P0671 Cylinder 1 Glow Plug Circuit

The P0671 – Cylinder 1 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 1.

This code is primarily found on diesel engines, where glow plugs are used to help generate the heat needed for reliable combustion during cold starts. P0671 is a cylinder-specific code, meaning the system has identified an abnormality in the circuit associated with cylinder 1.

The problem does not necessarily mean that the glow plug itself has failed. The fault can also be caused by the wiring, connector, fuse, glow plug relay, or glow plug control module.

What Does P0671 Mean?

In simple terms:

The engine control module has detected abnormal electrical behavior in the cylinder 1 glow plug circuit.

A simplified circuit can be thought of as:

Battery → Fuse → Glow Plug Control Module/Relay → Cylinder 1 Glow Plug

When the glow plug system is activated, the engine control system monitors the electrical response.

If the expected voltage, resistance, or current is not detected, P0671 may be stored.

Depending on the vehicle, the fault may 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, it does not normally produce a spark. Instead, its heating element becomes extremely hot and raises the temperature inside the combustion chamber.

This helps diesel fuel ignite more easily when the engine is cold.

Modern diesel engines may also operate the glow plugs briefly after the engine starts. This can improve cold combustion, reduce smoke, stabilize idle quality, and help control emissions.

What Does the Cylinder 1 Glow Plug Do?

The cylinder 1 glow plug provides additional heat inside cylinder 1 during cold starting.

The engine control system determines when the glow plug should operate and how much heating is required.

If the cylinder 1 circuit fails, cylinder 1 may not receive sufficient preheating.

The engine may still start normally in warm weather, so the problem can sometimes remain unnoticed until temperatures become lower.

What Causes P0671?

Possible causes include:

• Failed cylinder 1 glow plug

• Open circuit

• Shorted 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 should be confirmed through testing.

Can a Bad Glow Plug Cause P0671?

Yes.

A failed cylinder 1 glow plug is one of the first components that should be checked.

The heating element can fail because of:

• Internal open circuit

• Excessive resistance

• Electrical damage

• Age

• Excessive heat

However, replacing the glow plug immediately is not always the correct solution. The complete circuit should be tested first.

Can Broken Wiring Cause P0671?

Yes.

A broken wire between the glow plug controller and cylinder 1 can interrupt the electrical supply.

Inspect the harness for:

• Broken conductors

• Chafing

• Melted insulation

• Corrosion

• Pinched wires

• Poor previous repairs

Glow plug wiring is exposed to high temperatures and engine vibration, so harness damage is possible.

Can a Bad Connector Cause P0671?

Yes.

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

Inspect the cylinder 1 connector for:

• Corrosion

• Loose terminals

• Burn marks

• Bent contacts

• Water contamination

• Poor terminal tension

Can a Blown Fuse Cause P0671?

Yes, depending on the vehicle.

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

If the fuse is blown, the underlying reason should be identified before installing a replacement.

Can a Bad Glow Plug Relay Cause P0671?

Yes, if the vehicle uses a conventional glow plug relay.

Some diesel engines use a relay to control multiple glow plugs.

Modern systems often use an electronic Glow Plug Control Module (GPCM) that can control and monitor individual glow plug circuits.

Can a Bad Glow Plug Control Module Cause P0671?

Yes.

A defective control module may fail to provide the correct current or control signal to cylinder 1.

However, if P0671 is the only glow plug code stored, it is generally better to check the cylinder 1 glow plug, connector, and wiring before replacing the control module.

Can Low Battery Voltage Cause P0671?

Yes, potentially.

Glow plugs can draw substantial current.

A weak battery or excessive voltage drop during starting can affect glow plug operation and the control module's ability to monitor the circuit.

Check:

• Battery condition

• Battery voltage

• Cranking voltage

• Charging voltage

Can a Bad Alternator Cause P0671?

Potentially, but it is less likely if cylinder 1 is the only affected circuit.

A charging-system problem generally affects electrical voltage throughout the vehicle.

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

Symptoms of P0671

Possible symptoms include:

• Check Engine Light

• Glow plug warning light

• Difficult cold starting

• Extended cranking

• Rough idle after startup

• Engine vibration

• White smoke

• Increased emissions

• Poor cold-start performance

Once the engine reaches operating temperature, the symptoms may become much less noticeable.

Can P0671 Cause a No-Start Condition?

Usually not by itself.

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

Starting problems become more likely when:

• Several glow plugs have failed

• Ambient temperature is very low

• Battery voltage is low

• Engine compression is poor

• There are additional fuel-system problems

Can P0671 Cause Hard Starting?

Yes, particularly in cold weather.

Without sufficient preheating in cylinder 1, combustion may be less efficient during the first few engine cycles.

This can result in:

• Longer cranking

• Rough startup

• Uneven combustion

• Temporary engine vibration

Can P0671 Cause White Smoke?

Yes.

If the combustion chamber remains too cold, diesel fuel may not burn completely during startup.

The resulting unburned fuel can appear as white or light-colored exhaust smoke.

Can P0671 Cause Rough Idle?

Yes, especially when the engine is cold.

If cylinder 1 does not receive adequate glow plug assistance, combustion in that cylinder may initially be weaker.

This can cause:

• Rough idle

• Engine vibration

• Uneven combustion

• Temporary misfire-like symptoms

Can P0671 Affect Fuel Economy?

Potentially.

One failed glow plug normally has little effect once the engine is fully warmed up.

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

Is P0671 Related to the Fuel Injector?

Not directly.

P0671 identifies a problem with the cylinder 1 glow plug circuit.

A defective injector can also cause rough running, smoke, or difficult starting, but it normally produces different DTCs.

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

How Is P0671 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

If several glow plug codes are stored, investigate the common power supply or control module.

2. Inspect the cylinder 1 glow plug

Check its physical condition and electrical connection.

3. Test glow plug resistance

Measure resistance according to the manufacturer's specifications.

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

4. Check the wiring

Inspect the complete circuit between cylinder 1 and the glow plug controller.

5. Inspect the connector

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

6. Check the fuse

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

7. Check control-module power and ground

Confirm that the glow plug controller receives the required power and has a reliable ground.

8. Check the control signal

Verify that the controller is correctly commanding cylinder 1.

9. Test the glow plug control module

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

Can a Multimeter Diagnose P0671?

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 or electronically regulated current, so a basic 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 cylinder 1 glow plug.

This can help determine whether the glow plug is actually receiving and drawing the expected current.

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

• Missing control signals

• Abnormal voltage patterns

• Irregular current behavior

What If P0671 Appears With Other Glow Plug Codes?

This is an important diagnostic clue.

If only P0671 is stored, focus first on:

Cylinder 1 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

• Glow plug relay

A common electrical problem may be affecting multiple cylinders.

What If P0671 Appears With P0670?

P0670 generally concerns the glow plug control module/control circuit, while P0671 identifies the cylinder 1 glow plug circuit.

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

What If P0671 Appears Only in Cold Weather?

This is an important clue.

A weak glow plug or marginal electrical connection may operate adequately in mild temperatures but fail to provide sufficient heating during a cold start.

Cold-weather symptoms can include:

• Longer cranking

• Rough startup

• White smoke

• Engine vibration

What If P0671 Appears After Glow Plug Replacement?

Verify:

• Correct glow plug type

• Correct voltage rating

• Proper installation

• Connector seating

• Terminal condition

• Wiring condition

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

What If P0671 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 problem occurs only under specific temperatures, additional cold-start testing may be necessary.

What If P0671 Appears After Engine Repair?

Inspect the wiring around cylinder 1 carefully.

Engine repairs can result in:

• Disconnected connectors

• Pinched wires

• Damaged harnesses

• Loose grounds

• Incorrectly routed wiring

A damaged harness can produce a cylinder-specific glow plug code even when the glow plug itself is good.

Is P0671 a Serious Code?

Usually, P0671 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 problem becomes more significant as temperatures decrease.

Can You Drive With P0671?

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, particularly if:

• Cold starting becomes difficult

• Several glow plug codes are present

• Excessive smoke appears

• The engine runs very roughly after startup

How Is P0671 Repaired?

The repair depends on the confirmed cause.

Possible repairs include:

• Replacing the cylinder 1 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 P0671 Require Replacing All Glow Plugs?

Not necessarily.

If testing confirms that only cylinder 1 has a failed glow plug, replacing all glow plugs may not be necessary.

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 diagnosis should distinguish between:

Failed cylinder 1 glow plug

and

Fault in the cylinder 1 glow plug circuit

A new glow plug will not repair a broken wire, damaged connector, or faulty control module.

Common Diagnostic Mistakes

Replacing the glow plug without checking the wiring

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

Replacing all glow plugs immediately

P0671 specifically identifies cylinder 1.

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

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

P0671 vs. Other Cylinder Glow Plug Codes

The primary difference is the cylinder identified by the code.

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 diagnostic principle is similar for each cylinder-specific glow plug code.

How Do You Clear P0671?

The correct process is:

Scan the vehicle → Record all DTCs → Inspect cylinder 1 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 P0671 returns, the cylinder 1 glow plug circuit still has an unresolved problem.

Final Thoughts on P0671

P0671 – Cylinder 1 Glow Plug Circuit indicates an electrical problem involving the glow plug circuit for cylinder 1.

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 P0671 does not automatically mean the glow plug itself is defective. The complete circuit should be tested before replacing components.

The most effective diagnostic sequence is:

Test the cylinder 1 glow plug → Inspect the connector → Check wiring → Check fuse and power supply → Verify ground → Test the control signal → Evaluate the glow plug control module.

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