U0106 Lost Communication With Glow Plug Control Module (GPCM)

The diagnostic trouble code U0106 – Lost Communication With Glow Plug Control Module (GPCM) indicates that another control module in the vehicle has lost expected communication with the Glow Plug Control Module.

This code is primarily a communication/network fault, not a direct statement that the glow plugs themselves have failed.

On diesel engines, the GPCM manages the operation of the glow plugs and, depending on the vehicle, may also monitor glow-plug current, temperature, voltage, and operating conditions. It communicates with other control modules so that glow-plug operation can be coordinated with engine temperature, injection strategy, emissions control, and starting requirements.

When communication with the GPCM is interrupted, the vehicle may store U0106 and may also record additional glow-plug, voltage, or network-related codes.

What Is the Glow Plug Control Module?

A diesel engine uses glow plugs to assist combustion during cold starting and, on many modern engines, during certain periods after startup.

Older systems often used a relatively simple glow-plug relay. Modern diesel engines can use a dedicated Glow Plug Control Module (GPCM) that provides much more precise control.

Depending on the engine, the GPCM may:

• Control individual glow plugs

• Regulate glow-plug current

• Control pre-glow operation

• Control post-glow operation

• Monitor electrical load

• Detect glow-plug faults

• Communicate with the ECM/PCM

• Adjust glow-plug operation according to engine temperature

• Support emissions-related strategies

The exact design differs between manufacturers.

Some vehicles integrate glow-plug control functions into another module rather than using a separate GPCM, so the exact module arrangement should always be confirmed for the particular vehicle.

What Does “Lost Communication” Mean?

The critical part of U0106 is “Lost Communication.”

The code means that a control module expected messages from the GPCM but those messages were not received within the required time or were otherwise unavailable.

That can happen because:

• The GPCM has lost power.

• The GPCM has lost its ground.

• A fuse has failed.

• The communication wiring is damaged.

• A connector has poor contact.

• CAN communication is interrupted.

• Another module is disrupting the network.

• The GPCM has an internal electronic fault.

• Battery voltage is too low or unstable.

• There is a software or configuration problem.

Therefore, U0106 should not automatically be interpreted as “bad glow plugs.”

How the GPCM Communicates With the Engine Computer

Modern vehicles commonly use a CAN (Controller Area Network) to allow electronic control modules to exchange information.

The GPCM may communicate with the:

• Engine Control Module (ECM)

• Powertrain Control Module (PCM)

• Body Control Module

• Instrument cluster

• Gateway module

• Other powertrain or emissions-related modules

The network architecture is manufacturer-specific.

A communication failure can therefore originate outside the GPCM itself.

For example, a damaged CAN wire may prevent the ECM from communicating with the GPCM even though the GPCM and its glow-plug outputs are physically functional.

Symptoms You May Notice

U0106 can produce different symptoms depending on the diesel engine and the severity of the communication failure.

Possible symptoms include:

• Check Engine light

• Glow plug warning light

• Difficult cold starting

• Extended cranking

• Rough running immediately after startup

• Increased white smoke during cold start

• Reduced glow-plug operation

• Poor cold-weather starting

• Reduced engine performance

• Increased emissions

• Multiple communication-related DTCs

• Intermittent no-start condition in some vehicles

A warm engine may start normally even when a GPCM communication problem exists.

This can make the fault appear to be temperature-dependent.

Why Cold Starting Is Especially Important

Glow plugs are particularly important during cold starting because diesel combustion depends heavily on the temperature inside the combustion chamber.

When the engine is cold, the GPCM can energize the glow plugs before cranking.

This is commonly called pre-glow or preheating.

The system may also continue controlling the glow plugs after the engine starts. This can improve combustion stability and reduce emissions during the warm-up period.

If communication with the GPCM is lost, the engine may still start under favorable conditions, but cold-start performance can deteriorate.

In severe cases, especially at low ambient temperatures, the engine may crank for a long time or fail to start.

Common Causes of U0106

GPCM power supply problem

The first question should be whether the GPCM is actually receiving the electrical power it needs.

Possible causes include:

• Blown fuse

• Damaged power wire

• Loose connection

• Corroded fuse terminal

• Failed relay where applicable

• Voltage supply problem

If the module has no power, it cannot communicate with the ECM.

Poor GPCM ground

A damaged or corroded ground connection can cause the GPCM to malfunction or disappear from the network.

A ground can sometimes appear acceptable during a simple continuity test but develop excessive voltage drop when the module is operating.

CAN wiring fault

A communication wire may be:

• Open

• Shorted to ground

• Shorted to power

• Shorted to another communication circuit

• Physically damaged

• Corroded

• Poorly connected

The correct wiring diagram should be used to identify the GPCM's network connections.

Connector problems

The GPCM connector should be inspected for:

• Corrosion

• Moisture

• Bent pins

• Loose terminals

• Pushed-back terminals

• Poor terminal tension

• Damaged locking mechanisms

• Heat damage

The physical location of the GPCM can be important. Depending on the vehicle, it may be exposed to engine heat, water, road contamination, or vibration.

Low battery voltage

Diesel engines generally require substantial electrical power during starting.

A weak battery or poor battery connection can cause modules to reset or become temporarily unavailable.

If U0106 appears together with several unrelated communication codes, checking battery voltage, charging performance, and major grounds is particularly important.

Internal GPCM failure

The GPCM itself can fail.

Electronic components inside the module can develop faults caused by heat, moisture, vibration, electrical overload, or age.

However, the module should not be condemned until its power, ground, communication circuits, and connectors have been properly checked.

U0106 Does Not Mean the Glow Plugs Are Bad

This distinction is particularly important.

A defective glow plug can produce a glow-plug circuit or individual glow-plug fault code.

U0106, on the other hand, concerns communication with the control module responsible for the glow-plug system.

You could therefore have:

• Good glow plugs + faulty GPCM communication

• Good GPCM communication + defective glow plug

• Defective GPCM + good glow plugs

• Wiring fault affecting both communication and glow-plug control

The diagnostic approach is different in each case.

A Practical Diagnostic Approach

A good diagnosis starts with the entire vehicle rather than U0106 alone.

Scan every available control module

Use a scan tool capable of communicating with the vehicle's different modules.

Check whether the GPCM is visible and accessible.

Also record all other DTCs.

If multiple modules report lost communication with different systems, a general network or power-supply problem may be more likely than several modules failing simultaneously.

Check whether the GPCM responds to the scan tool

This is an important diagnostic step.

If the GPCM communicates normally with the scan tool but another module reports U0106, an intermittent communication problem or a specific network path may need further investigation.

If the GPCM cannot be accessed at all, check its power, ground, network connections, and the module itself.

Inspect fuses and power supplies

Use the vehicle-specific fuse and wiring information.

Do not assume that a single fuse supplies the entire glow-plug system.

Some vehicles use separate circuits for module electronics and high-current glow-plug operation.

Test power and ground

Verify the required voltage at the GPCM under the conditions specified by the manufacturer.

Also verify the ground circuit.

Voltage-drop testing can be particularly useful when a ground or power connection is suspected.

Inspect the CAN network

After confirming power and ground, investigate communication circuits.

Depending on the manufacturer's procedure, this may involve checking:

• CAN wiring continuity

• Shorts to ground

• Shorts to power

• Shorts between CAN circuits

• Network termination

• Communication activity

• Connector integrity

There is no single universal voltage or resistance measurement that should be applied to every vehicle. CAN architecture and diagnostic procedures vary.

Check the Glow Plug System Separately

Once communication with the GPCM has been restored, the actual glow-plug system should be evaluated if starting problems or glow-plug codes remain.

Depending on the vehicle, diagnosis may include:

• Individual glow-plug condition

• Glow-plug resistance or current

• GPCM output

• Wiring to individual glow plugs

• Electrical connections

• Battery condition

• Engine coolant temperature information

• Commanded glow time

• Actual system behavior

Glow-plug resistance specifications vary by design, so manufacturer-specific information should be used rather than relying on a universal resistance value.

U0106 With a Glow Plug Warning Light

If U0106 appears together with a glow-plug warning light, the GPCM communication problem may be directly related to the warning.

However, the warning light alone does not prove that the GPCM has failed.

The system may illuminate the warning because the ECM cannot confirm that glow-plug control is available.

This is similar to other networked automotive systems: a module can report a problem because it cannot communicate with another module, even though the other module's controlled component may still be physically functional.

Intermittent U0106

An intermittent U0106 can be particularly difficult to locate.

The vehicle might:

• Start normally on some mornings

• Crank longer when cold

• Show the glow-plug warning only occasionally

• Lose communication after driving over rough roads

• Develop the code during very cold weather

• Clear the problem after cycling the ignition

In these cases, inspect connectors and wiring carefully.

A loose terminal can maintain contact while the vehicle is stationary but lose contact because of vibration or engine movement.

Moisture intrusion can also create intermittent network faults.

Can U0106 Cause a No-Start Condition?

It can, depending on the vehicle.

Some diesel engines can start with reduced or limited glow-plug assistance when temperatures are moderate.

Others depend more heavily on correct glow-plug operation during cold starting.

At very low temperatures, loss of glow-plug control can make starting significantly more difficult.

If the engine cranks normally but does not start, U0106 should therefore be investigated alongside:

• Fuel pressure

• Injector operation

• Engine speed signal

• Rail pressure

• Engine temperature data

• Air intake conditions

• Compression

• Other stored DTCs

A no-start condition should not automatically be attributed to the GPCM.

Could a Bad Battery Cause U0106?

Yes.

Low system voltage can affect electronic control modules and communication networks.

This is especially relevant on diesel vehicles because starting places a substantial electrical demand on the battery.

A weak battery may cause:

• Module resets

• Temporary communication loss

• Multiple U-codes

• Glow-plug control problems

• Slow cranking

• Intermittent starting problems

If several unrelated modules suddenly report communication faults after a weak-starting event, battery and power supply checks should be performed early in the diagnosis.

Should the GPCM Be Replaced?

Not simply because U0106 is stored.

Before replacing the module, establish that:

• The module has the correct power supply.

• The module has a reliable ground.

• Its fuses and related circuits are intact.

• CAN communication wiring is sound.

• Connectors and terminals are in good condition.

• There is no network fault elsewhere.

• The module is actually inaccessible or failing its manufacturer-specific tests.

Only then does GPCM replacement become a stronger conclusion.

Some vehicles may also require programming, configuration, initialization, or relearning after installing a replacement GPCM.

Driving With U0106

The risk depends on how the vehicle is behaving.

If the engine starts and runs normally, the vehicle may remain driveable, particularly in warmer conditions.

However, ignoring the fault can leave the vehicle vulnerable to difficult cold starts or a no-start condition when temperatures fall.

If the engine is hard to start, stalls, runs very roughly, or produces excessive smoke, the vehicle should be diagnosed promptly.

A diesel that repeatedly requires extended cranking can also place additional strain on the battery and starter.

The Most Important Diagnostic Principle

U0106 is a communication code, so the diagnosis should begin with communication and module availability rather than automatically replacing glow plugs.

The correct sequence is generally to determine:

Can the GPCM communicate?

If not:

Does it have proper power and ground?

If power and ground are correct:

Is the CAN network intact?

If the network is healthy:

Does the GPCM itself pass the manufacturer's diagnostic tests?

Only after these questions are answered should the investigation move toward module replacement.

The glow plugs themselves may still need testing, but U0106 primarily tells you that the vehicle has lost the expected communication with the Glow Plug Control Module. Finding out why that communication disappeared is the key to repairing the fault rather than simply replacing parts.