Start with the pump lid o-ring.

Probably an air leak on the suction side. That’s the most common cause.

A pump losing prime means air is getting in somewhere between the skimmer and the pump. The lid o-ring is the first suspect — it dries out, gets brittle, or just isn’t seated right. Wipe it down, apply some pool lube, and make sure the lid is tight. Nine times out of ten that’s it.

If that doesn’t work, check the drain plug, the pump basket lid, and any valves or fittings on the suction line. Even a pinhole leak can let in enough air to kill the prime, especially when the pump is running. Listen for a hissing sound or look for tiny bubbles in the pump lid window.

Worst case: the water level in the pool dropped below the skimmer. Just top it off and re-prime. But if you’ve checked everything and it still loses prime, you might have a crack in the suction line underground — and that’s a pro job.

Lube that o-ring. Saves a headache every time.