The U0120 Diagnostic Trouble Code (DTC) indicates that communication has been lost with the Starter-Generator Control Module (SGCM) over the High-Speed Controller Area Network (HS-CAN). When this communication failure occurs, the Engine Control Module (ECM), Hybrid Control Module (HCM), and other electronic control units can no longer exchange essential operating data with the Starter-Generator Control Module. As a result, the vehicle stores U0120 and may activate warning indicators or enter a reduced-performance operating mode.
Modern vehicles equipped with integrated starter-generator (ISG), mild hybrid (MHEV), or hybrid systems rely on continuous CAN Bus communication between the Starter-Generator Control Module and the powertrain network. The SGCM manages engine starting, regenerative braking, battery charging, torque assistance, and communication with other vehicle modules. If communication is interrupted, the vehicle may lose start-stop functionality, charging efficiency, regenerative braking performance, and hybrid assistance.
Unlike mechanical failure codes, U0120 is a communication fault. In most cases, the root cause is damaged CAN Bus wiring, loose or corroded connectors, blown fuses, poor ground connections, low system voltage, or an internal Starter-Generator Control Module malfunction.
How the Starter-Generator Communication Network Operates
During normal operation, the Starter-Generator Control Module continuously exchanges information including generator speed, charging current, regenerative braking status, battery state of charge, engine start requests, torque assistance, and system temperatures. If CAN communication stops, the network immediately detects the fault and stores Diagnostic Trouble Code U0120.
Modules That Communicate With the Starter-Generator Control Module
| Module | Primary Function |
|---|---|
| Engine Control Module (ECM) | Powertrain management |
| Starter-Generator Control Module (SGCM) | Controls ISG/MHEV operation |
| Hybrid Control Module | Energy management |
| Body Control Module (BCM) | Vehicle network coordination |
| Instrument Cluster | Displays system warnings |
| Diagnostic Scan Tool | Reads DTCs and Live Data |
Symptoms of U0120 Code
The symptoms of U0120 depend on the vehicle's electrical architecture and whether the Starter-Generator Control Module is responsible for start-stop operation, regenerative braking, charging management, or mild hybrid assistance. In many vehicles, the communication loss immediately disables the integrated starter-generator system.
| Symptom | Severity |
|---|---|
| Check Engine Light (MIL) | ★★★★★ |
| Hybrid / MHEV Warning Light | ★★★★★ |
| Start-Stop System Disabled | ★★★★★ |
| Reduced Charging Performance | ★★★★☆ |
| Loss of Regenerative Braking | ★★★★☆ |
| Reduced Engine Assistance | ★★★★☆ |
| Multiple Communication DTCs | ★★★★☆ |
Common Causes of U0120
U0120 is generally caused by a loss of network communication rather than a mechanical failure of the starter-generator itself.
- Open circuit in CAN High wiring.
- Open circuit in CAN Low wiring.
- Short to ground on CAN Bus circuits.
- Short to battery voltage.
- Corroded or loose Starter-Generator Control Module connector.
- Blown fuse supplying the SGCM.
- Poor chassis or engine ground.
- Low 12V or 48V battery voltage.
- Damaged wiring harness.
- Internal Starter-Generator Control Module failure.
How Serious Is U0120?
Because the Starter-Generator Control Module controls charging, engine restart, regenerative braking, and hybrid assistance, U0120 is considered a high-severity communication fault.
✔ Start-stop system may stop functioning.
✔ Mild hybrid assistance may be disabled.
✔ Charging performance may decrease.
✔ Immediate diagnosis is recommended.
Estimated Repair Cost
Repair costs depend on whether the fault is related to the CAN Bus network, wiring, power supply, or replacement of the Starter-Generator Control Module.
| Repair | Estimated Cost (USD) |
|---|---|
| Professional Diagnosis | $100–200 |
| Repair CAN Bus Wiring | $100–450 |
| Connector Repair | $50–200 |
| Fuse or Relay Replacement | $20–80 |
| Module Programming | $150–400 |
| Replace Starter-Generator Control Module | $700–2,500+ |
Diagnostic Procedure
Diagnosing U0120 requires confirming that the Starter-Generator Control Module (SGCM) has proper battery voltage, ignition power, ground, and uninterrupted High-Speed CAN Bus communication. Most U0120 faults are caused by wiring, connectors, blown fuses, or network issues rather than an actual Starter-Generator Control Module failure.
↓ Read All Stored DTCs
↓ Save Freeze Frame Data
↓ Check 12V / 48V Battery Voltage
↓ Inspect SGCM Fuses & Relays
↓ Verify SGCM Power Supply
↓ Verify Ground Circuit
↓ Measure CAN High Voltage
↓ Measure CAN Low Voltage
↓ Inspect Connectors & Harness
↓ Repair Fault
↓ Clear DTCs
↓ Road Test
Electrical Inspection
Electrical testing should begin directly at the Starter-Generator Control Module connector. Verify battery power, ignition voltage, ground integrity, and CAN Bus communication before replacing the control module.
| Inspection Item | Normal Specification |
|---|---|
| Battery Voltage | 12.4–12.8 V |
| Charging Voltage | 13.5–14.8 V |
| 48V Battery (If Equipped) | Manufacturer Specification |
| Ground Resistance | Below 0.2 Ω |
| CAN Network Resistance | Approximately 60 Ω |
| CAN High Voltage | ≈2.5–3.5 V |
| CAN Low Voltage | ≈1.5–2.5 V |
Live Data Analysis
A professional scan tool should verify that the Starter-Generator Control Module remains online and is actively exchanging data with the Engine Control Module, Hybrid Control Module, and Battery Management System.
| Live Data Parameter | Normal | Fault Condition |
|---|---|---|
| Starter-Generator Module Status | Online | Offline |
| CAN Communication | Normal | Communication Error |
| Starter-Generator Speed | Normal | Unavailable |
| Generator Output Current | Within Specification | No Data |
| Battery State of Charge | Normal | Unavailable |
| Battery Voltage | Stable | Low Voltage |
Professional Diagnostic Tips
- Scan every module before disconnecting the battery.
- Record Freeze Frame data before clearing DTCs.
- Inspect SGCM wiring near the starter-generator for heat or vibration damage.
- Perform voltage-drop testing on both power and ground circuits.
- Measure CAN Bus resistance with the ignition OFF.
- Perform a harness wiggle test while monitoring Live Data.
- Inspect connectors for corrosion, moisture, or loose terminals.
- Check manufacturer Technical Service Bulletins (TSBs) before replacing the Starter-Generator Control Module.
Typical Wiring Diagram
The Starter-Generator Control Module (SGCM) receives battery power, ignition power, and ground while communicating with the Engine Control Module, Hybrid Control Module, and other network controllers through the High-Speed CAN Bus. Exact wiring colors and connector layouts vary between manufacturers.
Typical Starter-Generator Control Module Pinout
The following pinout represents a typical High-Speed CAN configuration. Always verify the manufacturer's wiring diagram before performing electrical measurements.
| Pin | Circuit | Expected Value |
|---|---|---|
| 1 | Battery Power (B+) | 12V / 48V |
| 2 | Ignition Power | Battery Voltage (Key ON) |
| 3 | Ground | Below 0.2 Ω |
| 4 | CAN High | ≈2.5–3.5 V |
| 5 | CAN Low | ≈1.5–2.5 V |
| 6 | Generator Control Signal | Manufacturer Specific |
| 7 | Battery Feedback Signal | Manufacturer Specific |
CAN Communication Signal Flow
Connector Inspection Checklist
- Inspect connectors for corrosion, oxidation, or moisture intrusion.
- Check for loose, bent, recessed, or overheated terminals.
- Verify terminal tension using the correct terminal tester.
- Inspect the harness near the starter-generator for heat or vibration damage.
- Look for crushed wiring, insulation wear, or rodent damage.
- Perform a harness wiggle test while observing Live Data.
- Repair damaged terminals before replacing the Starter-Generator Control Module.
Recommended Diagnostic Equipment
| Tool | Purpose |
|---|---|
| Professional Scan Tool | Read DTCs & Live Data |
| Digital Multimeter | Voltage & resistance testing |
| Dual-Channel Oscilloscope | Analyze CAN Bus waveforms |
| Back-Probe Test Leads | Non-invasive circuit testing |
Oscilloscope Waveform Analysis
A dual-channel automotive oscilloscope is the most effective diagnostic tool for diagnosing U0120. Monitoring High-Speed CAN Bus activity directly at the Starter-Generator Control Module (SGCM) allows technicians to identify open circuits, short circuits, intermittent wiring faults, and network interference that cannot always be detected using a multimeter.
Connect Channel A to CAN High and Channel B to CAN Low at the SGCM connector. Compare captured signals with the reference waveforms below.
1. Normal High-Speed CAN Communication
CAN Low 2.5V ──╮────╮ ╭────╮────╮ 1.5V ──╯ ╰────╯ ╰────╯
Diagnosis: Normal communication. The Starter-Generator Control Module is correctly exchanging CAN messages with other control modules.
2. CAN High Open Circuit
CAN Low 2.5V ─╮──╮──╮──╮──╮──╮──╮── 1.5V ─╯──╰──╯──╰──╯──╰──╯──
Diagnosis: Open circuit in the CAN High communication wire.
3. CAN Low Open Circuit
CAN Low 2.5V ─────────────────────────
Diagnosis: CAN Low circuit is disconnected or has excessive resistance.
4. CAN Bus Short to Ground
CAN Low 0V ───────────────────────────
Diagnosis: One or both CAN communication circuits are shorted directly to ground.
5. CAN Bus Short to Battery Voltage
CAN Low 5V ───────────────────────────
Diagnosis: CAN wiring is shorted to battery voltage or another powered circuit.
6. Intermittent CAN Communication
CAN Low ╰╯╰╯────╰╯╰────╰╯╰
Diagnosis: Loose connectors, poor grounding, vibration damage, or partially broken CAN wiring causing intermittent communication loss.
Professional Oscilloscope Tips
- Use a dual-channel oscilloscope to capture CAN High and CAN Low simultaneously.
- Back-probe connectors to avoid damaging wiring insulation.
- Record waveforms with ignition ON, engine running, and during a road test.
- Use trigger mode to capture intermittent communication failures.
- If one CAN signal remains fixed while the other is active, inspect the inactive communication circuit.
- Random voltage spikes often indicate poor grounding or electromagnetic interference.
- Always compare repaired waveforms with known-good reference patterns before returning the vehicle to service.
Recommended Repairs
After identifying the root cause of U0120, repair the communication fault before replacing the Starter-Generator Control Module (SGCM). Most U0120 cases are caused by CAN Bus wiring, poor connections, power supply issues, or grounding faults rather than an internal module failure.
| Repair Procedure | Difficulty | Success Rate |
|---|---|---|
| Repair CAN High / CAN Low wiring | Medium | ★★★★★ |
| Repair or clean SGCM connectors | Easy | ★★★★★ |
| Repair ground circuit | Easy | ★★★★☆ |
| Restore battery or ignition power supply | Medium | ★★★★☆ |
| Replace fuse or relay | Easy | ★★★★☆ |
| Reprogram Starter-Generator Control Module | Advanced | ★★★☆☆ |
| Replace Starter-Generator Control Module | Advanced | ★★☆☆☆ |
Common Repair Mistakes
- Replacing the Starter-Generator Control Module before checking CAN Bus communication.
- Ignoring low battery voltage or charging system problems.
- Failing to inspect fuse box terminals for corrosion or overheating.
- Overlooking loose engine or chassis ground connections.
- Clearing DTCs before recording Freeze Frame data.
- Ignoring additional U-series communication codes.
- Using only continuity testing instead of voltage-drop testing.
- Installing a replacement SGCM without required programming or calibration.
Manufacturer-Specific Notes
Starter-generator systems differ between manufacturers. Some vehicles use a 12V integrated starter-generator, while others use a 48V mild hybrid architecture with a dedicated Starter-Generator Control Module.
| Manufacturer | Typical Starter-Generator System |
|---|---|
| Mercedes-Benz | 48V Integrated Starter Generator (ISG) |
| Audi | 48V Mild Hybrid Starter Generator |
| BMW | 48V Belt Starter Generator |
| Hyundai / Kia | MHEV Starter Generator System |
| Volvo | Integrated Starter Generator |
| Land Rover | 48V Mild Hybrid System |
| Ford | Integrated Starter Generator (selected models) |
Vehicle Applications
The U0120 Diagnostic Trouble Code is commonly found in vehicles equipped with an Integrated Starter-Generator (ISG), 48V Mild Hybrid (MHEV), or hybrid charging system. While the code definition remains the same, the Starter-Generator Control Module design, connector locations, and communication architecture vary by manufacturer.
| Manufacturer | Starter-Generator System | Primary Inspection Area |
|---|---|---|
| Mercedes-Benz | 48V ISG | CAN Bus & SGCM power supply |
| Audi | 48V Mild Hybrid | Grounds & communication wiring |
| BMW | Belt Starter Generator | Gateway communication |
| Hyundai / Kia | MHEV System | Starter-generator wiring |
| Volvo | Integrated Starter Generator | Power & CAN circuits |
| Land Rover | 48V Hybrid System | Harness inspection |
| Ford | Integrated Starter Generator | Connector & network inspection |
Related Diagnostic Trouble Codes
Communication failures involving the Starter-Generator Control Module are frequently accompanied by additional U-series communication codes. Diagnose all stored communication faults before replacing any control module.
- U0100 – Lost Communication With ECM/PCM "A"
- U0101 – Lost Communication With Transmission Control Module
- U0115 – Lost Communication With ECM/PCM "B"
- U0119 – Lost Communication With Fuel Cell Control Module
- U0121 – Lost Communication With ABS Control Module
- U0122 – Lost Communication With Vehicle Dynamics Control Module
- CAN Bus Communication System
Professional Technician Tips
- Scan every control module before disconnecting the battery.
- Save Freeze Frame data and complete network topology before clearing DTCs.
- Inspect SGCM wiring near the starter-generator for heat, oil contamination, or vibration damage.
- Always perform voltage-drop testing on both power and ground circuits.
- Use a dual-channel oscilloscope whenever intermittent CAN Bus faults are suspected.
- Verify Starter-Generator Live Data after completing repairs.
- Inspect connectors for moisture, oxidation, overheated terminals, or poor pin tension.
- Complete a full drive cycle and confirm that U0120 does not return.
Frequently Asked Questions (FAQ)
What does the U0120 code mean?
U0120 indicates that communication has been lost with the Starter-Generator Control Module (SGCM) over the High-Speed CAN Bus network.
Can I drive with U0120?
Yes, in many cases the vehicle remains drivable. However, the start-stop system, regenerative charging, or mild hybrid assistance may be disabled, and some vehicles may enter a reduced-performance mode.
Will U0120 prevent the engine from starting?
It can. On vehicles that rely heavily on an integrated starter-generator for engine starting, communication loss with the SGCM may result in a no-start condition or intermittent starting problems.
Can a weak battery trigger U0120?
Yes. Low 12V battery voltage, low 48V battery voltage (where equipped), or charging system faults are common causes of communication errors involving the Starter-Generator Control Module.
Is U0120 caused by a bad starter motor?
No. U0120 is a network communication fault. Although the starter-generator assembly may be involved, the code is usually caused by CAN Bus wiring, connectors, power supply issues, or a faulty control module.
Can corrosion cause U0120?
Yes. Corroded connectors, moisture intrusion, loose terminals, and damaged wiring frequently interrupt CAN Bus communication and trigger U0120.
Will clearing the DTC fix U0120?
No. Clearing the code only removes it from memory. If the communication problem still exists, U0120 will return after the next ignition cycle or vehicle self-test.
What should CAN Bus resistance measure?
With the ignition OFF and the battery disconnected, resistance measured between CAN High and CAN Low should be approximately 60 Ohms.
Is an oscilloscope necessary?
A professional scan tool and multimeter can identify many faults, but a dual-channel oscilloscope provides the fastest and most accurate diagnosis of intermittent CAN Bus communication failures.
Does a replacement Starter-Generator Control Module require programming?
Yes. Most manufacturers require programming, coding, calibration, or security synchronization after replacing the Starter-Generator Control Module.
Conclusion
The U0120 Code – Lost Communication With Starter-Generator Control Module is a High-Speed CAN Bus communication fault that interrupts data exchange between the Starter-Generator Control Module and other electronic control units. Accurate diagnosis should begin with battery voltage verification, power and ground testing, CAN Bus inspection, connector examination, Live Data analysis, and oscilloscope waveform testing before replacing expensive components. A systematic diagnostic approach minimizes unnecessary repairs and restores reliable starter-generator operation.
Key Takeaways
- U0120 is a communication fault, not a mechanical starter-generator failure.
- Most repairs involve CAN Bus wiring, connectors, fuses, or ground circuits.
- Always verify power, ground, and CAN Bus integrity before replacing the SGCM.
- Professional scan tools, Live Data, and oscilloscope testing provide the most accurate diagnosis.
- Record Freeze Frame data before clearing Diagnostic Trouble Codes.
- Perform a complete drive cycle after repairs to confirm that U0120 does not return.
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