Check your sump pump every three months by pouring water into the pit until the pump turns on, then watching it drain the water completely

A sump pump sitting idle for months can fail silently—you won't know until heavy rain or snowmelt fills your basement. The simplest check takes five minutes and tells you whether the pump will actually run when water arrives. You pour water into the sump pit, listen for the pump to start, and watch it empty the pit. If it doesn't turn on or doesn't drain fully, you have found a problem before it costs you.

This check works because it mimics what the pump does in a real flood: it detects rising water and removes it. If the pump fails this test, the cause is usually something you can fix yourself—a tripped breaker, a clogged discharge line, or a float switch stuck in the wrong position. Sometimes it means the pump itself has failed and needs replacement. Either way, you learn this before water is actually in your basement.

Key Takeaways

  • Pour water into the sump pit until the pump motor starts; if it doesn't turn on within a few seconds, check the power cord, outlet, and circuit breaker first.
  • Watch the water drain completely after the pump starts; if the water level stays high, the discharge line is likely clogged or the pump is too weak for your pit size.
  • The float switch—a ball or arm that bobs on rising water—must move freely; if it is stuck or tangled in the pit, the pump won't start even if power is good.
  • Test your pump every three months and always before heavy rain season, because a pump that worked last month can fail without warning.
  • If the pump runs but water drains slowly, the check valve in the discharge line may be stuck closed, trapping water in the pipe.

What to check before you pour water

Before you test the pump, walk through a quick safety and setup check. Make sure the power cord is plugged in and the outlet has power—plug a lamp into the same outlet to confirm. Look at the circuit breaker panel and verify the breaker for the sump pump is in the ON position, not tripped to OFF or the middle. A tripped breaker is the most common reason a pump doesn't run, and it takes ten seconds to fix.

Look into the sump pit itself. The pit should be mostly empty or have only an inch or two of standing water. If the pit is already half full or more, do not add water; instead, call a plumber or pump contractor, because the pump should have drained that water already. A pit that stays full means the pump is failing or the discharge line is blocked, and adding water will only make the problem worse.

Check that nothing is blocking the top of the pit—no boxes, tools, or debris covering the opening. The pump needs air to cool itself, and a covered pit can overheat the motor. Also make sure the discharge line (the pipe that carries water away from the house) is not kinked, crushed, or frozen in winter. A kinked line will block water even if the pump runs.

How to run the water test

Fill a bucket with clean water from a hose or faucet. Pour the water slowly into the sump pit, watching the water level rise. Within a few seconds of the water reaching a certain height—usually 6 to 12 inches, depending on where your float switch is set—you should hear the pump motor start. It will sound like a small electric motor humming or whirring. If you hear nothing after 15 to 20 seconds of pouring, stop and check the power and breaker again before continuing.

Once the pump starts, keep watching the pit. The water level should drop steadily as the pump pushes water out through the discharge line. The entire pit should drain within a minute or two. If the water drains slowly or stops draining partway down, the discharge line is clogged or the check valve is stuck. If the water level drops to a certain point and then the pump shuts off, that is normal—the float switch has dropped back down and turned off the motor.

After the pit is empty, the pump should stay off. If the pump keeps running even though no water is entering the pit, the float switch is stuck in the raised position and needs to be freed or replaced. If the pump cycles on and off repeatedly without you adding water, the check valve is leaking and allowing water to flow backward into the pit.

What the float switch does and why it matters

The float switch is the part that tells the pump when to turn on. It is usually a ball or an arm attached to a rod or cable that rises and falls with the water level. As water fills the pit, the float rises. When the float reaches a certain height, it triggers an electrical switch that starts the pump motor. As the pump drains the water, the float drops. When it falls below a set level, the switch turns the motor off.

If the float switch gets stuck—tangled in the pump cord, wedged against the pit wall, or clogged with sediment—the pump won't start even if power is good and water is rising. You can usually free a stuck float by hand: reach into the pit (if it is safe and the power is off) and move the float up and down gently a few times. Make sure it moves smoothly without hitting anything. If the float is cracked or the rod is bent, it needs to be replaced.

Some newer pumps use a pressure sensor instead of a float switch. A pressure sensor detects water weight rather than a physical float. These are less likely to stick, but they can fail if water or sediment clogs the sensor port. If your pump has a pressure sensor and won't start during the water test, the sensor port may be blocked.

Clearing a clogged discharge line

The discharge line is the pipe that carries water away from your house—usually to a storm drain, daylight opening, or dry well. If water drains slowly or not at all during your test, the line is probably clogged. You can often clear it yourself without calling a plumber.

First, turn off the pump and let any water in the pit settle. Locate where the discharge line exits your house or enters the ground. Look for ice, mud, leaves, or debris blocking the opening. If the opening is blocked, clear it by hand or with a small tool. In winter, ice can form inside the line; if you suspect ice, pour warm (not boiling) water down the line to melt it, or wait for warmer weather.

If the opening is clear but water still won't drain, the clog is inside the pipe. You can try running a plumbing snake (a flexible auger) down the line from the house end, or you can disconnect the line at the pump and blow compressed air through it. If neither works, the line may need to be replaced or professionally cleared. A clogged discharge line that you cannot clear yourself is worth a service call, because a backed-up pump will burn out quickly.

Understanding the check valve and backflow

Most sump pumps have a check valve in the discharge line, usually installed right where the line leaves the pump. The check valve allows water to flow out of the house but prevents water from flowing backward into the pit when the pump shuts off. If the check valve fails or gets stuck, water will drain back into the pit after the pump stops, and the pump will cycle on and off repeatedly.

You can test the check valve during your water test. After the pump drains the pit and shuts off, watch the pit for a few minutes. If water slowly reappears in the pit without you adding more, the check valve is leaking. A leaking check valve is not an emergency, but it wastes electricity and wears out the pump faster. It usually needs to be replaced.

Some check valves can be cleaned if they are stuck with sediment. If you are comfortable working with plumbing, you can unscrew the valve from the discharge line and rinse it with clean water, then reinstall it. If you are not sure, a plumber can replace it in under an hour. A new check valve costs between $50 and $150 depending on the type and size.

When to call a professional

If your pump won't start after you have checked the power, breaker, and float switch, the pump motor itself may have failed. Motors can burn out from age, overuse, or running dry (when the pit is empty but the pump keeps running). A failed motor usually cannot be repaired and the pump needs to be replaced.

If the pump starts but makes a grinding, squealing, or rattling noise, something inside the motor or impeller is damaged. Do not run the pump repeatedly if it sounds wrong; turn it off and call a plumber. Running a damaged pump can make the problem worse and may damage the discharge line.

If water is backing up into your basement even though the pump is running, the discharge line may be blocked or undersized, or the pump may be too small for the amount of water entering the pit. This requires a professional assessment. A plumber can measure the flow rate and recommend a larger pump or a second pump if needed.

How often to test and what season matters most

Test your pump at least every three months—spring, summer, fall, and winter. A pump that worked in summer can fail by fall without any warning. Regular testing catches failures before they flood your basement.

Test more often before and during heavy rain season in your area. If you live in a region with spring snowmelt or summer thunderstorms, run a test a week or two before the season starts. If your area has a wet season, test monthly during that time. After a heavy rain that actually triggered the pump, test it again within a day or two to make sure it recovered properly and is ready for the next rain.

In winter, test the pump indoors if possible, or test it on a mild day. Cold weather can affect how quickly the pump starts and how well water flows through frozen discharge lines. If you test in winter and the pump seems slow, that may be normal—but if it doesn't start at all, the problem is not the cold; it is the pump or power.

Frequently Asked Questions

What if the pump starts but the water doesn't drain?

The discharge line is almost certainly clogged. Check the line opening outside for ice, mud, or debris and clear it. If the opening is clear, the clog is inside the pipe. Try running a plumbing snake down the line or blowing compressed air through it. If neither works, call a plumber to clear or replace the line.

Can I test the pump in winter?

Yes, but test it indoors or on a mild day if possible. Cold water and frozen discharge lines can slow the pump down, but the pump should still start and drain within a minute or two. If it doesn't start at all in winter, the problem is not the cold—it is the pump, power, or float switch.

How do I know if the float switch is stuck?

During the water test, listen for the pump to start as soon as water rises in the pit. If water is clearly high but the pump doesn't turn on, the float switch is likely stuck. Turn off the power, reach into the pit carefully, and move the float up and down gently several times to free it. If it still won't move or the pump still won't start, the switch needs to be replaced.

What does it mean if the pump cycles on and off constantly?

The check valve is leaking, allowing water to drain back into the pit after the pump shuts off. The pump turns back on, drains the water again, and the cycle repeats. This wastes electricity and wears out the pump. The check valve should be replaced, which usually costs $50 to $150 and takes under an hour.

Should I test the pump if it is raining?

No. If it is actively raining, the pump is already working and you should not add more water. Wait until the rain stops and the pit has drained, then run your test. If the pump is running during rain and the pit is still filling up faster than the pump can drain it, you may need a larger pump or a second pump—call a plumber to assess.