Changing a light bulb is the stuff of dumb-blonde jokes. But pool lights can be a bit trickier.

A light that doesn’t work isn’t necessarily the result of a bad bulb. Rather, it can be the symptom of any number of problems, including a bad GFCI, a blown fuse or faulty fixture.

If your pool has a light out, follow this step-by-step process to properly diagnose the problem.

To complete these steps, you’ll need both a volt-ohm meter and a megger — two measuring devices essential to any service tech. You should be able to pinpoint the problem after a diligent walk-through.

Step 1: With the power on (circuit breaker and switches), test and reset the GFCI. Be sure to check both functions. And remember: Older GFCIs will not test without power, but they will reset. Conversely, newer GFCIs will turn off without power, but will not reset.

As long as both the test and reset functions operate, you’re ready to proceed to Step 2. If not, turn the power off, disconnect the GFCI loads, turn the power back on and repeat Step 1.

If both the test and reset functions operate now, you have a ground fault somewhere downstream and you must check the loads with a megger. If they don’t, you have a bad GFCI. Turn the power off, replace the GFCI and repeat Step 1 again.

Step 2: With the power off, disconnect the black (hot) and white (neutral) stranded wires of the light cord at the junction box. But leave the green (or green with a stripe — usually yellow) wire connected to the junction box ground bar. Set your volt-ohm meter to “continuity test” (or lowest ohm scale), and test between the black and white wires to the light (they connect to the bulb).

If your light bulb is in good condition, it should read about 3-20 ohms.

A reading of high ohms or “OFL” (overflow, open, no continuity, infinite ohms) usually indicates a bad bulb, though it also could be caused by a bad wire or fixture.

A reading of zero ohms (short circuit, no ohms) indicates a short circuit in the bulb, cord or fixture that should have tripped a circuit breaker or blown a fuse. Double-check your findings.

Regardless of the results of this step, it’s time to proceed to Step 3.

Step 3: With the power still off, test from the ground to black wire with the megger, and then the ground to white wire. If the light bulb is good, you will get the same reading on both wires. If the bulb is bad, you may get different readings for each wire.

A reading of more than 100 megohms on both wires is considered good, meaning the fixture has no ground fault. Continue to Step 4.

By definition, 40 to 100 megohms is a ground fault. But the fixture still may function reliably without tripping the GFCI, especially if the ground fault is in the neutral line. If the ground fault is in the hot line, it most likely will trip the GFCI at some point, potentially causing nuisance tripping. It’s recommended that fixtures testing in this range be replaced (see illustration).

On the other hand, 20 megohms is a major ground fault, and you’re probably best off disconnecting the fixture. Place wire nuts on the load wires from the GFCI until the fixture can be replaced.

Step 4: If the GFCI passes the test in Step 1, the bulb tests poorly in Step 2, and the fixture tests well (over 100 megohms on both legs) in Step 3, then — and only then — is it time to replace the bulb and gasket. It is still possible, however, that the fixture is the problem and not the bulb, so test the bulb for continuity when you remove it.

If the bulb tests well (3-20 ohms) the fixture is probably bad. Try a new bulb without closing the fixture. It probably will not light, and the fixture will have to be replaced.

If the bulb tests poorly (overflow, open, no continuity, infinite ohms), the light most likely will work fine with a new bulb and gasket.