Just replaced the hotend on my Mk3S and ran calibration, passed the self-test. Then I did the belt test - it failed on X-axis, reported a number 243, and said "axis length". The extruder head is all the way left against the Z-axis carrier. On autohome there is a 127mm space (about 0.5").
Does the 243 mean the belt is too loose? According to the printing handbook numbers between 40 AND 240 are good, 243 is not that far out of range.
RE: X-axis and Y-axis belt tension
Adjusted X-axis belt tension a bit and now it passes self-test, However now the Y-axis is acting up. I have as mentioned the MK3s, but the Y-axis belt holder is the original fixed type (I've looked in the E-shop and don't see the adjustable belt holder - there is another belt holder LINK - but I don't see that it is adjustable).
I've loosened the front idler frame a turn or so on the screws, no joy. I returned it to tight, then loosened each screw one turn and it passed.
I ran the calibration self-test and all axes passed, bed and hot end passed as well.
I should add that the Prusa Knowledge Base was a big help!
So, I guess I solved my problem. Sometimes it helps just to write down the steps!
Throw the display #'s away, and print a tension gauge
The numbers don't mean belt tension. Ignore them. Download the prusa gcode for the belt tension gauge, print it in Prusament PETG and you will have a good tool for the job.
RE: X-axis length
Thanks Dan - just downloaded, will print soon.