U0120 Lost Communication With Starter-Generator Control Module

U0120 Lost Communication With Starter-Generator Control Module

OBD-II DTC Datasheet

Lost Communication With Starter-Generator Control Module

What does this mean?

This is a generic communication system diagnostic trouble code that applies to most non-hybrid vehicles, but can also be found on makes and models of hybrid vehicles.

This code means the starter alternator control module (SGCM) and other control modules on the vehicle are not communicating with each other. The circuitry most commonly used for communication is known as Controller Area Bus communication, or simply the CAN bus.

Without this CAN bus, control modules cannot communicate and your scan tool may not receive information from the vehicle, depending on which circuit is involved.

The SGCM detects how hot or cold the batteries get and changes the charging system to ensure the batteries are in the correct state of charge. They also monitor the starting system to determine the likelihood of a malfunction. This information is sent to the PCM to turn on the Malfunction Indicator Light (MIL) or, in the case of a HV, the HV warning light.

Troubleshooting steps may vary depending on the manufacturer, the type of communication system, the number of wires, and the colors of the wires in the communication system.

Severity and symptoms

The severity in this case depends on the system. Because this powertrain control system ensures safety in all operating conditions, safety is a concern when diagnosing these systems. In addition, safety is important when servicing these systems. ALWAYS refer to the service information before disassembling / diagnosing these systems.

Symptoms of a U0120 engine code may include:

• Malfunction Indicator Light (MIL) is on
• Hybrid mode indicator is on, if applicable
• The car cannot start or run
• The car can run, but on a gasoline engine, only if the hybrid

reasons

Usually the reason for installing this code is:

• Open in CAN + bus circuit
• Open in the CAN bus - electrical circuit
• Short circuit to power in any CAN bus circuit
• Short to ground in any CAN bus circuit
• Loss of power or ground to the SGCM - the most common
• Rarely - the control module is faulty

Diagnostic and repair procedures

A good starting point is always to check the Technical Service Bulletins (TSB) for your particular vehicle. Your problem may be a known issue with a known manufacturer-released fix and could save you time and money while troubleshooting.

First, look for other DTCs. If any of these are bus or battery / hybrid related, diagnose them first. Misdiagnosis is known to occur if you diagnose the U0120 code before any of the major codes are thoroughly diagnosed and rejected.

If your scan tool can access DTCs and the only code you are pulling from other modules is U0120, try accessing the starter-alternator control module. If you can access the codes from the SGCM then code U0120 is either intermittent or a memory code. If the codes for the SGCM module cannot be accessed, then code U0120 set by other modules is active and the problem already exists.

The most common failure is a loss of power or ground to the SGCM.

Before going any further, give a little caveat: if this is a hybrid vehicle: this is a high voltage system! If the warnings are not heeded and / or the manufacturer's protective and diagnostic measures are not followed, damage to the vehicle is VERY likely and could result in injury / personal injury to you. If you are not sure about any stage of the diagnostics, it is strongly recommended that you leave the diagnostics of this code in this system to someone who has been trained in it.

Check all fuses supplying the SGCM on this vehicle. Check all grounds for SGCM. Locate grounding attachment points on the vehicle and make sure these connections are clean and secure. If necessary, remove them, take a small wire bristle brush and baking soda / water solution and clean each one, both the connector and the place where it connects.

If any repairs have been made, clear the DTCs from memory and see if U0120 returns or you can contact the SGCM. If no code returns or connectivity is restored, the problem is most likely a fuse / connection issue.

If the code returns, look for the CAN bus connections on your specific vehicle, especially the SGCM connector.

WHEN RUNNING THE HYBRID, DISCONNECT THE HIGH VOLTAGE SYSTEM FOLLOWING ALL OF THE MANUFACTURER'S PRECAUTIONS AND PROCEDURES.

Disconnect the negative battery cable before disconnecting the connector on the SGCM. Once detected, visually inspect the connectors and wiring. Look for scratches, scuffs, exposed wires, burn marks, or molten plastic. Disconnect the connectors and carefully inspect the terminals (metal parts) inside the connectors. See if they look burnt or have a green tint indicating corrosion. If you need to clean the terminals, use an electrical contact cleaner and a plastic bristle brush. Allow to dry and apply electrical grease where the terminals touch. Reconnect all connectors. Clear all codes.

If all tests pass and communication is still not possible, or you were unable to clear DTC U0120, the only thing you can do is seek help from a trained automotive diagnostician as this will indicate a faulty SGCM. Most of these SGCMs must be programmed or calibrated for the vehicle in order to be properly installed.

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