There’s trouble at Ms. Wilson’s pond. Her pump is making a funny noise, the amount of water flowing out of it is reduced, or it’s just plain not working at all, and you get the call to come fix it. There’s a step-by-step process for figuring out just what the problem is.

When Andy Wassmann, pond development supervisor for Wisconsin Lake & Pond Resource in Eldorado, Wisconsin, gets a call like that, it’s usually because debris is blocking the flow, either entirely or partially.

If the pump is running, but the flow is reduced, the first step is to check the filter and skimmer box. The skimmer flap might be jammed or blocked by debris, or the skimmer mat may need cleaning.

Ms. Wilson may have noticed a steady, but gradual, decline in flow over a fairly short period of time. (A little marking on a pipe, showing the usual output level, is helpful as a flow-checking device.)

“It could be a linear decline, where one week, the water’s flowing at ten gallons a minute, nine the next week, and eight the next, for instance,” said Bradden Kerr, senior fishery biologist at Spring Creek Aquatic Concepts’ Portland, Oregon branch.

If there’s no blockage, and the pump’s still not running, check the power. It could be something as simple as the circuit breaker or ground fault circuit interrupter (GFCI) having been tripped.

“A family pet or rodent could have chewed the power cord just enough to expose some copper, and moisture got in there,” said Wassmann. “A GFCI, for the most part, isn’t going to trip off of a seized motor, or something not turning. It’s designed for detecting moisture.”

If that’s not it, next test the outlet and the terminals on the power cord’s plug. If there is power, then we can move on to the next step.

At this point, Frayne McAtee, director of sales at OASE in West Palm Beach, Florida, suggests a ‘feel’ test. “After you’ve tested that GFCI to make sure it’s working, you plug in the pump, put your hands in front of it, and see if you feel any water coming out. If not, put your hands on the pump itself. Do you feel any hum or vibration? If you do, then it’s getting power.”

The next step is to unplug the pump and remove it. “I want to see if the impeller underneath can freespin,” said Wassmann. “If it can’t, it’s probably got something lodged against it. It could be a pine cone, a piece of mulch bark, a stick, or a bunch of leaves, especially the large, leathery kind.”

A clog could also be caused by a big blob of filament algae, or accumulated fish wastes, especially if there are a lot of them swimming in the pond.

The impeller could have also been stymied by lime calcification, especially if the area has heavily mineralized water. “It builds up slowly around the impeller, so that it finally just binds it up,” said McAtee. If you find this, you can fix it by soaking the impeller in white vinegar for five minutes.

If the impeller does spin free, then the next step is to hook the pump back up to the supply line. If it still can’t pump the water to the top of the waterfall, the line might be clogged with algae or something else. Once that’s cleared, and the pump still doesn’t have the ‘oomph’ it needs, it may be that the unit has grown weaker, and it’s time for a new one.

If the intake screen or skimmer basket becomes too clogged, the pump has to work too hard without enough water going through it. This causes cavitation and heat buildup, which can burn out seals and bearings.

Cavitation could also have caused ‘vapor lock.’ This happens when an air bubble becomes trapped in the volute, the pump’s main internal space. The bubble will keep water from flowing, even if the impeller is spinning. Simply tilt the pump underwater with the intake on top, and let the bubble escape.

“If you plug a pump in and it makes a loud squealing noise, that indicates bearing damage,” said Brandon Dwyer, vice president, product development at Atlantic Water Gardens in Mantua, Ohio.

Burned-up bearings can sometimes be repaired, but often means it’s time for a new pump. “If the pump hums, but the impeller doesn’t spin, that typically indicates capacitor failure which, in most cases, can be replaced.”

“That’s usually caused by overheating, either by a pump’s not being fully submerged or low power draw,” Dwyer said. “You’d need to inspect both of those things.” Low power draw can be caused by a pump running off a too-long extension cord, loose wiring, or overloaded circuits.

If you’ve checked the power, cleared any clogs and the impeller still isn’t spinning, it may have seized up. While unplugged, try giving it a hard push with a screwdriver or other tool; this might get it going again. Restore the power, and see if the impeller spins. If it does, you’re back in business. You can unplug the pump and reinstall it.

If the impeller isn’t frozen but still doesn’t spin under power, try it in a different GFCI outlet. If it still won’t work, or trips the GFCI breaker, the pump has an internal problem. But if the pump works in the new outlet, something may be amiss with the outlet at the pond site. That’s a job for an electrician.

There are other causes of pump failure, besides obstructions and power problems. “It can happen when a pump’s being used in the wrong application,” said Dwyer. “If you need to pump a mass volume of water to a high head height, such as a ten-foot fountain, you would probably need a direct drive pump. But people often buy a pump based on its flow rate, not its application.”

There are three basic styles of pond pumps. Magnetic (mag) drive pumps are typically used in low head-pressure applications; asynchronous drives for medium applications, and direct drives, for high.

All pumps are rated by head pressure, a combination of the vertical height that you’re going to pump the water to, and the friction loss within the tubing you’re pumping it through. The calculations are simple, and can be found online. Adding the friction loss, horizontal and vertical, gives you your head pressure.

Wassmann said the next big reason he gets a call is the sheer age of a unit. It’s reached the end of its working lifespan, or its repair-versus-replace ratio is too high, and it’s time for a new one. “When they get to about eight years old, that’s the ‘witching hour.’ Something’s probably going to happen at that point, or soon after. It should be on your radar.”

But there really are no hard-and-fast expiration dates for pumps.

They can last a very long time, if they’re the right sizes and types for their applications, and are well-maintained. Wassmann takes all of his pumps out in the fall, pressure-washes them, and stores them inside, in a climate-controlled area, so the rubber seals won’t freeze in the Wisconsin winters.

Maintain the ponds and their pumps well, and Ms. Wilson, and your other clients, will be enjoying their backyard oases for years to come.