U0103 Lost Communication With Gear Shift Control Module (GSC)

U0103 Lost Communication With Gear Shift Control Module (GSC)

OBD-II DTC Datasheet

Lost Communication With Gear Shift Control Module (GSC)

What does this mean?

This is a generic communications DTC that applies to most makes and models of vehicles, including but not limited to Chevrolet, Cadillac, Ford, GMC, Mazda, and Nissan. This code means the gear shift control (GSC) module 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.

Without this communication scheme, the GSC does not know which gear / position the driver wants to select. In most cases, the default is the last position; otherwise, the default will be neutral.

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.

symptoms

Symptoms of a U0103 engine code may include:

• Malfunction Indicator Light (MIL) is on
• The car does not switch
• Vehicle stays in one gear (usually neutral; no forward gears)

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
• 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 communication or battery / ignition related, diagnose them first. Misdiagnosis is known to occur if you diagnose the U0103 code before any of the major codes are thoroughly diagnosed and rejected.

If your scan tool can access trouble codes and the only code you are getting from other modules is U0103, try contacting the GSC module. If you can access the codes from the GSC module, then code U0103 is either intermittent or a memory code. If the GSC module cannot be contacted, then code U0103 set by other modules is active and the problem already exists.

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

Check all fuses supplying the GSC module on this vehicle. Check all grounds for the GSC module. Locate ground anchorage 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 all modules that set the code in memory and see if U0103 returns or you can contact the GSC module. If no code is returned or communication with the GSC 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 GSC module connector. Disconnect the negative battery cable before disconnecting the connector on the GSC module. 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 dielectric silicone grease where the terminals touch.

Perform these few voltage checks before connecting the connectors back to the GSC module. You will need access to a digital volt/ohmmeter (DVOM). Make sure you have power and ground on the GSC module. Access the wiring diagram and determine where the main power and grounds enter the GSC module. Connect the battery before continuing with the GSC module still disabled. Connect the red lead of your voltmeter to each B+ (battery voltage) power supply plugged into the GSC module connector, and the black lead of your voltmeter to a good ground (if unsure, battery negative always works). You should see the battery voltage reading. Make sure you have a good reason. Connect the red lead of the voltmeter to the battery positive (B+) and the black lead to each ground circuit. Once again, you should see the battery voltage every time you connect. If not, repair the power or ground circuit.

Then check the two communication circuits. Locate CAN C+ (or HSCAN+) and CAN C- (or HSCAN - circuit). With the black wire of the voltmeter connected to a good ground, connect the red wire to CAN C+. With the key on and engine off, you should see about 2.6 volts with little fluctuation. Then connect the red wire of the voltmeter to the CAN C- circuit. You should see about 2.4 volts with little fluctuation.

If all tests pass and communication is still not possible, or you were unable to clear DTC U0103, the only thing that can be done is to seek help from a trained automotive diagnostician, as this will indicate a faulty GSC module. Most of these GSC modules must be programmed or calibrated for the vehicle in order to be properly installed.

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