U0120 — Lost communication with starter/generator control module

The vehicle’s main computer is not receiving signals from the starter/generator control module. This usually happens because of wiring issues, a faulty control module, or network communication problems.

Common causes

The cause is almost always one of a few things. Here they are — U0120 rarely comes from anything outside this list:

• Damaged or loose wiring/connectors to the control module
• Faulty starter/generator control module
• Network communication fault or bus error
• Blown fuse related to the control module
• Corroded or dirty electrical connections

Where to start

In the order a sensible mechanic would work — the cheap checks first, the expensive replacements only after:

1. Check and secure wiring and connectors to the control module
2. Inspect and replace any blown fuses
3. Clean corroded electrical connections
4. Test and repair network communication lines if needed
5. Replace the starter/generator control module if faulty

That is what holds for U0120 in general. From here it depends on your engine — tell me which car it is and what it does, and I will give you the steps for it: real procedures, torque figures and diagrams instead of general advice.