RV macerator toilet not flushing: the hum, the clog, and the power

Owners describe a macerator fault by its sound rather than by its mechanism. "It just hums" is the most common way this gets written down, and it is a useful description, because a pump that is running and moving nothing is a different problem from a pump that is silent. Decide which of these three you have before touching a part, because they have three different answers and only one of them is electrical.

Worth knowing while you decide whether this page is even yours: Thetford draws the line itself. "A permanent toilet uses gravity to evacuate waste into the holding tanks while a macerating toilet generally have a pump with a macerator that breaks down and pushes waste into the holding tanks." If your toilet has a pedal and no pump noise at all, the gravity toilet page is the right one.

The three faults, and which one you have

It hums and the bowl will not empty. The pump runs, the waste does not move. That is a blockage, and the maker's troubleshooting table starts at the pump inlet.

Water arrives and the pump does not run. The bowl fills and nothing else happens. Thetford's table gives this combination two causes: the motor, or the control panel. That is the table's entry for one symptom rather than a complete diagnosis, which is why the voltage check below comes first.

It flushes, and then the bowl drains dry. That is not a fault. It is siphonage, and it is the one item here that costs nothing.

The pump inlet comes first

The maker's table has two entries before it reaches anything electrical, and both are free. The first is "waste tank is full", with the instruction to empty it before using the toilet again, so check the tank and its gauge before you touch the toilet at all. The second is "clog at pump inlet", and the instruction is to clear the clog. That ordering is the point: the inlet comes ahead of the motor, ahead of the panel and ahead of the wiring, which is the opposite of how most people diagnose a pump that is making noise.

Before you open anything, two warnings apply. The one people do not expect is biological: Thetford's own words are "human fecal wastewater contains substances that can be hazardous to your health, such as bacteria, viruses and fungi," and its instruction is to "always treat fecal wastewater from human activities as hazardous liquid, and use appropriate personal protection equipment when performing work in areas contaminated by wastewater." Gloves, eye protection, and somewhere to wash afterwards. Thetford's words for the parts below the bowl valve are "there is a serious injury risk when touching the sharp macerator knives located below the toilet bowl valve", followed by "use extreme caution when removing obstructions out of the macerator house". The macerator is disabled while the bowl valve is open, which is the maker's stated protection, and the manual's order for an obstruction is to open the bowl valve, switch off the power at the control board, and then remove the blockage. Power off is not optional here. If you are not certain of the sequence on your own model, isolate the power first rather than second, and note that the maker's servicing rule puts this work with an approved competent person, which is where this page hands it over.

A solid object in the macerator

If the inlet is clear and the pump still will not move waste, the next entry is "solid object in macerator", and the remedy is in three parts: disable the power, attempt to remove the object, and if that is unsuccessful, contact Thetford. That third part is not a formality. It is the manual telling you where the owner's work stops.

There is a second symptom worth knowing because it points at the same cause: if the toilet has become noisy, the 2015 Tecma manual says the pump may be partially blocked if a solid object other than toilet paper has been flushed. A partially blocked pump is one explanation the manual gives for unusual noise, and it is a separate entry from the pumping check above.

Where this stops being owner work

Thetford recommends that plumbing and electrical work be performed by a licensed tradesperson, and its own troubleshooting guide describes itself as a basic service aid rather than a repair procedure, adding that "if the suggested actions above do not resolve the issue, it may be necessary to bring unit in for professional service". Past clearing a blockage with the power off, the honest answer is a technician. If you would rather hand it over, the directory lists mobile technicians and repair centres by state, and many of them work on macerating toilets at your site.

The maker names two causes: the motor or the control panel

The troubleshooting table entry reads "water added, but pump fails to flush", and the cause it gives is short: a potential problem with the motor or the control panel. Two candidates, and the manual does not pick between them for you, which is the point at which diagnosis needs a meter and a wiring diagram rather than a description from a page.

Before either: voltage, and the 10 percent drop

There is one measurement worth taking first, because it is free and it is in the same manual. The supply should be 12 volts plus or minus 2, and there should be no more than a 10 percent decrease in voltage while the macerator is running. If it drops further than that, the manual's conclusion is a wiring problem in the RV rather than a fault in the toilet, and that changes what you are fixing and who pays for it.

And one thing the same manual does not settle. Thetford's two documents for this product family disagree about the circuit, and neither this page nor the manual resolves the difference:

• The 2015 Tecma manual states that "every toilet requires a 12-VDC/40-AMP dedicated circuit with 8-gauge wire and 40-AMP breaker or fuse between the main bus or battery and a terminal located near the toilet".
• The later Silence Plus 2G manual carries a wiring chart headed "12 VOLTS - Install", giving 10, 8 or 6 gauge wire by run length, against a 30 amp fuse where the 2015 manual specifies 40.

Both are Thetford documents, eight years apart, and a 40 amp circuit and a 30 amp one are not the same claim. If you still have the manual that came with the toilet, that is the one to wire against. Both documents do treat the toilet as its own circuit, and the later one spells it out: "every toilet must have its own circuit breaker or fuse." It attaches a fire hazard warning to using the recommended rating rather than something else: "always use recommended circuit breakers or fuses. Failure to follow the recommendations can result in fire." A macerating toilet is not a load to share a circuit with something else.

The discharge hose has been pulled down

This one reads like a fault and is not. The manual's entry is "bowl drains dry after flush", and its cause is "water is siphoning out of bowl due to discharge hose pulled down". The remedy is to straighten the hose so that the top of the discharge line is in line with the toilet nozzle.

Why it matters that this is here. A bowl that will not hold water is the classic symptom of a failed seal on a gravity toilet, and it is what sends people looking for a part. On a macerating toilet the same symptom can be a hose that has settled, been pushed down behind the toilet, or been re-routed during something else. That is a five minute fix, and it is the reason this check sits above anything involving a part.

Two figures from the installation instructions, for anyone re-routing that hose. The Silence Plus 2G manual specifies a 1.5 inch internal diameter sanitation hose or pipe, allows rigid PVC, and asks that flexible hose avoid sharp bends that could kink it, and the same manual says the toilet should ideally sit higher than the holding tank with a 6 foot rise the maximum that will not affect performance. Thetford's support page for that toilet says a 12 volt macerator pump moves waste up to 120 feet. Those are the maker's figures for those models rather than for the whole range, so hose diameter, rise and run are worth checking against the manual for the toilet actually installed.

The maker's own list is specific, and it is worth reading once because it is the cause of the fault most likely to happen again: "only human waste and toilet paper should be put into the toilet. Never flush foreign materials (e.g., paper towels, pre-moistened wipes, condoms, feminine hygiene products, dental floss, household garbage, etc.)."

The wipe sentence is the one people get wrong, and the iNDUS manual is blunter about it than the others: "never use wet wipes, moist toilet tissues or anything similar, even if it says it is safe to flush. Any wet wipe will block the product." The rule as the maker writes it is absolute: any wet wipe will block the product. The same manual adds tampons, hair and cotton swabs to the list.

Cleaning matters for the same reason, and the 2G manual is direct about it: "bleach, petroleum-based products, strong acids and abrasives can cause irreversible damage to the toilet system and components." Its instruction is to "use only non-abrasive, non-petroleum products to clean the toilet." The same manual names household cleaners specifically as a source of both.

Last, one behaviour that reads as a fault and is a design decision. When the tank sensor reads full, the toilet disables the water inlet so the bowl will not fill, and the maker's note is that the macerator pump remains active during that lockout. A toilet that adds no water while the tank gauge is red is working as intended.

Nothing on this page has a price attached to it. A figure for a part or a service call that no maker document supports would be a number somebody made up, so the shape of the cost is what this section gives.

At the bottom is the work that costs nothing but a pair of gloves and an hour: clearing a clog at the pump inlet with the power off, straightening a discharge hose that has been pulled down, and reading the voltage while the pump runs. Two of the three faults on this page sit here, which is why the page puts them first.

In the middle is anything that needs a meter and a decision: a wiring problem, a control panel, or the pump itself. That is work for a technician, and what it costs depends on the fault and the diagnostic time rather than on the price of a part.

At the top is a replacement, and the reason it is at the top is not the price of the toilet. A macerating toilet sits on a discharge line and a circuit, so a replacement means both, and the maker's own servicing rule, all servicing must be carried out by an approved competent person, is a fair description of why. Water use for reference, from the manual: 0.1 to 0.7 gallons per flush, depending on the flush setting.

The gravity toilet is the other half of this subject and it is here. The tank behind both of them is covered in the tank sensor guide, and the smell that a macerating toilet can produce when its vent is blocked is in the sewer smell guide. For the maker's own documents behind this page, and the rest of the component manuals, there is a sanitation and tanks page in the manuals section.