If your access point is connected to in-line power, do not connect the power module to the access point. Using two power sources on the access point might cause the access point to shut down to protect internal components and might cause the switch to shut down the port to which the access point is connected. If your access point shuts down, you must remove all power and reconnect only a single power source.
On power up, the access point is placed into low power mode (both radios are disabled), Cisco IOS software loads and runs, and power negotiation determines if sufficient power is available. If there is sufficient power then the radios are turned on; otherwise, the access point remains in low power mode with the radios disabled to prevent a possible over-current condition. In low power mode, the access point activates the Status LED low power error indication, displays a low power message on the browser and serial interfaces, and creates an event log entry (see the "Checking the Autonomous Access Point LEDs" section and "Inline Power Status Messages" section).
and also verify the similar post:
https://supportforums.cisco.com/discussion/11171191/wlc-5508-and-cisco-aironet-1242-lwapp-air-lap1242ag-k9
