U0120: Lost Communication With Starter/Generator Control Module

What it means

A control module has stopped receiving expected messages from the starter/generator control module. The effects depend on the vehicle and may include loss of start-stop or starter/generator functions.

At a glance

• Usually lights the warning light of the system that lost communication
• Affects emissions; the car can fail an emissions test

How it works

The starter/generator control module manages functions such as integrated starting and, on some vehicles, energy recovery or stop-start operation. Other modules expect it to send status messages over the vehicle network. U0120 sets when a module does not receive those messages, so starter/generator functions may be disabled even if the engine still starts normally.

Common causes

Most likely first.

1. Starter/generator module power or ground fault
2. Damaged network wiring or connector
3. Starter/generator control module failure
4. Low system voltage

Symptoms

• Start-stop or starter/generator functions may be unavailable
• Warning lights or messages
• Other communication codes

How to diagnose it

1. Scan all modules and save freeze-frame data and related codes to see whether U0120 is isolated or part of a broader communication or low-voltage problem.
2. Check battery condition and charging-system operation, since low system voltage can interrupt communication with the starter/generator module.
3. Inspect the starter/generator module’s power, ground, connectors, and nearby harness for loose connections, corrosion, water entry, or damage.
4. Use a scan tool to check whether the starter/generator module responds and whether other modules report it as offline.
5. If the module has power and ground but remains offline, test its network wiring and connections against the vehicle’s wiring diagram and manufacturer’s specification.

Common fixes

• Charge or replace a weak battery and correct any charging-system fault.
• Clean and secure a loose or corroded starter/generator module connection.
• Repair damaged network wiring or a poor connector terminal.
• Restore the module’s power or ground circuit.
• Replace the starter/generator control module if its supplies and network circuits test correctly but it does not communicate.

Can I keep driving?

If the vehicle starts and drives normally, a short trip may be possible, but stop-start or starter/generator assistance may be unavailable. Avoid relying on those functions; if starting, charging, or other electrical warnings appear, stop in a safe place and arrange help.