P0670 Glow Plug Control Module Circuit Malfunction

The P0670 – Glow Plug Control Module Circuit Malfunction diagnostic trouble code (DTC) indicates that the engine control module has detected a problem with the electrical circuit or operation of the Glow Plug Control Module (GPCM).

This code is primarily associated with diesel engines. The glow plug control module controls the operation of the glow plugs and determines when they should be activated, how long they should remain active, and in some systems, how much electrical current each glow plug receives.

Unlike cylinder-specific codes such as P0671, P0672, or P0673, P0670 is generally related to the glow plug control module itself or its power, ground, communication, or control circuits.

What Does P0670 Mean?

In simple terms:

The engine control module has detected a malfunction in the glow plug control module circuit.

The basic system can be represented as:

Battery → Fuse → Glow Plug Control Module → Glow Plugs

The ECM/PCM communicates with or controls the glow plug module.

If the control module does not respond correctly, loses power or ground, or its communication/control circuit has a problem, P0670 may be stored.

Depending on the vehicle, the fault can involve:

• Glow plug control module

• Module power supply

• Module ground

• Fuse

• Wiring

• Communication circuit

• Control signal

• Internal module failure

What Is the Glow Plug Control Module?

The Glow Plug Control Module (GPCM) is an electronic component responsible for controlling the glow plugs in a diesel engine.

Older diesel engines may use a relatively simple glow plug relay, while modern diesel engines often use an electronic control module.

A modern GPCM can perform several functions, including:

• Activating the glow plugs

• Controlling glow plug heating time

• Monitoring glow plug current

• Adjusting heating according to engine temperature

• Protecting the glow plug system

• Communicating with the ECM/PCM

• Detecting individual glow plug circuit problems

What Is the Difference Between a Glow Plug Relay and a Glow Plug Control Module?

A conventional glow plug relay primarily switches electrical power to the glow plugs.

A modern Glow Plug Control Module is considerably more sophisticated.

It can electronically regulate and monitor the glow plugs rather than simply switching them on and off.

Some systems can control individual glow plugs independently.

This is why modern diesel vehicles can store both:

P0670 – Glow Plug Control Module Circuit Malfunction

and cylinder-specific codes such as:

P0671 – Cylinder 1 Glow Plug Circuit

What Causes P0670?

Possible causes include:

• Faulty glow plug control module

• Failed glow plug relay

• Blown glow plug fuse

• Low battery voltage

• Poor battery connection

• Poor module ground

• Damaged power wire

• Damaged control wire

• Corroded connector

• Loose connector

• Communication failure between ECM and GPCM

• Shorted wiring

• Open circuit

• Water intrusion

• Internal module failure

The exact cause depends on the vehicle's design.

Can a Bad Glow Plug Control Module Cause P0670?

Yes.

A defective GPCM is one of the main possible causes of P0670.

Internal electronic failures can prevent the module from:

• Activating the glow plugs

• Controlling current correctly

• Communicating with the ECM

• Reporting glow plug status

• Responding to control commands

However, the module should not automatically be replaced simply because P0670 is stored.

Power, ground, wiring, fuses, and communication should be tested first.

Can a Blown Fuse Cause P0670?

Yes.

The glow plug control module can require a high-current power supply.

If its fuse or power protection device fails, the module may become completely inoperative.

Always determine why the fuse failed before replacing it.

A repeatedly blown fuse may indicate:

• Short circuit

• Damaged wiring

• Failed module

• Excessive current draw

Can Low Battery Voltage Cause P0670?

Yes.

Glow plug systems can draw significant electrical current.

Low system voltage can prevent the glow plug control module from operating correctly.

Check:

• Battery voltage

• Battery condition

• Cranking voltage

• Battery terminals

• Ground connections

• Charging voltage

A weak battery can sometimes create electrical DTCs that appear to be module failures.

Can a Bad Alternator Cause P0670?

Yes, indirectly.

An alternator that does not maintain the correct charging voltage can cause low system voltage.

This can interfere with the operation of the GPCM.

If P0670 appears together with battery or charging-system codes, the charging system should be checked before replacing the glow plug module.

Can Bad Ground Cause P0670?

Yes.

A poor ground connection can prevent the control module from functioning correctly.

Inspect:

• GPCM ground connection

• Engine ground strap

• Battery negative terminal

• Chassis ground

• Ground wiring

A corroded or loose ground can cause intermittent or permanent module faults.

Can Damaged Wiring Cause P0670?

Yes.

Wiring problems are an important part of the diagnosis.

Inspect the harness for:

• Broken wires

• Chafing

• Melted insulation

• Corrosion

• Pinched wires

• Short circuits

• Poor previous repairs

Engine-bay wiring is exposed to heat, vibration, moisture, and contamination.

Can a Bad Connector Cause P0670?

Yes.

A poor connector can interrupt power, ground, or communication.

Check for:

• Corrosion

• Loose terminals

• Bent pins

• Water intrusion

• Burn marks

• Damaged locking mechanisms

• Poor terminal tension

Symptoms of P0670

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

The symptoms can vary considerably depending on whether the glow plugs are still being controlled normally.

Can P0670 Cause a No-Start Condition?

It can, depending on the engine and environmental conditions.

A complete glow plug control failure can make starting extremely difficult on some diesel engines, particularly in very cold temperatures.

However, many modern diesel engines can still start without fully functioning glow plugs under moderate conditions.

A no-start condition should therefore not automatically be attributed to P0670 alone.

Fuel pressure, injection, compression, crankshaft speed, and other starting requirements should also be checked.

Can P0670 Cause Hard Starting?

Yes.

If the glow plug control module cannot activate the glow plugs correctly, the combustion chambers may not receive enough heat during a cold start.

This can cause:

• Extended cranking

• Delayed ignition

• Rough startup

• Temporary engine vibration

• White smoke

Can P0670 Cause White Smoke?

Yes.

Insufficient glow plug heating can result in incomplete combustion during cold starting.

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

If the smoke continues after the engine warms up, however, other problems should be investigated.

Can P0670 Cause Rough Idle?

Yes, particularly when the engine is cold.

If several glow plugs are not receiving the required heating current, combustion can be uneven immediately after startup.

The engine may:

• Shake

• Vibrate

• Idle roughly

• Produce white smoke

The condition may improve as the engine warms.

Can P0670 Affect Fuel Economy?

Potentially.

A glow plug control failure can increase fuel consumption during cold starts because combustion may be less efficient.

The effect depends on how frequently the vehicle is started cold and whether the glow plug system remains completely or partially inoperative.

Is P0670 Related to the Fuel Injectors?

Not directly.

P0670 concerns the glow plug control system, not the fuel injection system.

However, injector problems can create similar symptoms, particularly:

• Hard starting

• Rough idle

• White smoke

• Poor cold running

If glow plug operation is confirmed to be normal, fuel-system diagnosis may be necessary.

How Is P0670 Diagnosed?

A systematic diagnosis is important because replacing the GPCM without testing the circuit can be expensive and unnecessary.

1. Scan for all DTCs

Check for:

• Cylinder-specific glow plug codes

• Communication codes

• Voltage-related codes

• Battery codes

• ECM/PCM codes

• Other engine performance codes

Multiple glow plug codes combined with P0670 can point toward a common control-module or power-supply problem.

2. Check the battery

Verify battery condition and voltage.

3. Check the glow plug fuse

Inspect all relevant fuses and high-current protection devices.

4. Check GPCM power supply

Verify that the module receives the required voltage.

5. Check the GPCM ground

Perform a proper ground and voltage-drop test.

6. Inspect the connector

Check for corrosion, loose pins, water intrusion, and heat damage.

7. Inspect the wiring

Check power, ground, control, and communication wiring.

8. Check communication with the ECM

Depending on the vehicle, the GPCM may communicate over a vehicle network such as CAN.

A communication failure can prevent normal operation.

9. Test glow plug outputs

Verify that the module actually provides the required control signal or current to the glow plugs.

10. Test or replace the GPCM only after circuit testing

If power, ground, wiring, and communication are correct but the module does not operate correctly, the GPCM may be defective.

Can a Multimeter Diagnose P0670?

Yes, to a certain extent.

A multimeter can be used to check:

• Battery voltage

• Fuse continuity

• Module supply voltage

• Ground voltage drop

• Wiring continuity

• Short circuits

• Connector voltage

However, a modern GPCM can use electronically controlled outputs, so simple voltage measurements may not be sufficient for complete diagnosis.

Can a Scan Tool Help Diagnose P0670?

Yes.

A capable diagnostic scanner can provide valuable information.

Depending on the vehicle, it may allow you to:

• Read additional GPCM codes

• View live data

• Monitor glow plug status

• Check module communication

• Perform actuator tests

• Command glow plug operation

• Read module-specific information

A basic code reader may only display P0670 without showing the underlying problem.

Can an Oscilloscope Help?

Yes.

An oscilloscope can be particularly useful when diagnosing modern electronically controlled glow plug systems.

It can help identify:

• Missing control signals

• Abnormal PWM signals

• Communication problems

• Irregular voltage patterns

• Abnormal glow plug current

What If P0670 Appears With P0671, P0672, P0673 and Other Glow Plug Codes?

This combination is significant.

If P0670 appears together with multiple cylinder-specific glow plug circuit codes, do not immediately replace every glow plug.

The common components should be checked first:

• Glow plug control module

• Main power supply

• Ground

• Main fuse

• Battery voltage

• Module wiring

• Communication circuit

A common failure can affect several glow plug circuits simultaneously.

What If Only P0670 Is Stored?

If P0670 is the only code, concentrate on the control system itself.

Check:

Battery → Fuse → GPCM power → GPCM ground → Connector → Wiring → Communication → Module operation

The glow plugs themselves should still be checked if there are starting symptoms, but the diagnostic focus should initially be on the control module circuit.

What If P0670 Appears After Glow Plug Replacement?

Replacing glow plugs can sometimes expose an existing electrical problem, especially if the plugs were removed or the wiring was disturbed.

Check:

• Correct glow plug specifications

• Glow plug connectors

• Harness routing

• Module output

• Fuse

• Battery voltage

If P0670 appeared immediately after repairs, inspect anything that was disconnected during the work.

What If P0670 Returns Immediately After Clearing?

If the code returns immediately after clearing, the problem is likely still present.

Focus on:

• Module power

• Module ground

• Fuse

• Communication wiring

• Open or shorted circuits

• Internal GPCM failure

If the code returns only during cold starts, temperature-dependent electrical faults should also be considered.

What If P0670 Is Intermittent?

An intermittent P0670 can be caused by:

• Loose connector

• Corroded terminal

• Broken wire inside insulation

• Poor ground

• Voltage fluctuation

• Intermittent CAN communication

• Internal module failure

Wiggling the harness while monitoring the circuit can sometimes help identify an intermittent connection, but this should be done carefully.

Can P0670 Be Caused by CAN Bus Problems?

Yes, on vehicles where the glow plug control module communicates over CAN.

A communication problem between the ECM and GPCM can prevent normal operation.

Possible causes include:

• CAN wiring damage

• Poor connector contact

• Shorted CAN line

• Module power loss

• Ground problem

• Faulty control module

• Network communication fault

If other modules also report communication-related DTCs, the vehicle network should be investigated.

Is P0670 a Serious Code?

P0670 can range from a relatively minor cold-start problem to a significant starting problem.

If the glow plugs continue to operate normally, the vehicle may drive without obvious performance problems.

However, a complete control-module failure can cause:

• Difficult cold starts

• Extended cranking

• Increased battery load

• Increased starter load

• Rough cold operation

• White smoke

• Increased emissions

The problem is particularly important in very cold climates.

Can You Drive With P0670?

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

P0670 does not necessarily mean the engine is at immediate risk of mechanical damage.

However, continuing to drive indefinitely without repairing the problem is not recommended.

If starting becomes increasingly difficult, the battery and starter can experience additional stress.

How Is P0670 Repaired?

The repair depends on the confirmed cause.

Possible repairs include:

• Replacing the glow plug control module

• Replacing a faulty glow plug relay

• Repairing damaged wiring

• Repairing a poor ground

• Replacing a blown fuse after correcting the underlying fault

• Repairing corroded connectors

• Repairing communication wiring

• Replacing damaged terminals

• Correcting low system voltage

• Repairing battery or charging-system problems

The GPCM should only be replaced after confirming that its power, ground, wiring, and communication circuits are functioning correctly.

Does P0670 Mean All Glow Plugs Are Bad?

No.

P0670 refers primarily to the glow plug control module circuit.

It does not automatically mean that every glow plug has failed.

If individual glow plug codes such as P0671, P0672, or P0673 are also present, each circuit should be tested separately.

Does P0670 Mean the Glow Plug Control Module Must Be Replaced?

Not necessarily.

This is one of the most important points when diagnosing P0670.

A faulty fuse, poor ground, damaged wiring, low battery voltage, or communication problem can produce the same code.

The module should be condemned only after its supporting circuits have been verified.

Common Diagnostic Mistakes

Replacing the GPCM immediately

P0670 does not automatically prove that the module itself is defective.

Ignoring the battery

Low voltage can interfere with module operation.

Ignoring the ground

A poor ground can create apparently mysterious electronic faults.

Replacing all glow plugs

P0670 is a control-module circuit code, not a code saying that all glow plugs have failed.

Ignoring additional DTCs

Other codes may reveal the actual cause of the problem.

Testing only with a basic code reader

Advanced diagnosis may require live data, actuator testing, voltage-drop testing, or waveform analysis.

P0670 vs. Cylinder-Specific Glow Plug Codes

These codes should not be confused.

P0670 → Glow Plug Control Module Circuit Malfunction

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 cylinder-specific codes point toward individual glow plug circuits, while P0670 points toward the control module circuit/system.

How Do You Clear P0670?

The correct process is:

Scan the vehicle → Record all DTCs → Check battery voltage → Check fuses → Check GPCM power → Check GPCM ground → Inspect connectors → Inspect wiring → Check communication → Test glow plug outputs → Repair the confirmed fault → Clear DTCs → Perform a cold-start test → Rescan

If P0670 returns, the underlying glow plug control system still has an unresolved problem.

Final Thoughts on P0670

P0670 – Glow Plug Control Module Circuit Malfunction indicates a problem with the electrical circuit or operation of the diesel engine's glow plug control module.

Possible causes include a faulty GPCM, blown fuse, poor ground, damaged wiring, corroded connector, low battery voltage, communication failure, faulty relay, or internal module failure.

The most important diagnostic point is that P0670 does not automatically mean the glow plug control module needs to be replaced.

A proper diagnosis should follow this sequence:

Check battery → Check fuse → Check GPCM power → Check ground → Inspect connectors → Check wiring → Check communication → Test glow plug outputs → Evaluate the GPCM.

If P0670 appears together with multiple cylinder-specific glow plug codes, the common power supply, ground, wiring, or control module should be investigated before replacing multiple glow plugs.