U0105 Lost Communication With Fuel Injector Control Module (FICM)

U0105 is a control-module communication fault indicating that communication with the Fuel Injector Control Module (FICM) has been lost.

This code is different from a conventional injector circuit code such as P0201 or P0202. Those codes generally concern the electrical control of an individual injector circuit. U0105 is broader: it indicates that the vehicle's electronic control system is no longer receiving the expected communication from the FICM.

That distinction is important because U0105 does not automatically mean that the fuel injectors are defective.

The problem may be in the FICM's power supply, ground, communication network, wiring, connector, another module affecting the network, or the FICM itself.

What Is the Fuel Injector Control Module?

The FICM is an electronic controller used on certain vehicles to control or assist with fuel-injection operation.

Its exact design varies considerably between manufacturers and engine families.

Depending on the application, the FICM may:

• Control fuel injectors directly

• Provide the electrical power required by injectors

• Generate or regulate injector drive voltage

• Receive commands from the ECM/PCM

• Control injection timing

• Control injector duration

• Monitor injector operation

• Report injector-related information to other modules

The terminology is particularly common on some electronically controlled diesel engines, although the exact architecture is vehicle-specific.

Some engines have a physically separate FICM, while others integrate injector-control functions into another engine controller.

U0105 Is Not an Injector Failure Code

This is one of the most important points when diagnosing this DTC.

U0105 says:

Communication with the FICM has been lost.

It does not say:

Injector number X has failed.

A vehicle can have perfectly good injectors and still set U0105 if the FICM loses its power supply.

Likewise, an FICM can be fully functional but unable to communicate because of a damaged CAN circuit.

This is why replacing injectors after seeing U0105 would usually be an inappropriate first step.

How the FICM Communicates With the Vehicle

Modern vehicles contain multiple electronic control modules that exchange information over a communication network.

Depending on the vehicle, the FICM may communicate with:

• ECM

• PCM

• Transmission Control Module

• Body Control Module

• ABS/ESC module

• Gateway module

• Instrument cluster

• Other engine-management controllers

CAN communication is commonly used, although the exact network architecture varies.

The engine-control system may depend on communication from the FICM to determine whether injector operation can safely continue.

If those messages disappear, the ECM may activate a fail-safe strategy.

What Can Happen When FICM Communication Is Lost?

The symptoms depend on the engine and control architecture.

Possible symptoms include:

• Check Engine Light

• Engine warning message

• Hard starting

• Extended cranking

• Rough running

• Misfire

• Loss of power

• Poor acceleration

• Engine stalling

• No-start condition

• Reduced-power mode

• Limited RPM

• Increased smoke

• Poor fuel economy

On some diesel engines, a complete FICM failure can result in a crank-no-start condition because the injectors cannot be controlled correctly.

On other vehicles, the engine may continue running using a limited strategy.

Common Causes of U0105

There are several possible causes, and the correct diagnosis starts with the electrical and communication system.

Loss of FICM power

The FICM cannot communicate if it does not receive the required power.

Possible causes include:

• Blown fuse

• Failed relay

• Open power wire

• Corroded fuse connection

• Damaged harness

• Ignition-power problem

• Loose electrical terminal

A missing power supply is one of the first things that should be investigated.

Poor FICM ground

A controller can have voltage available at its power terminal and still fail to operate because its ground circuit is inadequate.

Possible causes include:

• Corroded ground connection

• Loose ground

• Broken ground wire

• Excessive resistance

• Engine-to-chassis grounding problem

Ground testing under load is more meaningful than simply checking whether a ground wire has continuity.

CAN communication failure

If the FICM communicates over CAN, damage to the network can prevent communication.

Possible problems include:

• Open CAN-H

• Open CAN-L

• Short to ground

• Short to voltage

• Excessive resistance

• Damaged connector

• Poor terminal contact

• Harness damage

The correct CAN test procedure depends on the vehicle's network architecture.

Connector problems

FICM connectors can be affected by:

• Vibration

• Engine movement

• Heat

• Moisture

• Oil contamination

• Previous repairs

A connector may look connected while one terminal inside has poor contact.

Harness damage

The wiring can become damaged through:

• Chafing

• Engine vibration

• Heat

• Incorrect routing

• Rodent damage

• Previous engine repairs

• Collision repairs

An intermittent harness problem may cause U0105 only under certain operating conditions.

The First Thing to Determine: Can the FICM Be Accessed?

A professional scan tool can provide an extremely useful clue.

Try to establish whether the scan tool can communicate directly with the FICM.

There are two very different situations.

The FICM communicates

If the module responds normally but U0105 is stored, the problem may be intermittent.

Focus on:

• Network integrity

• Power stability

• Ground quality

• Connector condition

• Harness movement

• Freeze-frame information

• Conditions under which communication was lost

The FICM does not communicate

If the scan tool cannot access the FICM at all, investigate:

• FICM power

• FICM ground

• Fuses

• Relays

• Communication wiring

• Connector condition

• Network faults

• FICM internal operation

This distinction can dramatically shorten the diagnostic process.

Scan Every Module

Do not scan only the ECM.

A complete vehicle scan can reveal whether U0105 is an isolated problem or part of a larger network failure.

Look for:

• U0105

• Other U-codes

• Low-voltage codes

• ECM communication faults

• Transmission communication faults

• ABS/ESC communication faults

• Injector-related codes

• Engine-control faults

• Network gateway faults

If many modules report communication failures simultaneously, the FICM may not be the root cause.

There may be a common power, ground or communication problem.

Multiple Communication Codes Are a Major Clue

Suppose a vehicle stores:

• U0105

• U0100

• U0101

• U0121

and several modules are inaccessible.

Replacing the FICM immediately would make little sense.

The pattern suggests that the vehicle may have a broader electrical or network problem.

Possible causes include:

• Low battery voltage

• Failed main power supply

• Poor ground

• CAN network failure

• Gateway problem

• Water intrusion

• Shorted communication circuit

A module should not be blamed simply because its communication code appears in the scan report.

Check Battery and Charging Voltage

Electronic modules are highly sensitive to unstable electrical power.

Check:

• Battery condition

• Battery terminals

• Charging system

• Voltage during cranking

• Main engine grounds

• FICM supply voltage

A weak battery can cause modules to reset during starting.

A poor connection can produce voltage instability even when the battery itself is good.

If U0105 appeared immediately after a dead-battery event or jump-start, that history is worth recording.

However, a recurring U0105 under normal voltage conditions requires further diagnosis.

Do Not Rely on Resistance Testing Alone

A common mistake is to measure a wire with an ohmmeter and find low resistance.

That does not necessarily prove the circuit is healthy.

A communication wire can pass a static continuity test but fail under operating conditions.

Likewise, a connector terminal may have enough contact for a meter but not enough for reliable communication.

Depending on the manufacturer, diagnosis may require:

• Voltage-drop testing

• Loaded circuit testing

• Network resistance testing

• Oscilloscope testing

• CAN waveform analysis

• Module communication tests

The vehicle-specific service information should determine the appropriate procedure.

Check the FICM Power Supply Under Load

This is particularly important on systems where the FICM has demanding electrical functions.

Do not simply measure battery voltage with the connector disconnected and conclude that the power supply is good.

The circuit should be evaluated under the conditions specified by the manufacturer.

A poor connection can show normal voltage with virtually no load and then develop a significant voltage drop when the FICM operates.

Diesel FICM Systems Can Have Different Electrical Architectures

Not all FICM systems work the same way.

Some systems use the FICM to provide specialized electrical drive for injectors.

Others may have different arrangements involving the ECM/PCM and injector drivers.

Therefore, generic internet instructions such as:

"Measure this pin and expect exactly X volts"

can be dangerous or simply incorrect.

Always identify the exact engine and FICM architecture before performing electrical tests.

Can a Bad Injector Cause U0105?

A conventional injector failure does not normally cause a lost-communication code by itself.

An injector may be mechanically stuck, electrically open, shorted or contaminated and generate injector or misfire-related faults while the FICM continues communicating.

However, an electrical problem affecting a shared injector-control circuit can potentially damage or disrupt the FICM or cause it to shut down.

Therefore, if injector codes appear together with U0105, investigate the complete system rather than assuming the injectors are the primary cause.

Can an Injector Short Damage the FICM?

Depending on the system architecture, an injector or injector harness electrical fault can place abnormal electrical load on the injector driver circuitry.

This is one reason the complete diagnostic history matters.

If U0105 appeared immediately after an injector or engine-harness repair, inspect:

• Injector connectors

• Injector harness

• FICM connector

• Shared power circuits

• Grounds

• Wiring for shorts

• Previous repair work

The exact protection strategy varies by FICM design.

Can a Weak Battery Cause U0105?

Yes, potentially.

A battery that drops excessively during cranking can cause an electronic controller to reset or temporarily disappear from the network.

Other possibilities include:

• Corroded battery terminals

• Poor engine ground

• Loose main power connection

• Charging-system fault

• Damaged FICM supply circuit

However, a battery should not be blamed simply because the code is a communication code.

Measure the actual electrical conditions.

U0105 After Engine Repair

If U0105 appeared immediately after major engine work, inspect the areas that were disturbed.

Common possibilities include:

• FICM connector not fully seated

• Engine harness pinched

• Ground strap left loose

• Harness routed against a hot component

• Damaged CAN wire

• Connector terminal pushed backward

• Fuse or relay connection disturbed

This is often more productive than immediately ordering a replacement module.

U0105 After FICM Replacement

If the FICM has already been replaced and U0105 remains, the replacement itself should not automatically be declared defective.

Depending on the vehicle, the replacement may require:

• Programming

• Configuration

• Coding

• Software initialization

• Security synchronization

• Calibration

The exact procedure is manufacturer-specific.

A replacement module can also be the wrong part number or incompatible with the vehicle's engine configuration.

What About Used FICM Modules?

A used control module can create additional complications.

Compatibility may depend on:

• Engine type

• Model year

• Software version

• Calibration

• Hardware revision

• Vehicle configuration

Even if a used FICM physically plugs into the vehicle, that does not guarantee that it is correctly configured.

Always verify compatibility and programming requirements before installing a replacement.

Intermittent U0105

An intermittent U0105 deserves careful attention.

The vehicle may:

• Start normally

• Run normally

• Suddenly lose power

• Store U0105

• Recover after cycling the ignition

This can happen when a connection changes with:

• Engine vibration

• Temperature

• Moisture

• Harness movement

• Electrical load

A technician may need to reproduce the condition while monitoring FICM communication and power supply.

A fault that disappears after restarting is not necessarily a software problem.

U0105 With a No-Start Condition

A no-start condition combined with U0105 can be more serious.

If the FICM is responsible for injector control on the particular engine, loss of FICM communication or operation can prevent proper injector activation.

A useful diagnostic path is:

Battery and cranking voltage → FICM power → FICM ground → FICM communication → injector-control system → fuel delivery → engine synchronization

The exact sequence depends on the engine.

Do not replace the high-pressure fuel pump simply because the engine does not start.

A communication failure can prevent injection even when the fuel system itself is mechanically capable of producing pressure.

U0105 and Fuel Rail Pressure Codes

If U0105 appears with fuel-pressure codes, the relationship needs to be established carefully.

A communication failure may cause other control modules to lose expected data.

Conversely, a genuine fuel-system problem may create separate pressure codes while the FICM remains fully operational.

Look at:

• Commanded fuel pressure

• Actual fuel pressure

• Injector commands

• FICM status

• Engine RPM

• Synchronization

• Other communication DTCs

The scan data should be interpreted as a complete system rather than as isolated codes.

Do Not Confuse U0105 With P020x

These codes belong to different diagnostic categories.

P020x codes generally concern injector electrical control circuits.

U0105 concerns communication with the FICM.

For example, a P0203 might indicate a problem associated with an individual injector control circuit, while U0105 indicates that communication with the injector-control module has been lost.

If both appear, determine whether there is a common electrical cause.

A Practical Diagnostic Example

Consider a diesel vehicle that suddenly loses power and stores U0105.

The technician performs a complete scan and discovers that the ECM cannot communicate with the FICM.

Rather than replacing the FICM immediately, the technician checks its power supply.

One required supply is missing.

The fuse is good, so the harness is traced back toward the power relay.

A corroded relay terminal is found.

After repairing the terminal, FICM communication returns and the engine operates normally.

The FICM itself was never defective.

Another Example: Communication Lost Under Vibration

A different vehicle stores U0105 only while driving over rough roads.

At idle, the FICM communicates perfectly.

Inspection reveals that part of the FICM harness is rubbing against a bracket.

The insulation has worn through, allowing intermittent contact with the vehicle body.

When the engine moves, communication is interrupted.

This explains why the vehicle may work perfectly during a stationary diagnostic test.

Could the ECM Cause U0105?

It is possible, but the ECM should not be blamed simply because it is one of the modules communicating with the FICM.

The ECM should be considered after verifying:

• FICM power

• FICM ground

• Communication wiring

• Connector integrity

• Network condition

• Other modules

• Required software/configuration

If the FICM communicates with other diagnostic equipment but the ECM cannot receive its messages, the network and ECM-side circuits may require further investigation.

When Is the FICM Actually the Suspect?

The FICM becomes a stronger suspect when:

• Its power supply is correct

• Its grounds are correct

• Connectors are good

• Communication wiring is proven good

• The network is functioning

• No external module is disrupting communication

• Manufacturer-specific tests identify an internal fault

At that point, an internal FICM failure becomes much more credible.

Is U0105 Safe to Drive With?

That depends on how the vehicle is behaving.

If the FICM is responsible for active injector control and communication is unstable, the vehicle may experience:

• Sudden power loss

• Stalling

• No-start

• Reduced-power operation

• Poor combustion

A vehicle that is stalling or losing engine power unexpectedly should not be treated as normal to drive.

If the vehicle runs normally and U0105 is only a historical code, diagnosis can usually begin without assuming an immediate mechanical emergency, but the communication fault should still be investigated if it returns.

The Most Important Diagnostic Principle

The most important lesson with U0105 is simple:

A communication code is not automatically a failed-module code.

The module named in the DTC is the module that the vehicle has stopped hearing from. It is not necessarily the component that caused the problem.

For U0105, the fault could be:

• FICM power supply

• FICM ground

• Fuse

• Relay

• CAN wiring

• Connector

• Harness

• Low system voltage

• Another module interfering with the network

• FICM software/configuration

• Internal FICM failure

The diagnostic process should identify which of these possibilities actually explains the communication loss.

Final Diagnosis

U0105 – Lost Communication With Fuel Injector Control Module (FICM) indicates that the vehicle has lost expected communication with its fuel-injector control module.

It does not automatically mean that the fuel injectors or FICM have failed.

A proper diagnosis should begin by scanning all modules, determining whether the FICM can be accessed, checking battery and charging voltage, verifying FICM power and ground, inspecting connectors and harnesses, and testing the communication network according to the manufacturer's specifications.

If the vehicle uses a separate FICM to control diesel injectors, a communication failure can potentially cause severe symptoms including reduced power, stalling or a no-start condition.

The FICM itself should become the primary suspect only after its external electrical and communication circuits have been proven good. This approach prevents unnecessary replacement of expensive injectors, FICMs or other control modules and leads to a much more reliable diagnosis.