Code
U0120
HYUNDAI
U — Network/User
Lost Communication With Starter / Generator Control Module
Views:
UK: 24
EN: 75
RU: 61
AI status
Completed
Completed
100%
Causes
- Open or short in CAN high/low wiring between SGCM and the network
- Poor or corroded connector at the starter/generator control module
- Blown fuse or missing power/ground to the SGCM
- Dead or weak battery / low system voltage
- Failed starter/generator control module
- Faulty gateway/BCM/ECM or another CAN node disrupting bus
Symptoms
- DTC U0120 stored; other CAN communication codes may be present
- Charging system warning lamp or battery light illuminated
- Starter/generator may not operate properly or charging is erratic
- No response from SGCM when queried with a scan tool
- Intermittent start/charging issues or vehicle enters reduced functionality
- Electrical accessories may behave erratically if CAN bus is compromised
What to check
- Retrieve all stored codes and freeze frame data from all modules
- Check battery voltage and state of charge (battery should be >12.4 V at rest, >11 V cranking depending on vehicle)
- Visually inspect SGCM connector and wiring for corrosion, damage, or loose pins
- Verify fuses and power feeds to the SGCM and associated relays
- Check CANH/CANL wiring for continuity and shorts to battery/ground
- Use a scan tool to see if other modules are communicating on the CAN bus
Signal parameters
- Key ON (engine off): CANH and CANL idle ~2.5 V each (differential ~0 V).
- Dominant CAN state: CANH ~3.5 V, CANL ~1.5 V during frames (voltage swing depends on vehicle bus voltage).
- Bus activity: regular CAN frames visible on bus when other modules are active (frequency varies by network; many control messages every few ms to tens of ms).
- Supply voltage to SGCM: battery voltage present at module power pin with ignition in RUN/START
- Ground continuity: module ground should show low resistance to chassis ground (
- Termination resistance: ~60 ohms measured across CANH-CANL with ignition off
Diagnostic algorithm
- Verify and record all DTCs and instrument cluster warnings; note when issue occurred and any recent repairs or installations.
- Check battery state of charge and charging system health; charge or replace battery if necessary before CAN testing.
- Visually inspect SGCM connector and harness for corrosion, bent pins, or water intrusion; repair any obvious damage.
- Verify power and ground to the SGCM: check key-on power feed(s) and ground continuity at the module connector; check related fuses/relays.
- With a good quality scan tool, attempt to communicate directly with the SGCM; note if it responds or is completely absent.
- Backprobe CANH and CANL at the SGCM connector and observe with a multimeter/oscilloscope: confirm idle voltages (~2.5 V) and look for data frames (oscilloscope preferred).
- Measure resistance across CANH to CANL (expect ~60 ohms). If open or low, trace wiring for breaks or shorts; disconnect nodes one at a time to isolate a fault.
- Inspect and test termination resistors and check for any aftermarket devices tied into the bus; remove/disable non-factory devices if present.
- If wiring and power/grounds are good but no communication, consider replacing or reprogramming the SGCM; confirm replacement procedures and software calibration.
- After repairs, clear codes and perform a road test while monitoring CAN messages and charging system operation; verify no reoccurrence.
Likely causes
- Loose/poor connector or corrosion at SGCM
- Loss of power or ground to the SGCM (blown fuse, open feed)
- Damaged CANH/CANL wiring (chafe, pinched, short to ground or battery)
- Failed SGCM
- CAN bus termination problem or multiple nodes causing bus fault
Fault status
Status
U0120 — Lost Communication With Starter/Generator Control Module. The vehicle’s network is not receiving expected messages from the starter/generator control module. Investigate power/ground, connectors, CAN wiring, termination, and module integrity. Do not conclude a module replacement without confirming wiring and network health.
Repair difficulty: Medium
Diagnostic time: 1.0-3.5 hours
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