U0120

Universal

Lost Communication With Starter / Generator Control Module

Definition source: General OBD-II reference catalog
Definition verification: Not independently verified for display.
Time: Varies by diagnosis and vehicle | Diagnostic difficulty: Medium
Medium Severity

The vehicle has detected that another control module cannot communicate with the starter/generator control module. Depending on the vehicle, this module manages a unit that helps start the engine and generate electrical power. The communication loss may affect starting, charging, or related functions, but the code alone does not prove that the control module has failed.

Report an error with this code

⚠️ Before You Begin (Safety First)

If the vehicle will not start, displays a charging-system warning, loses electrical power, or behaves unpredictably, stop in a safe place and arrange assistance rather than continuing to drive. Some starter/generator systems use high voltage; do not touch high-voltage wiring or connectors. Have a qualified technician handle high-voltage or system-specific work.

Common Symptoms

  • There may be no noticeable symptoms if the communication loss is intermittent or the affected function is not currently needed.
  • The engine may be difficult to start or may not start if the vehicle depends on the starter/generator system.
  • A charging-system warning or electrical-system message may appear if charging functions are affected.
  • Electrical functions or engine operation may be limited if other modules cannot receive starter/generator system information.
  • Other communication or low-voltage warnings may occur alongside this code.

Common Causes

  • A loose, damaged, corroded, or wet connector or harness may interrupt communication.
  • A network wire or connection may be open, shorted, or otherwise unable to carry messages reliably.
  • A power-supply or ground problem may prevent the starter/generator control module from operating or communicating.
  • Low or unstable vehicle voltage may disrupt module communication.
  • The starter/generator control module may have an internal fault, but this should be confirmed before replacement.
  • A fault elsewhere on the vehicle network may prevent communication with the module.

Diagnostic Steps

  1. Note any warning messages, starting or charging problems, and when they occur; this helps distinguish an intermittent communication issue from a persistent one.
  2. Check the vehicle owner's manual for the meaning of the warning and whether it advises stopping or arranging service.
  3. With the vehicle safely parked and switched off, visually inspect accessible, low-voltage connectors and wiring for looseness or obvious damage. Do not unplug connectors or touch high-voltage wiring.
  4. Check the battery terminals for visible looseness or corrosion without disconnecting them; poor connections can cause voltage problems that affect module communication.
  5. If you have a suitable scan tool, record codes from all modules before clearing anything. Related codes can help identify whether the issue is isolated or affects the wider network.
  6. If the warning persists, the vehicle has starting or charging problems, or high-voltage components may be involved, arrange diagnosis by a qualified technician.

When to Call a Professional

A technician can scan all modules for communication codes, identify which modules can and cannot communicate, and check relevant power, ground, connectors, and network wiring using vehicle-specific service information. They should also consider low or unstable system voltage and confirm the fault before replacing a control module.

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Commonly Asked Questions

Does U0120 mean the starter/generator control module must be replaced?

No. It reports lost communication, not a confirmed failed part. Wiring, connectors, power, ground, low voltage, or another network fault can produce the same condition.

Can I drive with this code?

It depends on whether starting, charging, or other functions are affected. If the vehicle shows a charging warning, loses electrical power, will not start reliably, or behaves unpredictably, stop safely and seek assistance.

Why might there be no symptoms?

Communication may fail intermittently, or the affected system may not be needed during the drive. A stored code can remain even when the issue is not currently noticeable.

How is the cause usually narrowed down?

A technician checks codes across the vehicle, determines which modules can communicate, and tests the relevant power, ground, connectors, and network circuits using vehicle-specific information.

Is it safe to inspect starter/generator wiring at home?

Only make basic visual checks of accessible, low-voltage areas. Some systems include high-voltage components, which should be handled only by qualified technicians using the correct procedures.

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