The OBD2 code B0490 for a 2003 Buick LeSabre corresponds to a problem with the HVAC control head rear auxiliary unit sensor circuit. This code indicates that there may be an issue with the sensor or its wiring. The sensor is likely failing to send proper signals to the HVAC control unit, which can affect the operation of the rear auxiliary unit in your car's climate control system.

To address this issue, you may need to inspect the sensor and its wiring for any visible damage or corrosion. It's essential to ensure that the sensor is properly connected and that there are no loose connections or broken wires. If everything looks intact, the sensor itself may need to be replaced.

Once you have addressed the sensor or wiring issue, you can reset the OBD2 code using a diagnostic scanner to see if the problem has been resolved. If the code persists, further diagnosis may be needed to pinpoint and fix the underlying issue with the HVAC control head rear auxiliary unit sensor circuit.