P0684 Glow Plug Control Module - PCM Communication Circuit Range/Performance

The P0684 – Glow Plug Control Module - PCM Communication Circuit Range/Performance diagnostic trouble code (DTC) indicates that the Powertrain Control Module (PCM) has detected a problem with the communication or control signal between the PCM and the Glow Plug Control Module (GPCM).

This code is primarily associated with diesel engines. The glow plug system helps a diesel engine start by heating the combustion chambers before and sometimes during engine startup.

When the PCM detects that communication with the glow plug control module is outside the expected operating range or is not performing correctly, it can store P0684.

What Does P0684 Mean?

In simple terms:

The PCM and glow plug control module are not communicating or responding within the expected electrical or operational range.

A simplified system looks like:

PCM → Communication/Control Circuit → Glow Plug Control Module → Glow Plugs

The PCM determines when glow plug assistance is required based on factors such as:

• Engine coolant temperature

• Intake air temperature

• Battery voltage

• Engine operating conditions

• Engine speed

• Starting conditions

The glow plug control module then manages the glow plugs according to the commands it receives.

If the PCM detects that the communication or control signal is outside the expected range, P0684 may be stored.

What Is the Glow Plug Control Module?

The Glow Plug Control Module (GPCM) is an electronic controller used on many diesel engines.

Its job is to control the glow plugs accurately rather than simply switching them on and off.

Depending on the vehicle, the module may control:

• Glow plug activation

• Glow plug heating time

• Post-glow operation

• Glow plug current

• Individual glow plug circuits

• Communication with the PCM

The exact design varies between manufacturers.

What Is the PCM Communication Circuit?

The communication circuit allows the PCM and GPCM to exchange information.

Depending on the vehicle, this may involve:

• Dedicated control wiring

• Serial communication

• CAN bus communication

• Other manufacturer-specific communication networks

The PCM may send commands to the GPCM and receive information about glow plug operation.

P0684 does not necessarily mean that the glow plugs themselves are defective.

The problem may be in the communication circuit, module power supply, wiring, or control module.

What Causes P0684?

Possible causes include:

• Faulty glow plug control module

• Damaged communication wiring

• Open communication circuit

• Shorted communication circuit

• Poor electrical connection

• Corroded connector

• Low battery voltage

• Poor GPCM power supply

• Poor GPCM ground

• PCM communication problem

• CAN bus fault

• Damaged fuse

• Incorrect wiring repair

• Water intrusion

• Aftermarket electrical modifications

• Internal PCM or GPCM failure

The actual cause must be confirmed through testing.

Can a Bad Glow Plug Control Module Cause P0684?

Yes.

A defective GPCM is one of the most likely possibilities.

The module may have:

• Internal electronic failure

• Communication failure

• Power-supply problems

• Internal overheating

• Damaged output circuitry

However, the module should not automatically be replaced without checking its power, ground, and communication circuits.

Can a Bad Glow Plug Cause P0684?

Usually not directly.

A failed glow plug can produce other glow-plug-related DTCs.

P0684 primarily concerns the communication or performance of the circuit between the PCM and GPCM.

However, severe problems within the glow plug system can sometimes result in additional faults.

Can Low Battery Voltage Cause P0684?

Yes.

Glow plug systems can draw substantial electrical current.

A weak battery or low system voltage can interfere with proper GPCM operation and communication.

Check:

• Battery condition

• Battery voltage

• Cranking voltage

• Charging voltage

Can a Bad Alternator Cause P0684?

Yes, indirectly.

If the alternator does not maintain the correct system voltage, the GPCM and PCM may not operate correctly.

Voltage instability can cause communication faults.

Can a Bad Ground Cause P0684?

Yes.

The GPCM requires a stable ground reference.

A poor ground can cause:

• Communication problems

• Module resets

• Incorrect voltage readings

• Glow plug control problems

Inspect:

• Battery grounds

• Engine grounds

• Chassis grounds

• GPCM ground connections

Can a Fuse Cause P0684?

Yes.

The GPCM normally receives power through one or more fuses.

A blown fuse or poor fuse connection can prevent the module from operating.

This may cause the PCM to lose communication with the GPCM.

Always determine why a fuse failed before replacing it.

Can a Wiring Problem Cause P0684?

Yes.

Wiring problems are important to investigate.

Look for:

• Broken wires

• Chafed insulation

• Corrosion

• Melted wiring

• Loose terminals

• Water intrusion

• Poor previous repairs

The glow plug control module is often located in the engine compartment, where wiring can be exposed to heat, vibration, moisture, and contamination.

Can a CAN Bus Problem Cause P0684?

Yes, on vehicles where the GPCM communicates with the PCM over CAN.

A problem with the communication network can prevent the PCM and GPCM from exchanging information.

Possible causes include:

• CAN High wiring fault

• CAN Low wiring fault

• Short to ground

• Short to power

• Open communication wire

• Poor connector

• Network termination problem

• Another module interfering with the network

Additional U-codes may be stored if the CAN network is affected.

Can the PCM Cause P0684?

Yes, but it is not the first component to suspect.

The PCM may have an internal communication-driver problem.

However, the technician should first verify:

Power → Ground → Wiring → Communication network → GPCM

before condemning the PCM.

Symptoms of P0684

Possible symptoms include:

• Check Engine Light

• Glow plug warning light

• Difficult cold starting

• Longer cranking time

• Rough cold idle

• White smoke during startup

• Reduced cold-start performance

• Reduced engine performance

• Increased emissions

• Intermittent starting problems

Some diesel engines may start normally in warm weather even though P0684 is stored.

Can P0684 Cause a No-Start Condition?

Yes, potentially.

Diesel engines depend more heavily on glow plug assistance during cold starts.

If the glow plug control system is unavailable, the engine may become difficult or impossible to start under very cold conditions.

However, many modern diesel engines can still start without fully functioning glow plugs when the ambient temperature is relatively high.

Can P0684 Cause Hard Starting?

Yes.

This is one of the most common symptoms on diesel engines.

A communication failure can prevent the GPCM from properly controlling the glow plugs.

This can result in:

• Extended cranking

• Rough startup

• Misfiring immediately after starting

• White exhaust smoke

Can P0684 Cause White Smoke?

Yes, particularly during cold starts.

If the combustion chamber is not heated adequately, fuel may not burn completely during initial combustion.

This can produce white or light-colored exhaust smoke immediately after startup.

Can P0684 Cause Rough Idle?

Yes, especially when the engine is cold.

Insufficient glow plug operation can cause incomplete combustion during the first moments after startup.

The engine may:

• Shake

• Misfire

• Idle unevenly

• Produce white smoke

The problem may disappear as the engine warms up.

Is P0684 Related to the Glow Plug Relay?

Some diesel vehicles use a dedicated glow plug relay, while others use an electronic glow plug control module.

The terminology differs by manufacturer.

If the vehicle uses a relay rather than a sophisticated GPCM, the technician should follow the manufacturer's wiring diagram to identify the actual component involved.

How Is P0684 Diagnosed?

A systematic diagnostic approach is recommended.

1. Scan all control modules

Record:

• Stored DTCs

• Pending DTCs

• Glow plug codes

• Communication codes

• U-codes

• Voltage-related codes

Other codes can provide important clues.

2. Check the battery

Verify battery condition and voltage.

3. Check charging voltage

Make sure the alternator is operating correctly.

4. Locate the GPCM

Identify the glow plug control module and its connectors.

5. Check GPCM power supply

Verify that the module receives the required voltage.

6. Check GPCM grounds

Perform voltage-drop testing where appropriate.

7. Inspect connectors

Look for:

• Corrosion

• Water intrusion

• Loose terminals

• Bent pins

• Damaged locks

8. Inspect communication wiring

Look for open circuits, shorts, and physical damage.

9. Check CAN communication

If the vehicle uses CAN communication, verify the network according to manufacturer specifications.

10. Check related fuses

Verify both the fuse and the electrical connection at the fuse.

11. Check PCM communication

Confirm that the PCM can communicate with the GPCM.

12. Test the GPCM

If power, ground, and communication wiring are correct, the GPCM may require further testing or replacement.

Can a Multimeter Diagnose P0684?

Yes, to a certain extent.

A multimeter can check:

• Battery voltage

• Charging voltage

• GPCM power supply

• GPCM grounds

• Wiring continuity

• Voltage drop

• Fuse voltage

However, communication-network faults may require more advanced diagnostic equipment.

Can an Oscilloscope Help?

Yes.

An oscilloscope can be particularly useful when diagnosing communication problems.

It can help identify abnormal signals on:

• CAN High

• CAN Low

• Dedicated communication lines

This can reveal problems that a simple continuity test may not detect.

Can a Scan Tool Help?

Yes.

A professional scan tool can determine whether the PCM can communicate with the GPCM.

If the GPCM cannot be accessed while other modules communicate normally, the diagnostic process should focus on:

GPCM power → GPCM ground → Communication wiring → GPCM

What If P0684 Appears With U-Codes?

This is an important clue.

If P0684 appears together with communication-related U-codes, investigate the vehicle's communication network.

A possible sequence is:

CAN/network fault → GPCM communication lost → PCM detects abnormal communication → P0684

Do not automatically replace the GPCM.

What If P0684 Appears With Glow Plug Codes?

If P0684 appears together with individual glow plug codes, there may be more than one problem.

For example:

GPCM communication problem + glow plug circuit faults

In this situation, first determine whether the GPCM itself is functioning and communicating before replacing individual glow plugs.

What If P0684 Appears Only in Cold Weather?

This is an important diagnostic clue.

Cold temperatures increase the importance of the glow plug system.

A marginal battery, poor connection, weak GPCM, or glow plug system problem may become more noticeable during cold starts.

Check battery condition and GPCM power supply carefully.

What If P0684 Appears After Battery Replacement?

Check:

• Battery terminals

• Battery ground

• Fuse box connections

• GPCM power supply

• GPCM connectors

A loose electrical connection can be disturbed during battery service.

What If P0684 Appears After Engine Repair?

Inspect the engine-bay wiring carefully.

Pay attention to:

• GPCM connectors

• Communication wiring

• Ground straps

• Harness routing

• Wiring near the glow plugs

A harness can easily be damaged, pinched, or left disconnected during engine work.

Is P0684 a Serious Code?

It can be moderately to seriously important depending on the vehicle and climate.

In warm conditions, the engine may continue to start and operate normally.

In cold conditions, however, a glow plug control problem can cause:

• Difficult starting

• Extended cranking

• Rough running

• Excessive smoke

• Increased battery and starter load

A persistent communication problem should therefore be diagnosed.

Can You Drive With P0684?

If the diesel engine starts normally and runs correctly, the vehicle may still be driveable.

However, the problem should not be ignored, particularly if:

• Cold starting is difficult

• The glow plug warning light is illuminated

• The engine stalls

• There are additional communication codes

• The vehicle has severe starting problems

How Is P0684 Repaired?

The repair depends on the confirmed cause.

Possible repairs include:

• Replacing the glow plug control module

• Repairing communication wiring

• Repairing CAN wiring

• Repairing damaged connectors

• Cleaning corrosion

• Replacing damaged terminals

• Repairing GPCM power supply

• Repairing GPCM grounds

• Replacing a blown fuse after correcting the underlying problem

• Repairing PCM communication circuits

• Replacing the PCM only when internal failure is confirmed

Does P0684 Require Glow Plug Replacement?

Not necessarily.

P0684 is primarily a GPCM-to-PCM communication/range-performance code.

Replacing all glow plugs without testing the communication circuit may not solve the problem.

Does P0684 Require Glow Plug Control Module Replacement?

Not automatically.

The GPCM is a possible cause, but power, ground, wiring, communication network, and connectors should be checked first.

Does P0684 Require PCM Replacement?

Rarely.

The PCM should only be considered after the GPCM and its complete electrical and communication circuits have been verified.

Common Diagnostic Mistakes

Replacing all glow plugs immediately

P0684 does not specifically identify a failed glow plug.

Replacing the GPCM without checking power and ground

A simple electrical supply problem can prevent a good module from communicating.

Ignoring CAN communication faults

If the vehicle uses CAN communication, network problems can cause P0684.

Ignoring battery condition

Low voltage can affect module communication.

Replacing the PCM too early

The PCM is usually one of the last components that should be condemned.

P0684 vs. Other Glow Plug Codes

P0684 is different from codes that specifically identify individual glow plug circuits.

P0670 generally concerns the glow plug module/control circuit.

P0671–P0678 generally relate to individual cylinder glow plug circuits.

P0684 focuses on the communication or range/performance relationship between the GPCM and PCM.

Therefore:

P0684 → GPCM/PCM communication or performance problem

P0671–P0678 → Individual glow plug circuit problems

How Do You Clear P0684?

The proper procedure is:

Scan all modules → Record DTCs → Check battery → Check charging system → Check GPCM fuses → Verify GPCM power → Verify GPCM ground → Inspect connectors → Test communication wiring → Check CAN network if applicable → Test GPCM → Verify PCM communication → Repair the confirmed fault → Clear DTCs → Cold-start test → Rescan

If P0684 returns, the communication or range/performance problem between the PCM and glow plug control system has not been properly corrected.

Final Thoughts on P0684

P0684 – Glow Plug Control Module - PCM Communication Circuit Range/Performance indicates that the PCM has detected an abnormal communication or control condition involving the Glow Plug Control Module (GPCM).

Possible causes include a faulty GPCM, damaged communication wiring, CAN network problems, poor power or ground connections, corroded connectors, low battery voltage, blown fuses, or, less commonly, a PCM problem.

The most important point is that P0684 does not automatically mean the glow plugs themselves are defective.

The best diagnostic sequence is:

Check battery → Check fuses → Verify GPCM power → Check GPCM grounds → Inspect connectors → Test communication wiring → Check CAN network → Test GPCM → Evaluate PCM.

If the vehicle has hard cold starting, excessive white smoke, rough startup, or repeated communication faults, the problem should be diagnosed before the next cold-weather starting cycle.