The compressor is located near the bottom CPAP machine at the back. To access it, you will need the case to be open, with the machine unplugged and switched off.

Before you can access the compressor, you first need to disconnect it from the inlet resonator.

With the resonator pulled slightly out, unscrew the clip that holds the hose in place using a flat-headed screwdriver. You can then ease off the hose and rotate the resonator towards you.

Next, detach the hose from the compressor port.

Then, undo the narrow black hose by pushing in the holding ring, then pulling the hose. Slide the inlet hose out of the clip at the top and swing the resonator back towards the machine.

Now, carefully roll the machine forward and rest it on its front side. Next, you will need to remove the four nuts at the base of the CPAP’s casing. You will need to use a 10-millimeter socket spanner, or you can use a socket attachment for an electric drill or driver.

Lift the CPAP machine back into the upright position. As you do so, be careful to collect the nuts as they fall from the base.

Carefully move the tubes and cables aside to make space to remove the compressor. You can then lift the compressor up and out and place it down beside the machine. It will still be attached by a couple of cables.

Undo the two white cable connectors to detach the compressor fully.

Now, we will open up the compressor.

First, you will need to remove the cylinder head. There are eight screws to undo using a T25 screwdriver. It is important to release these in a criss-cross pattern to avoid putting too much strain on the screws.

Make a mark across the cylinder head and down over the valve plate and onto the compressor body. This will help you place everything back the right way later.

Next, tap all around the cylinder head with the handle of a screwdriver or something similar. This helps loosen the head and stop the gaskets inside from tearing. You can now ease off the cylinder head and carefully place it to one side.

Next, lift off the two valve plates and place them to one side.

Next, we will remove the piston plates. Starting with one piston, use a T25 screwdriver to undo the screw at the center. As you apply pressure on the piston plate, the piston will go down. Use your hand to put pressure on the other piston to keep the side you are working on stable.

When the screw is out, you can lift out the cylinder sleeve together with the piston plate and the piston cups.

You can put the screw back and partially secure it for safekeeping if you wish.

Then, in the same way, remove the cylinder sleeve, piston, and the piston cups on the other side.

Inspect each of the piston sleeves. The surface should be smooth and even.

Wipe the inside of each sleeve with a cloth and check for signs of wear or physical damage. Over time, the sleeve wears down and stops being airtight. If you see any of these issues, replace both piston sleeves.

Take the chance to clean the piston plates.

Next, wipe the cup seals, then inspect these for cracks and worn areas.

Here, we can see that it has become shiny and worn from friction. If you find these signs of wear, replace both seal cups.

You should also check the O-rings in the valve plates. Check whether there are ridges or cracks. Have they become flattened or hard and brittle? If either of the O-rings are damaged or worn, it is best to replace all of them at once. Remove the old ones using tweezers or a similar tool.

Inspect the other side of the valve plate. The area that is in contact with the cylinder head gasket should be smooth and even. If not, this should be replaced.

Check the gaskets in the cylinder head, too.

If either of these gaskets is cracked, flattened, or hardened, replace both of them. If you are replacing the O-rings or head gaskets, first prise out the old ones using tweezers or another pointed tool.

Place each replacement O-rings or gaskets into its groove. Ensure that it’s not twisted, then carefully press it into position.

Next, place a finger on each piston and apply some pressure. Turn the rotor to check that the pistons are moving freely up and down.

Now, you can put the cylinder sleeves back into position.

Next, press each cup seal and piston plate back together. These need to be oriented in a specific way.

The small metal pin on the bottom of the plate needs to line up with the hole on the piston.

Also, the dimple on the top side of the piston plate needs to line up with the screw on the valve plate, which will be on top of it.

Make sure the piston is in the raised position. Then carefully lower the piston cup and cup seal into place.

Place the screw in the center hole and tighten using a T25 screwdriver. As you do up the screw, ensure that the piston plate cannot be twisted.

If it can, the pin is not lined up correctly, and you should undo the screw fully and start again.

Then turn the rotor to check that the piston moves smoothly up and down. Make sure the cup seal fits snugly without a gap around it.

Place the valve plate back into position. Check again that the screw lines up with the dimple in the piston plate. Check that it is aligned properly by holding the plate down while turning the rotor. It should turn smoothly.

If there is a knocking sound on each turn, the plate needs to be aligned properly so the screw doesn’t hit the piston plate. Then raise the piston on the other side and put the piston cup and cup seal back in the same way.

Again, take care to align the pin with the hole and the screw to the dimple.

Next, put the compressor head in position. Be careful to get it the right way round by following the marks you made earlier.

Place the screws back into the corners of the compressor head. Then do up the screws almost the way in a crisscross pattern using a T25 screwdriver.

When the screws are all in, finish screwing in each of them, then fully tighten them by hand.

Check that the piston still moves smoothly by turning the rotor.

Also, place your finger over the inlet port while turning the rotor. If it is assembled correctly, you should feel suction at the port. You can now place the compressor back in the CPAP machine and reconnect the tubes and cables.

Then reassemble the casing and turn it on to check whether the issue is resolved.