# How to Troubleshoot T12 Tubes
The T12 is a cylindrical fluorescent light bulb used in many household and industrial fixtures. The "T" stands for "tube," while "12" indicates the diameter of the tubular bulb in eighths of an inch; a T12 tube is 12/8, or 1.5, inches in diameter. These bulbs and their fixtures don't have many parts, so troubleshooting a malfunctioning T12 fluorescent is a relatively simple matter.
Instructions
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Look at the ends of the T12 tube. If one or both ends have turned black on the inside, the bulb is burned out.
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Watch the glow of the bulb when you turn it on. If the bulb isn't as bright as usual, or if it flickers, the electrodes or filaments may have failed or may be in the process of failing.
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Check out the ballast if the bulbs that you are having problems with are new. If you detect a burning odor from the ballast, or if the fixture buzzes or hums loudly, the ballast -- not the bulb -- may be the problem. If the socket's contacts have shifted because of damage caused by rough insertion or removal of a T12, the bulb may blink intermittently or the fixture may stop working altogether, in which case it will have to be replaced.
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