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U0120 — Lost Communication Starter Generator Control Module

Generic OBD-II · all makesNetwork / communication

U0120 indicates the powertrain control module (PCM) has stopped receiving data from the Starter/Generator Control Module (SGCM) over the CAN bus. The SGCM manages the integrated starter-generator unit, often used in mild-hybrid or advanced start-stop systems. This loss of communication disables cooperative functions such as regenerative braking or assisted launch.

Symptoms

What causes it — most likely first

1. Wiring or connector fault in CAN or power circuits

Corroded, bent, or moisture-intruded pins in the SGCM harness, or opens/shorts in the high-speed CAN+ and CAN- lines, are the most frequent source of intermittent or total signal loss.

2. Internal failure of the Starter Generator Control Module

The SGCM may suffer internal circuit board damage, voltage regulator failure, or CAN transceiver faults that prevent it from responding to network requests.

3. Low system voltage or battery degradation

Weak battery condition or poor charging system performance can drop reference voltage below the SGCM's operational threshold, causing it to drop off the bus.

4. CAN bus network interference or termination error

A short to ground or power in the data bus, missing termination resistors, or conflicting modules flooding the network can isolate the SGCM from the PCM.

5. PCM internal network driver failure

Although uncommon, a malfunction in the PCM's CAN transceiver could theoretically fail to acknowledge valid messages from the SGCM; this should only be considered after wiring and the SGCM are proven good.

How it's diagnosed

  1. Visually inspect the SGCM connector and harness for corrosion, pin push-out, or chafing; verify ground integrity
  2. Check battery state of charge and charging system output; correct any voltage deficiencies before further testing
  3. Using a wiring diagram, measure CAN bus voltage (approx 2.5V dominant/recessive) and resistance (60 ohms between CAN+ and CAN- at the SGCM) to verify network integrity
  4. Scan for additional U-codes; if multiple modules are offline, suspect a network-wide fault rather than the SGCM alone
  5. If wiring and voltage are correct, attempt reprogramming or updating SGCM software if applicable; substitute a known-good SGCM to confirm failure
  6. Only after confirming power, ground, and network integrity to the SGCM, and ruling out the module itself, evaluate the PCM for internal CAN driver faults

Code facts & related codes

SystemNetwork / communication
StandardizationSAE generic — identical on every make
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Frequently asked

Is it safe to drive with code U0120?

Short trips are usually possible, but start-stop functionality and assisted torque may be disabled. If the starter-generator is the sole cranking source, the engine may not restart once stopped; repair is advised before relying on the vehicle.

Do I need to replace the engine computer (PCM)?

Rarely. U0120 almost always stems from wiring, power/ground issues, or the SGCM itself. PCM replacement is typically unnecessary and should be the final step only after the SGCM and network wiring are verified.

Will disconnecting the battery clear the code permanently?

Disconnecting the battery may erase the code temporarily, but it will return immediately or after the next drive cycle if the underlying open circuit or module fault remains.

Can a weak battery alone cause U0120?

Yes. Low voltage can cause the SGCM to shut down or drop off the CAN bus. Always perform a battery load test and charging system verification before replacing modules.