The sieve beds can be found inside the machine at the front. They are the big silver cylinders running down each side.

The sieve beds extract nitrogen from the air, leaving it enriched in oxygen. If the sieve beds have become contaminated, you should replace both of them at the same time.

Before accessing the sieve beds, make sure the machine is unplugged and switched off.

To remove the sieve beds, first detach the tubes from the bottom. Undo the screws that hold the hose clamps in place.

You can then prise off the hose from the connector on the variable synchronized unit. You should then be able to prise the hose off the bottom of the sieve bed. Carefully use a blunt object like a screwdriver head if you need to.

Next, detach the tube from the port at the top of the sieve bed.

You should be able to remove the sieve beds without undoing the ties, so you can leave them in place for later.

Starting with the left-hand sieve bed, slide it up through the ties and out through the top.

To remove the right-handed sieve bed, you may first need to remove the filter above the sieve bed. If this is in the way, remove the tubes and rotate the filter until it comes away.

You can then remove this sieve bed in the same way as the other one.

To install the first new sieve bed, slide it down through the two ties and get it roughly into position. Repeat this for the other sieve bed.

Put the valve back in place above the right-hand sieve bed. Rotate this into position until it is secure.

Next, reattach the tube to the valve.

You can then feed the tube in behind the circuit board and reconnect it to the left-hand sieve bed.

Then, reattach the tubes that connect the valve to the right-hand sieve bed.

Then reassemble the CPAP machine, turn it on, and test whether the issue is resolved.