A dishwasher not draining is one of the fastest ways to bring a service to a standstill. Racks back up on the outlet bench, the tank sits full of grey water, and every cycle after that washes plates in yesterday's soil. The good news: in a commercial kitchen, most drainage failures trace back to a handful of predictable causes — and several can be found and fixed by an operator in under fifteen minutes, without tools or a callout fee. This guide works through the likely culprits in order, from blocked scrap trays to a failed dishwasher drain pump, covers the checks that are safe to do yourself, and is honest about where the do-it-yourself line ends and a licensed technician begins. Along the way you'll learn how to unclog a dishwasher that won't drain and which DIY fixes are genuinely operator-safe. It applies to underbench, pass-through and rack-conveyor machines alike — the full range is in our commercial dishwashers collection if you end up needing a replacement rather than a repair.
Safety First: Isolate Before You Investigate
Before you touch anything inside the machine, isolate it properly. Commercial dishwashers combine 240V or 415V power, water at pressure, and rinse temperatures in the 80–90°C range — a bad combination for improvised troubleshooting.
- Switch off and isolate power. Turn the machine off at the panel, then switch off the isolator or unplug it. Hard-wired three-phase machines have a dedicated isolation switch.
- Close the water supply valve. Usually under or behind the machine.
- Let the tank water cool. Wash tanks typically run at 60°C or more — give it time, and wear rubber gloves regardless.
- Know your boundary. Anything involving wiring, the pump motor, heating elements, solenoid valves or fixed drainage plumbing is work for a licensed electrician, plumber or commercial appliance technician. In Australia, DIY electrical and most plumbing work is illegal as well as dangerous — and it will void your warranty. The checks below stop short of that line.
Why Is Your Dishwasher Not Draining? Six Common Causes
When a commercial dishwasher isn't draining, the fault is rarely exotic. If you open the door to standing water in the bottom of the dishwasher after a cycle, work through these in order — they run from most likely (and easiest to fix) to least likely. In our experience, the majority of "broken machine" calls turn out to be cause one or two.
| Symptom | Likely cause | Action |
|---|---|---|
| Water pools in the tank; drains slowly or not at all | Blocked scrap trays / tank filters | Remove, empty and rinse all filters; clear the sump inlet |
| Machine hums at drain stage but water level barely drops | Kinked, crushed or blocked drain hose | Inspect hose run behind/under machine; straighten kinks, check for internal blockage |
| No drain sound at all, or a laboured buzz then silence | Drain pump jammed or failed | Check impeller access point for debris (glass, cutlery, cable ties); if clear, call a technician |
| Water drains, then seeps or gurgles back into the tank | Air gap / air break fault, or hose sitting too low | Check the hose loop height and air break fitting; re-seat the hose |
| Machine drains fine but the sink or floor waste backs up | Blocked sink standpipe, floor waste or grease trap downstream | Test the sink drain independently; check grease trap service records |
| "Standing water" but only in patches; poor wash results | Wash-arm debris masking as a drainage fault | Remove and rinse wash/rinse arms; clear the jets |
1. Blocked scrap trays and tank filters
The single most common cause of a commercial dishwasher not draining is also the simplest — a clogged dishwasher filter. The scrap trays and tank filters get loaded with food solids, napkin fragments and glass chips. Once the filter mesh blinds over, water can't reach the sump fast enough for the drain pump to do its job — and whatever bypasses the filter ends up in the pump itself. Pull every filter (most machines have a coarse scrap tray plus a finer sump filter beneath it), empty and rinse them, and check the exposed sump inlet for anything sitting over it before refitting.
2. Drain hose kinks and blockages
The dishwasher drain hose is the flexible line running from the machine to the standpipe or waste connection. Two things go wrong with it. First, it gets kinked or crushed — machines get pushed hard against the wall during cleaning, and a flattened hose throttles flow to a trickle, so water can't drain properly. Second, it blocks internally: grease, rice and detergent scale build up over months, particularly along low, sagging sections where water sits between cycles. Slide the machine forward enough to sight the full hose run, straighten any kinks, and confirm the hose rises and falls the way the installer left it. A hardened or caked hose is a cheap replacement — but disconnecting fixed plumbing is a job for your plumber.
3. Drain pump problems
Most commercial machines use a powered drain pump rather than gravity drainage. If the pump's impeller jams — glass shards, a stray teaspoon, a cable tie — the motor hums but moves no water, and many machines then display a drain error code. Some machines allow impeller inspection via the sump once filters are out; if you can see and safely remove an obstruction with the machine fully isolated, do so. If the impeller is clear and the pump still won't run, the motor, capacitor or control board is at fault — technician territory, full stop. A failed dishwasher drain pump is a common, well-priced repair on an otherwise healthy machine.
4. Air gap and air break faults
Commercial installations use an air gap or air break to stop waste water siphoning back into the machine — a backflow protection requirement under Australian plumbing standards. If the drain hose has slipped down the standpipe, lost its high loop, or the air break fitting has clogged with grease, you'll see water drain and then come flowing back into the dishwasher, often with a gurgle. Re-seat the hose at the correct height, not so far into the standpipe that it sits below the water line. If the air break device itself is blocked or damaged, have your plumber service it — it protects your potable water supply.
5. Blocked sink, standpipe or grease trap downstream
Sometimes the machine is draining perfectly — into a drain that can't take the water. If the adjacent sink is slow, the standpipe overflows during the drain cycle, or the floor waste bubbles, the blockage is downstream of the dishwasher. The usual suspect is the grease trap (more on that below), but a fat-choked standpipe or branch line does the same thing. Quick test: tip a bucket of water down the sink the dishwasher shares a drain with — if that backs up too, stop blaming the machine.
6. Wash-arm debris masquerading as a drainage fault
A surprisingly common false alarm: staff report "water left in the machine" when the real issue is blocked wash and rinse arms dumping water unevenly, leaving pooled patches, dirty plates and detergent residue. Most machines hold some water in the sump between cycles, so a little residual water at the bottom of the tub is normal. Pull the wash and rinse arms (they usually unclip without tools), rinse the jets clear, spin them by hand to check they rotate freely, and refit. If wash results improve and the tank still empties at end of day, there was never a drainage fault at all.
Step-by-Step: Troubleshoot the Drainage Problem Safely
Here's the sequence as a single operator-safe routine — the cheapest way to fix a dishwasher that won't drain, before anyone pays for a callout. Total time: 10–20 minutes.
- Isolate. Power off at the isolator, water valve closed, tank water cooled. Gloves on.
- Empty and rinse every filter. Scrap trays, sump filters, and any pump inlet screen. Check the sump for glass, cutlery and packaging fragments.
- Check the wash and rinse arms. Remove, rinse the jets, spin to confirm free rotation, refit.
- Sight the drain hose. Pull the machine forward if needed. Straighten kinks, confirm the high loop, check the standpipe connection is seated correctly.
- Test the drain path downstream. Run water down the shared sink; watch the floor waste. If it backs up, the problem is the drain, not the dishwasher.
- Restore and run a test cycle. Power and water back on, run an empty cycle, and watch the drain stage: listen for the pump, watch the water level fall, and check nothing returns to the tank afterwards.
- Dishwasher still won't drain? Stop. Note any error code, photograph the sump and hose run, and call your service technician with those details — they'll often arrive with the right part.
One habit to avoid: pouring caustic drain cleaner into the tank or sump to unclog the dishwasher drain. Caustic products attack pump seals and hoses and linger into the next wash cycle. If a line is truly blocked, mechanical clearing by a plumber is the right fix.
How to Remove and Clean Your Dishwasher Filter Properly
Because a blinded filter is the leading cause of dishwasher drainage problems, it's worth doing properly rather than a quick shake over the bin. With the machine isolated, open the dishwasher, lift out the scrap trays, then remove the dishwasher filter assembly from the sump — on most machines the whole set lifts out without tools. Rinse each dishwasher filter by hand, working detergent scale and food particles out of the mesh; an old toothbrush is ideal for the fine screen, where hardened grease restricts water flow before the mesh looks blocked. Wipe out the sump and the inside of your dishwasher so nothing washes straight back in, then refit firmly. On a busy line, cleaning the dishwasher filter every service period is the difference between clean dishes all night and a mid-service standing water problem.
Dishwasher Not Draining After Every Check? It's Probably the Grease Trap
If the dishwasher is not draining even though it passes every internal check, look downstream. In most Australian commercial kitchens, the dishwasher discharges through a grease trap (grease arrestor) required by the local water authority as a condition of the trade waste agreement. When the trap is overdue for pump-out, the accumulated fat, oil and grease leave no working capacity — and hot, high-volume dishwasher water is precisely what a full trap can't absorb. The tell-tale pattern is a machine that drains fine first thing in the morning but backs up during peak service, when the trap is dealing with sinks, floor wastes and the dishwasher at once.
Check the service sticker or your trade waste records for the last pump-out date. Most venues sit on a schedule set by their water authority, and a busy kitchen can outrun it. If the trap is due or overdue, book the pump-out before you book an appliance technician — it's the cheaper first move and very often the actual fix. Grease trap servicing must be done by a licensed liquid waste contractor.
How the Dishwasher Connects to the Drain: Disposals, Sink Drains and Drain Valves
Knowing the parts of a dishwasher's drain path makes troubleshooting faster. All commercial dishwashers have a drain route out of the tank: most use a powered pump, while some door-type machines drain by gravity through a drain valve — if that valve seizes or fouls with debris, the water stays put. From there, the drain hose from the dishwasher connects to a standpipe, a sink drain tailpiece or a floor waste. Where the wash-up sink has a food waste disposer, the dishwasher may share that drain line — a garbage disposal packed with scraps will keep water from draining out of the machine entirely. Try running the disposal with cold water flowing before you condemn the dishwasher. Wherever it discharges, the machine can only drain as fast as the drain it connects to.
Dishwasher Still Not Draining? When It's Time to Call a Professional
If the dishwasher is still not draining after every check above, the remaining causes sit past the point of safe DIY repair. One last thing is worth trying: resetting your dishwasher — switch it off at the isolator for a couple of minutes and power back up; a latched drain error may clear, fixing the issue outright. Beyond that, a faulty drain pump, a control fault or a clogged drain line inside the wall needs a licensed technician or plumber to diagnose and repair. A tech who deals with these dishwasher issues daily will usually sort the fault in one visit — and if it sits in the plumbing rather than the dishwasher, you've saved an appliance callout fee. Note what you checked, any error codes, and when the water from your dishwasher backs up.
How to Prevent Dishwasher Clogs Before They Start
Nearly all of these draining issues are preventable with a routine your team can run without thinking. Build these into your opening and closing checklists:
- Scrape before you rack — every plate, every time. The scrap bin and pre-rinse spray exist so food particles and solids never enter the tank. This is the single highest-leverage habit, and a properly set up dishwasher inlet sink and outlet bench with a scrap hole and pre-rinse unit makes it the path of least resistance rather than a chore.
- Empty scrap trays through service, not just at close. On a busy night, filters blind over long before the last docket.
- Drain the tank and clean the dishwasher filters at end of day. Never leave the tank full overnight — the full closing sequence is covered in our guide to what should be done with the commercial dishwasher at the end of the day.
- Run a hot flush weekly. Running hot water through an empty cycle at full temperature (use the self-clean function if fitted — a domestic dishwasher cleaning tablet is no substitute) helps move grease out of the machine and hose before it settles.
- Descale on a schedule. In harder-water areas especially, scale narrows jets, hoses and pump passages. Use a manufacturer-approved descaler at the recommended interval, or have it done at a service visit. Our full commercial dishwasher cleaning guide covers the daily and weekly detail.
- Keep the grease trap on schedule. Put the pump-out date on the same calendar as your pest control and fire equipment checks.
Repair or Replace? Making the Call on an Ageing Machine
A jammed pump or blocked hose on a five-year-old machine is a repair, no question. But if you're clearing the same fault monthly, the machine is eight-plus years old, parts are hard to source, or the repair quote is heading past roughly half the cost of a new dishwasher, the economics flip. Older machines also cost you twice — in callouts, and in water and energy, because modern warewashers rinse with far less fresh water per cycle. Factor in the revenue lost every time the dishwasher won't drain mid-service, and a planned replacement usually beats a forced one.
If you're weighing it up, these are two examples of current-generation machines where the drainage hardware is built in rather than bolted on:
For smaller venues, compare the underbench dishwasher range; high-throughput kitchens working rack-to-rack should look at pass-through dishwashers instead.
Frequently Asked Questions
Why is my commercial dishwasher not draining?
The most common causes of a dishwasher that refuses to drain are, in order of likelihood, blocked scrap trays and tank filters, a kinked or blocked drain hose, a jammed or failed drain pump, an air gap or air break fault, and a blockage downstream in the sink line or grease trap. Start with the filters — they account for the majority of drainage callouts.
How do I unclog a commercial dishwasher drain safely?
Isolate power and water and let the tank cool, then remove and rinse all scrap trays and filters, clear the sump inlet, and check the drain hose for kinks or internal build-up. Avoid caustic drain cleaners inside the machine — they damage pump seals and hoses. If a fixed drain line is blocked, have a licensed plumber clear it mechanically.
How do I know if the dishwasher drain pump has failed?
Listen at the drain stage of the cycle. A healthy pump makes a steady running sound and the water level drops quickly. A jammed pump typically hums or buzzes without moving water, and many machines show a drain error code. If the impeller area is clear of debris and the pump still won't move water, the pump or its controls have failed and a technician is needed.
Can a blocked grease trap stop a dishwasher from draining?
Yes. If the grease trap downstream is full, dishwasher discharge has nowhere to go and backs up into the machine, sink or floor waste. A machine that drains fine early in the day but backs up during peak service points strongly to the grease trap. Check the last pump-out date and book a licensed liquid waste contractor before paying for an appliance callout.
When should I call a technician instead of fixing it myself?
Call a licensed technician for anything beyond filters, wash arms, visible hose kinks and accessible debris — that includes pump motors, wiring, solenoid valves, heating elements, control boards and any work on fixed plumbing. In Australia, electrical and most plumbing work legally requires a licensed tradesperson, and unauthorised repairs can void your machine's warranty.
Why is there still water in my dishwasher after the cycle ends?
A small pool in the bottom of your dishwasher between cycles is normal — the sump holds some water by design. A rising level, grey water covering the tank floor, or water returning after the drain stage is not. If the level grows through service, start with the filters and drain hose.
How often should I clean the filter in a commercial dishwasher?
Empty the scrap trays and rinse the filters during every service period, then take the full filter set out for a thorough clean when you drain the tank at close. Cleaning little and often keeps food solids out of the drain pump and keeps the machine draining properly.
Keep Your Dishwasher Running Smoothly
Most drainage faults are solved with a rinsed filter, a straightened hose or an overdue grease trap pump-out — and the rest are a straightforward job for a licensed technician. If the maths on an ageing machine no longer stacks up, browse the full commercial dishwashers range, or contact our team for spec-matched advice on the right underbench or pass-through unit for your venue.
