SEBO Automatic X1 height adjuster wheel seems to be slipping and " clunks".

http://www.fixya.com/support/t24326835-repair_front_left_wheel_height

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Yes its repairable, but your description of the fault means it needs looking at in a workshop. If you are in the UK you can look here for this: Sebo Vacuum Cleaner Repair Service National Approved Service Repair

You should have a dial or a lever to adjust the height. It is usually on the top of the unit or under the unit. Good luck!

problem with the keeper on the pedal. Look and see if you can adjust the level on the left lower side (the pedal !)

It's likely the circuit board that controls the height adjustment motor, or the motor itself. Your local sebo service center will be better able to test to see just what is wrong. Though it wouldn't hurt to take a look at the boards sometimes they're visibly burnt, or connections can be loose. Good Luck!

The belt on the motor shaft is probably slipping off. I have the same problem and that's what I saw when I opened the vacuum. I keep putting the belt back, but it slips off again. The belts aren't supposed to stretch. I am going to call Sebo and take mine to an authorized service place to make sure that it is just a stretched belt.

Assuming that the automatic height adjuster is working properly, most likely your Sebo needs a new brush roll because the bristles on the old one are worn to the limit. To test the height adjuster, put the machine in the normal operating position (with the handle not upright) then switch on - the green "down" light may come on briefly (or may stay on if the brush is worn). Then, leaving the machine switched on, move the handle to the locked upright position - the green "up" light should come on, and you may be able to see the whole machine tilt backwards slightly as it lifts the brush off the floor. If that happens, the adjuster is working properly and you definitely need a new brush roll.
I hope this helps. If you need more guidance, just post a reply here and I'll get back to you soon. Please remember to rate this answer as I do value feedback. Good luck!

Thanks to its automatic height-adjust feature, the Sebo X4 has two drive belts, one from the motor to the height-adjust sensor, and another from that sensor to the brush roller. To access them,first remove the base from the handle, undo the four recessed screws on the underside of the base, then turn it over and lift off the cover. You will then see the belts. The rest should be obvious (it's a while since I've done one of these, but I think you may have to loosen the screws holding the height-adjust sensor before you can slip the belt off).
I hope this helps. Any further problem, just re-post here and I'll do my best. Good luck!

Hi
If the wheel isn't moving when the upright handle is swung down normally then the hydrallic action will be broken. The wheel only adjusts to the carpet when the handle is swung down and the wheel goes to the highest position. HOWEVER if you have large pile / thick carpeting then the X1 will struggle to pick this up as it doesn't cope well with thick carpeting. Sebo's G1 series with a manual adjuster dial is better in that respect. Put back the wheel as normal and check that it is lowering once the handle is unlocked. If it doesn't show up with the lights on the front of the hood showing that the wheel is going down, then its either the carpet thickness or the hydrallic wheel mechanism is broken. Contact Sebo in your country and they can usually send out a line diagram or suggest a repair.

The X1 has a servo motor with gear box, this is what adjusts for height. Either something has jammed it, or
it has gone bad. You would probably be looking at a $80 to $100 repair, that is if the servo is bad. There also is a computer controller circuit board which could be the problem.