Saniflo Microswitch Problems – Won’t Start, Keeps Running, Turns On by Itself

Written by our independent Saniflo engineer, with over 20 years of repairing Saniflo toilets, macerators and shower pumps.

Most Saniflo “it won’t start” and “it won’t stop” calls come down to the same small part: the microswitch. But a damaged membrane, a sticking non-return valve, a dripping tap or a blockage can look almost exactly the same from the outside. This guide goes through each symptom in turn, explains what is actually happening inside the unit, and shows you how to tell the microswitch apart from the other usual suspects – so you only buy the part you need.

Quick answer: which fault have you got?

What the unit is doing	Most likely cause	Also check
Won’t start at all, no sound	Microswitch failed (not closing)	Power at the spur, or membrane
Hums or buzzes but won’t pump	Capacitor or a jammed blade	Failed motor
Keeps running, won’t switch off	Microswitch stuck on or membrane problem	Blocked outlet pipe, limescale
Turns on by itself every few minutes	Dripping tap or cistern, or non-return valve letting water back	Worn microswitch
Clicks on and off in short bursts	Leaky non return or constant flow of water from somewhere	Electrical short
Starts late – water backs up first	Stiff membrane or worn microswitch	Severe limescale on the membrane
Runs, then cuts out and won’t restart for a while	Thermal cut-off (overheating)	Hot water, long run times, blockage

How the microswitch works (in plain English)

Every Saniflo macerator and pump works on the same idea. When water flows into the tank, it traps a pocket of air in a small pressure chamber. As the water level rises, that air is squeezed and pushes up a rubber membrane. The membrane presses a small plastic paddle with a push-rod on to the microswitch, the switch clicks closed and the motor starts. When the water has been pumped away, the water level drops, the membrane relaxes, the switch clicks open and the motor stops.

So the microswitch never “sees” the water. It only reacts to the membrane. That is why the switch, the membrane and the pressure chamber are so closely linked – a fault in any one of them gives the same symptoms. If you want the full explanation, see how the Saniflo microswitch works.

Saniflo won’t start

You flush, or run the basin or shower, and nothing happens. The water level rises in the pan or tray and stays there.

Likely causes, most common first

The microswitch has failed open. The contacts inside are worn or burnt and no longer close when the membrane presses the lever. The motor never gets the signal to run. This is the classic microswitch fault on older units.
The membrane has split or gone stiff. If the membrane can’t flex, it can’t push the switch, even when the tank is full. Limescale and harsh cleaning chemicals are the usual reasons.
No power. A tripped breaker, a blown fuse in the fused spur, or the spur simply switched off. Always check this first.
The thermal cut-off has tripped. If the unit has been running a lot or hot water has gone through it which exceeds the recommended temperature, it may be cooling down. Leave it off for 30–60 minutes and try again.

How to tell which one it is

If the unit is completely silent, the problem is in the switching side – power, microswitch, membrane or pressure chamber. If it hums or buzzes without pumping, the switch is doing its job and power is reaching the motor; that points to a capacitor or a jammed blade instead or worse a failed motor.

If the unit starts after a long delay, or only after you flush a second time, the switch is usually still working but the membrane is stiff and needs more pressure than it should to push it.

Saniflo keeps running and won’t switch off

The motor keeps going long after the water has gone, sometimes until it cuts out on its own thermal protection.

Likely causes

The microswitch is stuck closed. The lever or the contacts stick in the “on” position, so the motor keeps running even with an empty tank. A tired switch often sticks some of the time rather than all the time.
The membrane is stuck up. Limescale or debris around the membrane can hold it against the switch lever. This is very common in hard water areas or due to chemical cleaner damage.
The outlet is partly blocked. If the discharge pipe is restricted or the non-return valve is blocked the pump can’t empty the tank properly so the level never drops and the switch stays on. You will often hear the unit labouring, and it may take much longer than usual to clear.
Water is still coming in. A cistern that keeps filling or a tap that’s been left on will keep topping the tank up, so the unit keeps running. Check this before you buy anything.

What to do

Switch the unit off at the spur. Check that no water is running into the toilet pan, basin or shower tray. If the unit stops as soon as everything is turned off but is running continuously again on its next cycle, the switch or membrane is sticking. In our experience, replacing the microswitch and the membrane together cures the large majority of “won’t start” and “won’t stop running” faults.

Saniflo turns on by itself

The unit starts for a few seconds every so often – often noticed at night, when the house is quiet – without anyone using the toilet or basin.

Likely causes, most common first

A slow drip from the cistern or a tap. A cistern fill valve that never quite shuts off, or a basin tap that drips, slowly fills the tank. When it reaches the switch-on level, the unit runs, empties, and the cycle starts again. Put a few drops of food colouring in the cistern: if the colour appears in the pan without flushing, the cistern is leaking into the toilet.
The non-return valve isn’t sealing. After each run, water that has just been pumped up the pipe should be held there by the non-return valve. If the valve is worn or dirty, that water runs straight back into the tank and triggers the switch again. A tell-tale sign is the sound of water running back just after the motor stops. See our non-return valves.
The microswitch is worn. A tired switch can become over-sensitive, shorting out and closes with less pressure than it should.
The membrane is tired. A membrane that has lost its spring can sit close to the switch lever and trigger it with very little water.

Saniflo clicks on and off in short bursts

Instead of one clean run per flush, the motor starts and stops several times in a row, sometimes with a clicking sound from the top of the unit. This is one of the clearest signs of a worn microswitch: the contacts are no longer making a firm connection, so the motor drops in and out. A stiff or lime-scaled membrane that wobbles on the switch lever can do the same, which is why we recommend changing both.

Saniflo starts late – water backs up first

The unit does start, but only once water has risen higher than normal – sometimes into the shower tray or bath. The switch is usually still working, but it is being pressed too late. The usual reasons are a membrane that has hardened with age, limescale on the membrane or a switch whose lever has become stiff. Left alone, this tends to get worse until the unit stops starting altogether.

Saniflo runs, then cuts out

If the motor runs for a while, stops on its own, and won’t restart until it has cooled down, that is normally the thermal cut-off protecting the motor – not a microswitch fault. It is often caused by hot water from a bath, dishwasher or washing machine, by the motor running for too long because of a blockage, or by a switch that is stuck on (see “keeps running” above). Let the unit cool for at least 30–60 minutes. If it keeps happening, find out why it is running so long, this problem is also a sign of a dying moter.

Microswitch or membrane?

This is the question we are asked most. The honest answer is that it is often both, because they wear out together and do the same job as a pair.

Points to the microswitch: the unit is completely silent when full; it clicks on and off in bursts; it sometimes works and sometimes doesn’t with no pattern.
Points to the membrane: the unit starts late; there are bad smells (a split membrane is a common cause); the membrane looks hard, cracked or covered in limescale.
Change both if: the unit is more than a few years old or you are opening it up anyway. They are inexpensive parts, and fitting one new part next to one old one often means opening the unit again soon after.

Our compatible microswitches and macerator membranes are listed by model.

Is it the switch, the non-return valve or a blockage?

Sign	Microswitch	Non-return valve	Blockage
Water level rising in the pan or tray	Sometimes (won’t start)	No	Yes
Runs for a few seconds by itself	Sometimes	Yes – water runs back after each cycle	No
Won’t stop running	Yes	Yes	Yes – pump can’t clear the tank
Silent when full	Yes	No	No
Grinding or labouring sound	No	No	Yes

Where is the microswitch?

On most Saniflo macerators and pumps, the microswitch sits in the control box on the side of the motor, directly above the rubber membrane and pressure chamber. You reach it by taking off the lid or top cover. The exact layout varies between models and ages, but the switch is always paired with the membrane underneath it.

The same switch is used across the range, which is why one compatible microswitch fits the Sanibroyeur, Sanitop, Saniplus, Sanipro, Sanivite, Sanislim, Sanishower, Sanibest, Sanichasse, Sanipack, Saniwall, Sanispeed, Saniaccess and Traymatic, as well as Grundfos (switch 463) and Broysan Planus units.

Checks you can do safely before buying a part

Check the power. Make sure the fused spur is on, the fuse is good and nothing has tripped at the consumer unit.
Check for incoming water. Look for a running cistern, a dripping tap or a shower left on. Use the food-colouring test on the cistern.
Listen. Silent, humming, grinding or clicking each point to a different fault (see the quick-answer table above).
Watch what happens after a cycle. If you hear water running back into the unit as the motor stops, suspect the non-return valve.
Let it cool. If it stopped after a long run or after hot water, leave it off for 30–60 minutes.
“Reset” it. Most domestic Saniflo units don’t have a reset button. Switching off at the fused spur for a minute and back on might help. If it does, it could point to a power supply problem.

Safety: never open the unit or touch the switch, wires or membrane while it is plugged in. If the unit is hot to the touch, smells of burning, or trips the electrics, switch it off and leave it off – that needs an expert engineer.

Replacing the microswitch

Replacing the switch is one of the simplest Saniflo repairs. In outline:

Switch off and unplug the unit at the fused spur.
Remove the lid or top cover to reach the control box.
Take a photo of the wires on the old switch so you know which goes where.
Unclip or unscrew the old switch and pull off the spade connectors.
Fit the new switch in the same position, with the wires on the same terminals.
Check the membrane while you are there – if it is stiff, split or limescaled, replace it at the same time.
Refit the cover, switch back on and test with a couple of flushes.

Most people do it in 15–20 minutes with a screwdriver. Our microswitch replacement guide covers this in more detail. If you are not confident working near mains electrics, ask a Saniflo engineer.

How to stop it happening again

Descale once a year, or more often in hard water areas. Limescale on the membrane and in the pressure chamber is behind most switching faults.
Don’t use bleach or harsh chemicals – they harden the membrane and rubber seals.
Only flush toilet paper and waste. Wipes, sanitary products and cotton buds cause blockages that make the unit run for longer.
Avoid very hot water going into units not designed for it.
Fix drips straight away. A dripping tap or cistern makes the unit start dozens of times a day and wears the switch out much faster.
Replace the switch and membrane together every few years on a heavily used unit.

Which part do I need?

Won’t start, won’t stop, keeps running: microswitch
Starts late or keeps running: membrane
Turns on by itself, water runs back after each cycle: non-return valve
Hums or buzzes but won’t pump: capacitor

Frequently asked questions

How long does a Saniflo microswitch last?

It depends on how often the unit runs and how hard your water is. On a busy family toilet in a hard water area, a switch can start to play up after a few years; on a lightly used unit it can last much longer. Constant small drips into the unit shorten its life considerably.

Can I test the microswitch?

Yes, with the unit unplugged and the switch disconnected, a multimeter on continuity will show whether the switch closes when you press the lever and opens when you release it. A switch that is intermittent, or that needs a hard press to click, should be replaced even if it passes the test sometimes – if you’re going to all that trouble to take it apart, replace it.

Can I just clean the microswitch?

Not really. The contacts are sealed inside the switch body. Cleaning limescale off the membrane can help for a while, but a worn switch will keep coming back.

Is it the same microswitch for every Saniflo?

The same switch is used across almost all of the range, plus Grundfos (switch 463) and Broysan Planus units. If your old switch looks different, contact us before ordering.

Will a new microswitch stop my Saniflo running all the time?

If the switch is sticking, yes. If the membrane is stuck up with limescale, or the outlet pipe is partly blocked, the unit will still run on. That is why we recommend checking the membrane and replacing it at the same time.

Why does my Saniflo start in the middle of the night?

Almost always because water is getting into the tank when nobody is using it – a cistern that doesn’t quite shut off, a dripping tap, or water running back past a worn non-return valve. Fix the water first; if it still happens, look at the switch and membrane.

Does the microswitch cause bad smells?

Not directly. Smells are usually a split membrane, a worn non-return valve, or waste building up inside the tank. See our Saniflo problems and troubleshooting guide for smells, noises, leaks and blockages.

Should I call an engineer?

If the unit trips the electrics, gets hot, smells of burning, or you are not comfortable working near mains electrics, yes. Otherwise a microswitch or membrane swap is a straightforward DIY job.

Still not sure?

Tell us your model and what the unit is doing, and we’ll point you to the right part. Contact us here. Order by 3pm for same-day dispatch with fast tracked delivery (weekends and bank holidays excluded).