Solenoid valve faults

It's 1am. The gripper on the palletiser won't close. The PLC output LED is on. The cylinder hasn't moved. The operator says it was "a bit slow" earlier, then it stopped.

Someone gives the valve a tap with a screwdriver handle. It shifts. Everyone cheers, and the line runs for an hour before it does it again.

A solenoid valve sits right on the line between electrical and mechanical. That's why it gets passed back and forth between the sparky and the fitter. Split the fault there first and you'll know whose job it is.

Split it at the coil

Use the half-split method. The coil is the halfway point.

Is the signal getting to the coil? Most valve plugs have an LED. If it's lit, the PLC output and the wiring are probably fine. If the PLC output is on but the plug LED isn't, the fault is between them. Look for a blown fuse, a broken core or a loose terminal. Our guide to PLC inputs and outputs covers that side.

If the plug is lit, check the coil. With the supply isolated and the plug off, measure the coil resistance and compare it with a good one of the same type. Open circuit means a burnt coil. Look for melted plastic and a burnt smell. Then check it's the right coil. A 24V DC coil and a 110V AC coil can look the same from the outside. Fit the wrong one and it either won't pull in or it will cook.

If you need to prove a voltage on a live coil, that's live work. HSE counts using a test instrument to measure voltage on a live control system as live work (HSG85, paragraph 3)*. Plan it, use proper probes, and don't do it alone on a mains-voltage coil.

Try the manual override, carefully

Most valves have a manual override button or screw. Pushing it tells you a lot. If the valve shifts and the cylinder moves on the override, the air side is working and the fault is the coil or the signal. If nothing moves on the override, the fault is on the air side.

But the override moves the machine. Treat it like any other start. Make sure the area is clear, nobody is inside the guard and you know which way the cylinder will go. Never leave an override locked in. Someone will forget it's there.

No movement on the override

Now you're on the air side. Start with the pilot. Most solenoid valves on machines are pilot-operated. The solenoid only moves a small pilot, and line pressure does the work of shifting the main spool. If the pressure at the valve is too low, the pilot can't shift it. Check the valve's minimum operating pressure on its label or datasheet against the gauge on the machine.

Then the spool. Dirty or wet air is the usual cause of a sticking spool. HSE's compressed air guidance lists dirty or "wet" air as something that can lead to corrosion and blocked valves (HSG39, paragraph 2)*. Water, rust from old pipework and varnish from the wrong oil all build up on the spool. A valve that needs a tap to shift is telling you this. Our guide to air preparation covers the filters and drains that should be stopping it.

Last, the exhaust. A blocked exhaust silencer can stop a valve from venting the cylinder, so it moves slowly or not at all on the return. Take the silencer off and try again.

A valve that hisses

A steady hiss from a valve exhaust, with the cylinder held still, means air is getting past a seal. It's either in the valve or the cylinder.

To tell which, you need to break into the circuit. Do that only with the air isolated and vented. HSE's guidance says pressurised components should be isolated from all pressure sources and completely vented to atmosphere before anyone dismantles them (HSG39, paragraph 169)*. Prop any load that could fall. Our guide to a sticking or slow cylinder explains how stored air can move a cylinder when you crack a fitting.

Replace like for like

This is where a quick fix can turn into a dangerous one.

Valves aren't all the same, even when they fit the same base. Some are spring return, some stay where they were last put. HSE's guidance says actuators that must go to a set position if the supply fails should be controlled by a spring return valve that goes to the safe position (HSG39, paragraph 114)*. Swap that for a valve that holds its last position and the machine behaves differently on a power loss or an e-stop.

Three-position valves are the same. The centre position might vent the outlets or seal everything. HSG39 warns that an all-ports-sealed centre can leave stored energy in the actuator, leading to unintended movement if piping is disconnected (HSG39, paragraph 128)*. Check the part number and the symbol on the valve against the circuit drawing before you fit anything.

Seal kits bring the same risk. HSE warns that kits often hold seals for a range of valves, and that incorrect seal replacement can completely alter the valve's function (HSG39, paragraph 170)*. Test a rebuilt valve on the bench before it goes back on the machine.

The cost of the tap

Here's our view. Tapping a valve to get it going is fine to keep the line moving for an hour. It isn't a fix. If the spool is sticking, it'll stick again, probably at a worse time. Swap the valve or clean it, then find out why the air was dirty.

The bigger cost is the wrong part. A night shift fitting "near enough" from the stores can change how a machine stops. That's not something you want to find out when someone's hand is in the gripper. If nobody on shift can read the circuit drawing and match the valve, wait for someone who can. Reset and run is cheaper for an hour and dearer for a month.

What to do on Monday

1. Label each valve on your problem machines to match the circuit drawing, and put a copy of the drawing in the panel.
2. Hold spare coils for each voltage you use, and mark the voltage on the stores bin.
3. Check your spare valves match what's fitted, including spring return or detented and the centre position.
4. Find out which machines have valves that need a tap to shift, and look at the air feeding them.

* Paragraph numbers correct as of 30 October 2026 and refer to HSG39, second edition (1998), and HSG85, third edition (2013). Check HSE's HSG39 page for the latest.