Sounds like the trapalert system, here's the tech bulliten on the system.
REMOTE KEYLESS ENTRY SYSTEMS -2001 Chevrolet Impala
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NOTE: All RKE transmitters recognized by RCDLR must be programmed during programming sequence. If system is placed in program mode, it will erase all previously programmed RKE transmitters upon receipt of programming signal from first transmitter.
NOTE: Scan tool must stay connected until all of transmitters are programmed.
RKE TRANSMITTER PROGRAMMING
1.Install a scan tool. Turn ignition switch to RUN position. Using scan tool, access Body Control Module (BCM) SPECIAL FUNCTION in PROGRAM KEY FOBS menu. 2.Press and hold LOCK button and UNLOCK button simultaneously on one transmitter. After 15 seconds, BCM will chime in order to confirm programming of that transmitter. 3.Repeat previous step in order to program up to 4 transmitters. Remove scan tool. Operate transmitter functions in order to verify correct system operation. 4.Remove scan tool. Operate transmitter functions in order to verify correct system operation.
Wednesday, February 20th, 2019 AT 11:00 AM