow Plug Codes?

This is an important distinction.

If only P0679 is stored, concentrate first on:

Cylinder 9 glow plug → Connector → Wiring → Individual control circuit

If several glow plug codes are stored simultaneously, investigate:

• Glow plug control module

• Main power supply

• Ground

• Fuse

• Battery voltage

• Control-module communication

A common problem may be affecting multiple circuits.

What If P0679 Appears With P0670?

P0670 generally concerns the glow plug control module/control circuit, while P0679 identifies the cylinder 9 glow plug circuit.

If both are stored, check the control module, its power supply, ground, and main wiring before replacing individual glow plugs.

What If P0679 Appears Only in Cold Weather?

This is an important clue.

A weak glow plug or marginal electrical connection may work sufficiently in mild temperatures but fail to provide adequate heating when the engine is very cold.

Check the glow plug and its circuit carefully.

What If P0679 Appears After Glow Plug Replacement?

Verify:

• Correct glow plug type

• Correct voltage rating

• Correct installation

• Proper connector seating

• Terminal condition

• Wiring condition

A physically compatible glow plug is not necessarily electrically correct for every engine.

What If P0679 Returns Immediately After Clearing?

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

Focus on:

• Open glow plug

• Shorted circuit

• Broken wire

• Poor connector

• Control-module output

If the fault occurs only under certain temperatures, additional cold-start testing may be necessary.

What If P0679 Appears After Engine Repair?

Inspect the engine-bay harness around cylinder 9.

Engine repairs can result in:

• Disconnected connectors

• Pinched wires

• Damaged harnesses

• Loose grounds

• Incorrectly routed wiring

Is P0679 a Serious Code?

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

However, it should be repaired because it can cause:

• Difficult cold starting

• Longer cranking

• Increased starter load

• Increased battery load

• Rough cold operation

• Increased emissions

• White smoke

The problem becomes more important as ambient temperatures decrease.

Can You Drive With P0679?

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

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

However, repair should not be postponed indefinitely, 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 P0679 Repaired?

The repair depends on the confirmed cause.

Possible repairs include:

• Replacing the cylinder 9 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 P0679 Require Replacing All Glow Plugs?

Not necessarily.

If testing confirms that only cylinder 9 has a failed glow plug, replacing every glow plug may not be required.

However, if the glow plugs are old and several show 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.

Testing the complete circuit is the best way to determine whether the fault is actually the glow plug.

A proper diagnosis should distinguish between:

Failed glow plug

and

Failed glow plug circuit

Common Diagnostic Mistakes

Replacing the glow plug without checking the wiring

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

Replacing all glow plugs immediately

P0679 identifies a specific cylinder circuit.

Using the wrong glow plug

Voltage and electrical characteristics must match the engine's specifications.

Ignoring the connector

A loose terminal can create excessive resistance.

Relying only on resistance

Some modern glow plug systems require current or waveform testing for accurate diagnosis.

P0679 vs. Other Cylinder Glow Plug Codes

The main 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 process is similar for each cylinder-specific code.

How Do You Clear P0679?

The correct process is:

Scan the vehicle → Record all DTCs → Inspect cylinder 9 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 P0679 returns, the cylinder 9 glow plug circuit still has an unresolved problem.

Final Thoughts on P0679

P0679 – Cylinder 9 Glow Plug Circuit indicates an electrical problem involving the glow plug circuit for cylinder 9.

The 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 P0679 does not automatically mean the glow plug itself is defective. The entire circuit should be tested before replacing components.

The best diagnostic sequence is:

Test the cylinder 9 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.