Last Updated on September 25, 2026 by Umar Farooq
A pressure washer check valve is a spring-loaded one-way valve inside the pump head — six of them on a three-plunger pump, an inlet and a discharge pair above each plunger — and when one stops seating, the machine does not simply die. It fails in one of four recognisable ways, and which way tells you which valve.
Those four, up front, because most people arrive at this page having already decided the pump is finished:
1. Pressure that never builds. Flow arrives, force does not. A discharge valve held open.
2. A pump that will not prime, or loses prime every time you release the trigger. An inlet valve held open.
3. A beat you can feel through the lance. One plunger’s stroke contributing nothing.
4. Low pressure that crept up on you across a season. Debris under a seat rather than wear.
Hardly anybody types “check valve,” which is part of why this fault is so thinly answered online. Going through what people actually write, the same six parts turn up as check valves, one-way valves, inlet and outlet valves, intake and exhaust valves, and — in the manufacturers’ own service manuals, which is where I spent most of this week — suction and delivery valves. The complaint attached to all of those is nearly always the same five words: it will not build pressure.
One redirect before we go further, because the vocabulary collides. If you came looking for the small brass fitting on the outside of the pump that dribbles hot water onto the driveway, that is a thermal relief valve, and it belongs on pressure washer relief valve — different part, different job, different failure. Check valves are internal, there are six of them, and you cannot see one without a wrench.
Six Valves, Three Plungers, One Direction of Travel
Start with why the part exists, because on this fault the mechanism does most of the diagnosis for you.
A plunger pump has no clever geometry hiding in it. Each plunger moves in and out of a cylinder, and a cylinder open at both ends will happily pull water in the front and push it back out of the front again. Everything that makes the machine a pump rather than an elaborate way to agitate water is done by two valves sitting above each plunger.
On the retract stroke the chamber pressure falls, so the inlet valve lifts off its seat and water follows the plunger in, while the discharge valve is pinned shut by the pressure standing in your hose. On the delivery stroke the two swap jobs: the inlet slams closed so the water cannot leave the way it came, and the discharge lifts and sends it down the line. Three plungers, two valves each, six valves, one direction of travel. Hold any one of them open at the wrong moment and that cylinder stops contributing.
They are small parts. Comet’s service manual describes a valve assembly as “the plastic cage, spring, valve seat, poppet and o’ring” (Comet plunger pump service manual) — five components, most of which would sit under a thumbnail, holding back 2,500 PSI a few thousand times a minute. Giant’s parts list says the same thing in a different dialect: six valve seats, six valve plates, six valve springs and six O-rings make up one valve kit for one pump (Giant MP4130/MP4135 service manual).
Two other valves get blamed for everything on this page and neither is a check valve. The unloader bolts to the outside of the pump and diverts water back to the inlet when you release the trigger — it redirects, it does not seal. The relief valve vents. A check valve only ever does one job, which is to refuse to let water travel backwards, and it does not need to break to stop doing it. It needs to be held about a hundredth of an inch off its seat. A grain of sand manages that.
Pressure That Never Builds: a Discharge Valve Held Open
Symptom one, and the one that sends most people to a search bar in the first place.
The machine starts, the pump sounds normal, water leaves the nozzle — and it leaves like a garden hose with ambition. No sting, no reach, nothing that cuts. Changing tips does not help, and neither does holding the trigger longer.
When a discharge valve cannot close, the chamber below it is no longer sealed against the high-pressure side. The plunger pushes water out on the delivery stroke, then reaches back on the return stroke and draws most of it straight back in through the valve that should have shut behind it. Water is moving. It is just moving in a small circle inside the pump head rather than down the hose. Comet’s troubleshooting guide sets “worn, dirty/blocked valves” against both “irregular pump pressure” and “drop in pressure,” and in both rows the remedy column says replace the check valves.
The tell that separates this from the usual suspects is that it ignores everything outside the machine. A blocked nozzle changes when you change the tip. A starved inlet changes when you open the tap. A valve sitting half a hair off its seat does not care about your garden hose, your tip, or your patience.
Two things it is not:
- An unloader stuck in bypass. Far more common, and it also gives you flow with no pressure. The tell is heat — in bypass the pump recirculates the same water and the head gets noticeably warm within a couple of minutes of light use. A leaking discharge valve runs comparatively cool, because most of the water is still leaving the machine.
- Pressure that builds properly and then collapses after a few seconds. That machine demonstrably sealed well enough to make pressure at all, which is the one thing a stuck-open valve cannot do, so the answer is in the supply or the unloader rather than here.
The Pump That Will Not Prime, or Loses It Every Time
Symptom two is the same failure one valve earlier in the cycle.
An inlet valve that cannot close turns the chamber into a tube that is open at the back. On the delivery stroke the plunger takes the easier route and pushes water back up the supply line instead of past the discharge valve. The chamber breathes in and out of the inlet, never develops suction, never fills, and the machine will do this cheerfully all afternoon at perfectly normal engine speed — which is why owners conclude the pump has seized when it is doing the exact opposite of seizing.
The awkward part is that a stuck inlet valve produces precisely the same nothing as a half-open tap, a blocked screen, a hose still coiled on its reel, or the spray tip left on the lance while you tried to purge. All of those are free to rule out and one of them is nearly always the answer, which is why the starting sequence lives on pressure washer won’t prime, and why that page puts the valves last. That page owns the procedure. This one owns the part.
What earns you the spanners is a specific combination and only that combination: water flows freely into the machine with the motor off, the screen is clean, the tap has been timed into a bucket, the nozzle was off the lance, the trigger was held for a full minute — and the wand still produces nothing while the pump makes no pumping note at all.
The variant worth naming separately is losing prime rather than never finding it. The machine works, you let go of the trigger to move a ladder, and when you squeeze again there is nothing there. A leaking inlet valve lets the column of water in the suction line drain backwards down the hose the moment the pump stops holding it, so every pause hands you a fresh pocket of air to clear. If that is your pattern, it is the valve rather than your technique.
A Beat Through the Lance With One Stroke Missing
Symptom three is the one you feel rather than measure.
With three plungers on one crankshaft the strokes overlap, and the delivery is smooth enough that you should feel nothing through the lance at all. Take one plunger out of the arithmetic — a valve on that cylinder not sealing, so its stroke delivers little or nothing — and you get two-thirds of the flow with a gap in it, arriving once per crankshaft revolution. At working speed that reaches your forearms as a hammer rather than as a rhythm you could count.
The symptom has its own page and the boundary is worth stating once: pressure washer pulsating owns the judder and works through all six things that cause it, from a partly blocked tip to a tired unloader. This page owns one of those six. If you have not yet ruled out the nozzle, the trapped air and the garden hose, start there — a valve is the fourth thing to suspect, not the first.
What points at a valve rather than the other five: the beat survives with the tip removed entirely, it is there on the first trigger pull of the day rather than arriving once the pump has warmed up, and it does not change when you improve the water supply. A knock that cleans itself up the moment you free a kinked inlet hose was never the valve — that is cavitation, a more urgent problem wearing very similar clothes.
Low Pressure That Arrived Slowly Is Usually Debris
Symptom four has no drama in it, which is exactly why it goes unfixed for so long.
Nothing failed on a Tuesday. The machine simply is not what it was. It takes two passes where it used to take one, the fan does not reach as far, and the owner has quietly revised their expectations downward twice without noticing either time. A valve does not have to be jammed wide open to do that. A poppet landing on a fragment of grit still seals against most of its seat and leaks past the rest, so the pump loses a slice of every stroke instead of all of it.
Every manufacturer troubleshooting table I read for this article lists the dirty version of the fault before the worn version, and gives cleaning as the first remedy rather than the fallback. Comet’s reads “worn, dirty/blocked valves.” Giant’s is more specific: against a drop in pressure and delivery it names a “fouled discharge valve,” and the remedy column says clean the valve assembly, not replace it.
Which brings me to the one opinion I will push here, and there is a number under it. On a homeowner machine, a check valve fault is almost never wear. It is dirt — and buying a valve kit before you have cleaned the old valves is paying for the expensive half of a job whose cheap half you skipped. Comet’s maintenance schedule puts “check suction/delivery valves” on a 500-hour interval. Somebody who gives their machine a solid afternoon once a month is doing fifteen to twenty hours a year, which places their first scheduled valve inspection somewhere in the late 2050s. Valves on that duty cycle do not wear out. They get something underneath them, or they perish from sitting dry through five winters, and neither of those is cured by a part number.
Valve Fault or Packing Fault? Four Ways to Tell
Valves and packings are the two things inside a pump head a homeowner can realistically change, they are sold as separate kits, and their symptoms overlap enough that people routinely order the wrong one. Four questions separate them and none of them needs a tool.
| Question | Points at valves | Points at packings |
| Is there water under the pump? | No — a leaking valve leaks internally | Yes, weeping from the seam where the head meets the crankcase |
| What does the oil look like? | Normal | Milky, with the level risen |
| Cold or warm? | The same from the first trigger pull | Fine cold, worse as it warms and the seal material softens |
| Does it beat, or just sag? | A beat you can feel | An even, general softening |
The short version: packings fail wet and valves fail dry. A packing is a seal wrapped tightly round a moving plunger, so when it goes, water escapes the pump — onto the floor, or past the plunger into the oil. A check valve seals a stationary joint inside the casting, so when it goes, the water never leaves the casting at all. A weak machine with a dry pump and clean oil is pointing at valves. A weak machine with a puddle under it or milk on the dipstick is pointing at seals.
One caveat from Comet’s own table, because it is the exception that catches people out: “water leak from head” is listed against dirty or blocked valves too. A valve cap that has been out and gone back without its O-ring, or with a cut one, will weep from the cap itself rather than from the seam. Work out where on the casting the water is appearing before you decide which kit to buy.
Clean a Pressure Washer Check Valve Before You Replace It
If you have got here honestly, the job is smaller than the diagnosis was.
Comet publishes the procedure in six steps and it is representative of most serviceable triplex pumps:
1. Take the caps off. On the AXD series that is a 17mm wrench and six caps along the manifold, three inlet and three discharge. Expect water. There is always more in there than you allowed for.
2. Look at the cap O-ring before anything else. The manual’s own second step is to “examine the valve cap o’ring for cuts or distortions. Replace if worn.” It is the part most likely to be damaged on the way out and the part most likely to go back in without a glance.
3. Draw the valve out with needle-nose pliers. The assembly normally comes out in one piece. If it comes apart in the hole, fish every fragment out rather than leaving one behind — a spring abandoned in a chamber will eventually find somewhere expensive to be.
4. Inspect all six, and this is where the job is decided. Lay them out in the order they came from. You are looking for grit trapped between poppet and seat, a spring that has lost its tension, a cracked cage, and the one finding that ends the afternoon: a seat that is pitted rather than polished. Anything merely dirty gets rinsed and refitted. Anything cratered is not a cleaning problem.
5. Seat it firmly. Comet’s wording is to press down firmly on the top of the valve assembly or riser tube with your finger. A finger. Not a socket and not a hammer.
6. Reassemble to the number rather than to feel. Comet specifies applying Loctite 243 to the valve cap and torquing it to 33 ft-lbs.
That last figure is worth having, because I went looking for it after watching three separate repair videos tighten these caps to “good and snug.” It matters in both directions. Brass caps in an aluminium manifold strip quietly when you lean on them, and a cap fitted dry and merely tight will work itself loose against the pulsation a plunger pump generates simply by existing, which is why the manual asks for threadlocker as well as a torque value. If your pump is not a Comet, the figure for yours is in your own service manual and it will not be far off.
Two things worth knowing before you order anything. A valve kit is priced per pump rather than per valve — Comet says one kit is needed for a complete valve change in one pump, and Giant’s kit contains six of each part — so there is no sensible version of this job where you fit one new valve and leave five tired ones next to it. And the whole procedure assumes a pump somebody designed to be opened. The sealed wobble-plate pumps in cheap electric machines have no caps to undo and no kit to buy; their valves are pressed in, and the pump is the part. Which one you own, and whether opening it is worth the afternoon, is the subject of repairing a pressure washer pump.
Grit Under a Seat Means Your Inlet Screen Failed First
If you opened the pump and found sand, the valve is not what failed. It is what caught the thing that failed.
There is a mesh screen in the inlet fitting behind where your garden hose screws on, and it exists for one reason: to be the part that gets dirty so the valves do not have to. A grain of sand under a poppet came through that screen, which means the screen is blocked, torn, fitted backwards, or has been sitting in a drawer since somebody cleaned it two summers ago. Fit fresh valves, leave the screen as it is, and you have bought the same fault again on a delay. Where the screen hides, what mesh it should be and how to get it out without tearing it are all on pressure washer inlet filter.
Comet’s maintenance schedule makes the relationship plain through how often it asks for each. “Check and clean inlet filter” sits on a 50-hour interval. “Check suction/delivery valves” sits on 500. The people who build the pump are telling you to touch the screen ten times for every once you open the head, and the reason is that doing the first job properly is what keeps the second one on schedule instead of on your Saturday.
There is a second route to a ruined seat and cleaning does nothing for it. A starved inlet makes the pump boil its own water, and those vapour bubbles collapse against the valve seats, removing metal in craters rather than wearing it smooth. A cratered seat leaks under a brand-new valve the moment it is torqued down. So if your valves come out looking sandblasted rather than dirty, the question has quietly stopped being which kit to buy.
FAQ
How do you know if a pressure washer check valve is bad?
By what the machine does rather than by looking at the valve. Four signatures point at one: full flow with no pressure, a pump that will not prime or loses prime at every pause, a beat through the lance that survives with the nozzle removed, and pressure that has faded gradually across a season. What they share is that none of them responds to anything outside the machine — not the tap, not the tip, not the hose.
Can you clean pressure washer check valves instead of replacing them?
Yes, and every manufacturer table I read lists cleaning first. Comet and Giant both name dirty or fouled valves as a cause of lost pressure and give cleaning the valve assembly as the remedy ahead of replacement. Rinse the poppet, the seat and the cage, check the spring still has tension, replace any O-ring that is cut or flattened, refit. A valve that is pitted rather than dirty is a different matter — nothing restores a sealing face.
What makes a pressure washer check valve stick?
Grit through a failed or missing inlet screen, mineral scale from hard water, dried detergent left in the pump after a soap job, and rubber that has gone hard sitting dry over winter. Three of those four are storage and filtration problems rather than mechanical ones, which is why valve faults cluster in the first session after a machine has been put away for months.
Where are the check valves on a pressure washer pump?
Inside the manifold — the front casting the water passes through — under a row of caps, with one inlet and one discharge valve above each plunger. On a triplex that is six caps and a spanner. On a sealed axial or wobble-plate pump they are pressed in with no caps at all, which is the design telling you it was never meant to be opened.
Is a check valve the same as an unloader valve?
No, and the confusion sends people to the wrong part constantly. Check valves are internal, there are six of them, and their only job is to stop water travelling backwards. An unloader bolts to the outside of the pump, responds to you releasing the trigger, and diverts flow back to the inlet so the pump is not fighting a closed nozzle. Both can produce flow with no pressure, which is where it starts.
Can one bad check valve stop the pump building pressure?
One discharge valve stuck fully open can cost most of your pressure on its own, because it opens a path back into a chamber the other cylinders are also feeding. A valve merely leaking past a fragment of grit usually costs a slice rather than everything, and shows up as a fade or a beat. Either way, take all six out — they have done the same hours in the same water.
Do electric pressure washers have check valves you can change?
Most have valves and almost none have valves you can change. The wobble-plate pumps in cheap electric machines are pressed together with no service caps, no parts diagram and no valve kit, so when a valve stops sealing, the replacement part is the pump. That makes the inlet screen the only maintenance on an electric machine that meaningfully protects anything.
Six Cheap Parts Guarding One Expensive Casting
The thing to hold on to is the ratio. Six valves, each a cage, a spring, a seat, a poppet and an O-ring, sold as one kit for one pump — guarding a machined manifold worth more than the rest of the machine put together, and fed by a screen that costs about as much as a coffee.
Every part of that chain is cheap except the one at the end. So take the caps off and look before you order anything, rinse whatever is only dirty, put the O-rings back properly, and torque the caps to the figure in the manual rather than to how your wrist feels about it. And whatever you find in there, go and check the screen on your way past. Something got through it.
