If your fluorescent bulb is flickering, switching on and off by itself, or dead altogether, test the bulb first. Testing a fluorescent tube is a relatively easy procedure that can be done without professional help. There are several steps involved that will allow you to determine the health of the bulb as well as the lamp. Keep in mind that most fluorescent bulbs have a fairly long working life, and there are several possible sources for the trouble.
Make sure electrical power is flowing normally to the fixture by checking the circuits in your electrical service panel.
Remove the bulb from the fixture by twisting the bulb and guiding the two small electrodes at either end out of their connectors. There is a small opening in either side of the lamp, just above the connectors, that will help you manoeuvre the bulb out. Examine both ends of the bulb. If the glass has darkened, the bulb is at or near the end of its life. Check also for missing or bent electrode pins.
If the bulb's appearance is normal, use a continuity tester on both pairs of electrodes. This is a simple device that checks for a functioning electrical circuit through the pins. A simpler method is to install the bulb in another lamp that you know is working normally. If the bulb lights as it should, the problem is in your fixture.
If you have an older fixture, the problem may lie with your starter. This is the small box that is placed on or near the lamp and primes the bulb with a high-voltage charge whenever you turn the switch on. (If your lamp always flickers a few times before going on, you have a starter system.) Remove the starter and then return it to its base or socket. If the bulb still malfunctions, your best move at this point is to replace the starter.
If the good bulb and new or replaced starter still don't work normally, your ballast may be malfunctioning. Remove the ballast cover with a screwdriver. If oil is leaking, replace the ballast.
If no oil is leaking, use the voltmeter on one of the white wires and one of the blue or yellow wires leading from the ballast to the connectors. If the voltmeter does not show an active circuit, replace the ballast. If a circuit is present, check the wires for loose connections, to the ballast and to the connectors. If there are no loose connections, replace the ballast. There are different sizes and two basic types: magnetic (the humming kind) and electric (the silent, more expensive types).
When removing bulbs, starters or ballasts, make sure power to the fixture is off and wait a few minutes for stored current to dissipate.
Handle the ballast with care. Old ones can leak oil, which contains toxic chemicals. Always handle fluorescent bulbs with care; broken bulbs can release toxic mercury.