You can change the battery yourself but will probably need to programme it to your alarm - now I know we have a thread covering this process somewhere on the forum - is your alarm the sigma M30?
this is for the M30
Reprogramming ProcedureArm and then Disarm the system using the working Radio Key.
Within 30 seconds of disarming, type into the keypad- *17856*91*2# where 2 is total number of radio keys to work the system Briefly press both radio keys in turn (the dash board LED will flash each time and the siren will bleep) Any radio keys not programmed here will no-longer operate the system. You should now be able to arm and disarm the system using all the radio keys programmed.
When using the keypad always observe the following points:
The PIN number should not be entered too quickly or too slowly - when a digit has been pressed the small LED built into the keypad will illuminate briefly as confirmation and only then should the next digit be entered.
If an incorrect digit is pressed or the delay between entering digits exceeds 10 seconds, then the LED above the keypad will flash rapidly and the PIN attempt should be abborted and started again after 30 seconds.
Due to the small size of the keypad some users may find a rubber tipped pencil easier to use. DO NOT use the sharp tip of a ball point pen etc, or press excessively hard on the keypad as this will only damage the membrane and cause a digit to remain pressed, causing a continual incorrect PIN.
The keypad allows only THREE PIN attempts and then will “Lock Up” if an incorrect number is entered a third time. This is a security feature to prevent random PIN attempts disarming the system.
If the remote(s) still don’t work then there are 3 possible reasons:
The remote battery is flat (the built in LED takes very little current to illuminate it and is not an indication that the battery voltage is healthy). The remote is damaged or faulty. It’s the wrong type of remote for the M30. There have been a number of Sigma remotes over the years that operate on different radio frequencies and use different encryption methods. The correct remote for the M30 is the AC38. This remote is easy to identify as it has a round battery “pod” on the rear of the remote with a screw-off cover.