U0109 and Related Engine Module Communication Codes
Lost-communication codes such as U0109, U0105, U0106, U0107, U0119, and U0120 record an expected message that was not received on the vehicle communication network. They do not prove a named controller has failed. Match each code to the year, model, and powertrain before linking it to a fuel pump, injector, glow plug, throttle, fuel cell, or starter generator control module, and treat stored diagnostic trouble codes as evidence to interpret, not a parts list.
Verify Each Code’s Meaning and the Vehicle’s Module Configuration
A U-code is only as useful as its vehicle-specific definition. Read the complete code, including any suffix, in service information for that year, model, and powertrain. Manufacturer-dependent labels can assign the same number to different modules, so a generic scanner description is not enough. A lost-communication code records that an expected message was not received on the vehicle communication network. It does not establish that the named module has failed, only that expected traffic was missing.
Identify which module reported the code, whether the fault is current or among stored diagnostic trouble codes, and which other communication or voltage codes sit beside it. Low system voltage and recent battery work can leave network faults that look like failed controllers. Record when symptoms appear and any recent electrical work before clearing anything. A basic reader may not reach every relevant module, so a limited scan can miss companion codes that change the path.
U0109: Fuel Pump Control Communication Symptoms and Possible Causes
Once the vehicle-specific definition is confirmed, U0109 is a communication fault involving the fuel pump control module, so fuel-pump network issues deserve first attention. Fuel delivery can stop or become intermittent if that controller never answers. Extended cranking, a no-start, stalling, or a warning light may appear, depending on the vehicle and whether the fault is active. An inactive U0109 with no drivability complaint still needs that confirmation of meaning; it is not a reason to replace the pump or module on the code alone.
Possible causes include interrupted module power and ground, low system voltage, connector damage, network wiring faults, and an internal module fault. Owners can note when cranking, stalling, or warning lights appear and inspect only safely accessible areas with the vehicle off. Do not open the fuel system, jumper circuits, or probe energized wiring. A shop may run a full module scan, power and ground tests, network checks, and fuel-delivery testing. If the engine stalls or cannot maintain power, stop safely and arrange assistance.
U0105: Fuel Injector Control Module Communication
Confirm the applicable definition and whether the vehicle uses a separate fuel injector control module before explaining U0105. Some powertrains integrate injector drive into another controller, so a generic label can be misleading. When a dedicated injector controller is present, lost communication can coincide with hard starting, rough running, or unexpected shutdown because injector commands never arrive. Those symptoms still do not identify a failed injector. Companion voltage or network codes often point elsewhere first.
Module supply faults, poor grounds, damaged connectors, and interrupted network communication can keep the fuel injector control module silent even when the injectors are intact. Professional electrical and communication testing should precede any replacement decision, including power and ground checks, connector inspection, and confirming whether other modules still talk on the same network. Injector circuit probing and fuel-system disassembly are not owner steps and are not required to interpret this code. Replacing injectors or the module on U0105 alone skips those wiring and voltage checks.
U0106: Glow Plug Control Module Communication
Verify the vehicle-specific definition and the applicable diesel glow plug system before treating U0106 as a cold-start aid problem. Lost communication with the glow plug control module can appear with hard starting in cold conditions and with warning indications, yet neither proves that individual glow plugs have failed. The plugs may still heat if the controller is powered but silent on the network, or they may be healthy while a supply or ground fault keeps the module offline.
Interrupted controller power, poor grounds, connection damage, and network faults are diagnostic possibilities for U0106. Controller communication and individual glow plug operation are separate questions and need separate confirmation. A communication code does not measure plug heat or current. High-current circuit testing belongs with a qualified technician. Until power, ground, and network integrity are verified, replacing glow plugs or the glow plug control module remains a guess rather than a diagnosis.
U0107: Throttle Actuator Control Communication
Match U0107 to the throttle control architecture on the specific vehicle before naming the silent controller. Some designs use a throttle actuator control module; others fold actuation into the engine controller, so the same number can describe different hardware. Reduced-power operation, limited accelerator response, and warning lights can accompany lost communication, but those symptoms have other causes, including sensor faults, low voltage, and mechanical binding. The code says an expected throttle-related message was missing, not that the throttle body has failed.
Power, ground, connector, and network faults can silence the throttle actuator control module without any need to replace the throttle body. Forced throttle movement and bypass procedures are unsafe and are not diagnostic substitutes. If accelerator response becomes unreliable, stop safely rather than driving through a reduced-power condition. Professional scan data and circuit testing can show whether the controller is powered, whether it still appears on the vehicle communication network, and whether companion codes point to voltage or wiring. Those checks belong before actuator or module replacement.
U0119: Confirm the Module Name Before Connecting It to Fuel Additives
Check the exact code and vehicle-specific service definition before connecting U0119 to fuel additives. It is commonly defined as lost communication with a fuel cell control module, not with a fuel-additive controller. A fuel additive controller is a separate system and must not be assigned to U0119 from an assumed code sequence or an unverified scanner label. If a tool reports a fuel additive module for this number, resolve that discrepancy against service information first. Until the named controller is confirmed, symptoms, causes, and replacement talk are premature.
When a fuel cell communication fault is confirmed, warning indications and possible propulsion restrictions are vehicle-dependent outcomes, not universal results of U0119. Some systems may limit drive or display a dedicated warning, while others may store the code with little owner-facing change. Diagnosis belongs with qualified personnel who can verify module power and ground, connector condition, and network traffic without opening hydrogen or high-voltage assemblies. Those repair instructions are outside owner work and are not implied by this communication code.
U0120: Starter/Generator Control Communication
Confirm the vehicle-specific definition and whether a starter generator control module is fitted before interpreting U0120. Hybrid and stop-start architectures may include that controller; many conventional starters do not. Where the design supports it, lost communication can coincide with starting trouble, failed auto-restart, or charging-related warnings, but only as that system is built. The code does not prove the starter, generator, or inverter has failed. It records missing traffic from the expected controller.
Low-voltage supply problems, ground faults, connector damage, or network interruptions can keep the starter generator control module from answering even when the rotating machine is healthy. Professional testing must match the vehicle's electrical architecture before any component is replaced, including low-voltage power and ground verification and network presence checks. High-voltage access, starter bypasses, and unsupported reset procedures are not owner diagnostics and can create additional hazards. Until those tests are done, U0120 remains a communication clue, not a reason to swap the starter-generator or its controller.