This bulletin applies to 2008-2013 Chevrolet Tahoe Hybrid models. You may find that there is a lack of brake assist (hard pedal) after disconnecting the battery or after the 12V battery has run low on voltage. This may also be followed by an ABS and red brake lamp with a possible DTC C121D sym00 or C056D sym00 being set in the EBCM. This may be caused by an improper 12V battery disconnect procedure.
After disconnecting the battery, you must wait for a period of at least one minute with the ignition in the OFF position prior to disconnecting the negative battery cable. During this one minute wait period it is critical that the key does not pause in the ACC mode. During this period it is also critical that the brake pedal is not depressed.
Due to programming parameters, it is necessary to allow the brake system pressure relief procedure to occur without interruption. Failure to follow these steps may result in warning lamps and/or DTCs being set pertaining to interruption of the pressure relief procedure.
Other vehicle systems may also be affected by not allowing a one minute duration for power-down. If the 12V battery is disconnected prior to the completion of the brake system pressure relief procedure, the next “ignition on” drive cycle may present a hard brake pedal with little or no power assist.
If the vehicle’s battery is disconnected prior to waiting the one minute period, follow these steps to resolve the concern:
1. Install the negative battery cable to the battery.
2. Rotate the lever lock clockwise until an audible click is heard in order to connect the negative battery cable to the battery.
3. Position the negative battery cable insulating cover over the negative terminal.
4. Start the vehicle without stepping on the brake pedal.
5. Shut off the vehicle for one minute.
6. Restart the vehicle and check brakes for proper operation.
7. Clear codes in the ECM and the EBCM.