U0120 Lost communication with starter/generator control module
What U0120 means
Mild-hybrid and some start-stop cars use a combined starter/generator (often belt-driven) with its own control module. U0120 means other modules stopped receiving messages from it. Start-stop, energy recovery and possibly charging may stop working.
U0120 is a generic SAE J2012 code: it means the same on every make. The exact cause still depends on the car, so read the freeze-frame data before replacing parts.
At a glance
- Severity
- Medium
- Typical repair cost
- 150–2500 €
- Workshop time
- 1.5–6 h
- Repair difficulty
- Hard
- DIY possible
- No
- Check engine light
- Yes
- Affects emissions test
- Yes
- Limp mode possible
- No
Parts and labour at an independent workshop in Europe. Main dealers charge more; the cause decides the bill, not the code.
Symptoms
- Hybrid or system warning light
- Start-stop not working
- Battery or charging warning
- Possible hard starting
Common causes
- Blown fuse or failed power supply to the module
- Damaged or corroded connector at the starter/generator
- CAN bus wiring fault
- Low 12 V or 48 V battery voltage
- Failed starter/generator or its control module
Can I keep driving with U0120?
You can usually drive, but have it diagnosed within one or two weeks. Watch for a charging warning; if the battery is not charging the car may stop running.
What to check first
- Check 12 V battery voltage and, on 48 V systems, any related battery codes
- Check fuses for the starter/generator module
- Inspect the connector at the starter/generator for corrosion
- Have the module and CAN wiring tested; 48 V work should be done by a trained technician
Related codes
- U0121 Lost communication with ABS control module
- U0126 Lost communication with steering angle sensor module
- U0127 Lost communication with tyre pressure monitor module
- U0109 Lost communication with fuel pump control module
- U0140 Lost communication with body control module
- U0184 Lost communication with radio
- U0046 Vehicle communication bus C fault
- U0252 Lost communication with rear lighting control module B
Data: TORKO fault-code catalogue, built on OBDex (CC0-1.0) and reviewed against workshop practice. Informational only — it does not replace a diagnosis by a mechanic.
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