For 1990-2009 cars

### Circuit Description

This DTC indicates an internal fault and is set within the vehicle communication interface module (VCIM). No external circuits are involved.

### Conditions for Running the DTC

The microprocessor runs the program to detect an internal fault during module power up and every 10 seconds thereafter until the module re-enters the sleep mode.

### Conditions for Setting the DTC

The VCIM detects an internal communication malfunction.

### Action Taken When DTC Sets

• | The OnStar® status LED turns RED.

• | The vehicle is unable to connect to the OnStar® Call Center.

### Conditions for Clearing the DTC

• | A current DTC clears when the malfunction is no longer present.

• | A history DTC clears after 50 malfunction free ignition cycles.

### Diagnostic Aids

• | The diagnostic procedure used for DTC U1500 in OnStar® systems with single module generations of OnStar® differ greatly from previous generations with 2 modules. While these older generations denoted a failure of the 3-wire bus between the modules by setting a U1500, single module OnStar® systems use this DTC to denote an internal module failure. The determination of whether a one or 2-module generation of OnStar® is used should be confirmed. Refer to OnStar Description and Operation .

• | This DTC may be stored as a history DTC without affecting the operation of the module. If stored only as a history DTC and not retrieved as a current DTC, do not replace the VCIM.

• | If this DTC is retrieved as both a current and history DTC, replace the VCIM.

Step | Action | Yes | No
--- | --- | --- | ---
1 | Did you perform the Cellular Communication Diagnostic System Check? | Go to Step 2 | Go to Diagnostic System Check - Cellular Communication
2 |

1. Install a scan tool.
2. Turn ON the ignition, with the engine OFF.
3. Retrieve DTCs from the vehicle communication interface module (VCIM).

Is the DTC retrieved as a current DTC?

Go to Step 3

Go to Diagnostic Aids

3 | Important: Perform the OnStar® setup procedure. Replace the VCIM. Refer to Communication Interface Module Replacement . Did you complete the replacement? | Go to Step 4 | --
4 |

1. Use the scan tool in order to clear the DTCs.
2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

Go to Step 2

System OK