⬢ prusa-profile-tuning

Phase 1 · The machine · L5

Selftest and the health check

Before you measure a filament, make the machine stop being a variable. Your printer will run most of this check for you — if you know where it lives and what it proves.

⏱ ~20 minutes + printer time• 🖨 Hands on the machine• Phase 1
◎ Why this lesson

You have three printers and a written guide full of numbers that were never measured on them. The first thing that has to happen isn't a temperature tower — it's establishing that each machine is mechanically what it claims to be. The selftest is the printer's own hardware check, it's free, it takes ten minutes, and until it passes, nothing you measure afterwards means anything. This is the floor of the calibration order.

A selftest is not a print-quality test. It is the printer checking that its own parts are connected, moving, heating and reading as designed. It answers questions like is the hotend fan actually spinning and are the X motor's wires on the right header — the kind of fault that produces bizarre print defects you will otherwise spend a weekend attributing to filament.

Run it in three situations: now, before you calibrate anything; after any hardware change (nozzle, belt, extruder, sheet, fan); and whenever a machine starts behaving in a way you can't explain.

1Run it on your machine

The path differs by firmware family, and so does what the test covers. Open your machine — only yours.

MK3S · Marlin firmware

Path: LCD → Calibration → Selftest. It also runs automatically on first power-on or after a factory reset, as the first part of the Calibration Wizard.1

What it checks, in order1:

  1. Extruder and print fan
  2. Heatbed and hotend wiring
  3. XYZ motor wiring and function
  4. XY axis length
  5. XY belt tension
  6. Loose belt pulley
  7. Filament sensor / IR sensor

Note items 4–6: the MK3S selftest is also your belt measurement. It writes a number you'll read in Lesson 6. That's why the MK3S health check and belt check are naturally one session.

Then run, in this order:

  1. LCD → Calibration → Calibrate XYZ — measures bed skew and axis lengths and corrects them in firmware. Results are readable afterwards at LCD → Support → XYZ cal. details.2
  2. LCD → Calibration → First layer calibration — Lesson 7 covers judging it.

If it fails: stop. Don't work around it. Prusa's Selftest failed (MK3/MK3S/MK3S+) article diagnoses failure by failure. A common one — an XY axis length error — is frequently a belt tension problem rather than a broken axis.3

MK4S · Buddy firmware

Path: the full Calibration Wizard runs automatically on first power-on or after a factory reset. Afterwards, each step can be run individually from LCD → Control → Calibrations & tests.4

What's different from the MK3S:

  • There is a loadcell calibration step. This is the one manual adjustment the loadcell needs — after it, first-layer height is measured automatically before every print during mesh bed levelling, and no Live Adjust Z is required.5 Do not skip it.
  • There is no belt-tension number written by the selftest the way the MK3S writes one. Belt tension on the MK4S is adjusted by feel and by the physical procedure in Lesson 6.
  • Input Shaper is present and is normally not something you calibrate on a stock printer — but it depends on the machine being mechanically sound, so it comes after belts, not before.6

If it fails: the firmware reports a numbered error code. Search that exact number in the Prusa Knowledge Base — the KB indexes them individually, which is far faster than describing the symptom.

Core One · Buddy firmware, CoreXY, enclosed

Path: same as the MK4 family — full wizard on first power-on or factory reset, individual steps afterwards at LCD → Control → Calibrations & tests.4

What's different from the MK4S:

  • Kinematics. A CoreXY gantry can be out of square in a way a bedslinger can't, because both belts drive both axes. Squareness is a belt-tension outcome here, which makes Lesson 6 more consequential on this machine than on the other two.
  • The enclosure. Before running anything thermal, decide the door and vent state and record it. A check run with the door shut and a print run with it open are two different machines. This matters most for PLA, whose glass transition is close to achievable chamber temperatures.
  • Loadcell, as on the MK4S — same automatic first layer, same one-time selftest step.5

If it fails: see Selftest failed (CORE One/+), and check the firmware variant matches your machine — the Core One, Core One+ and Core One L have separate KB articles with different numbers in them.

2What the selftest does not prove

A pass is a floor, not a certificate. It is easy to read "Selftest OK" as "the machine is fine" and walk straight into Phase 2 with three unmeasured faults still in place.

What a passing selftest establishes, and what it leaves open.
Proved by the selftestStill unproved — and which lesson closes it
Fans spin, heaters heat, thermistors readWhether temperature is stable under load — L8
Motors move and are wired correctlyWhether belts are at the right tension — L6
Axes reach their expected length (MK3S)Whether the gantry is square (Core One) — L6
The probe respondsWhether the first layer is the right height — L7
The filament sensor triggersWhether the nozzle is the size it claims to be — you

That last row is on you and nothing automated will catch it. A worn or clone nozzle is not 0.4 mm, and every number in this course is measured relative to a nozzle that is. If you can't say with confidence what's currently installed and roughly how many kilograms have gone through it, that's a fault to resolve before Phase 2 rather than a detail.

▲ Two more prerequisites while you're here

Firmware current, on all three. Not for features — for consistency. The Prusa KB documents behaviour per firmware version, and menu paths move between releases. If a path in this course isn't where it says, check your version first.
PrusaSlicer in Expert mode, with the Configuration Wizard run so the system profiles for MK3S, MK4S and Core One are installed with the nozzle sizes you actually own. Half the settings this course touches are hidden in Simple and Advanced mode.

3The health check as a repeatable ritual

The point of Phase 1 isn't to do this once. It's to have a check you can re-run in twenty minutes, whose results go in the tuning log, so that when a machine starts misbehaving in three months you can compare against a baseline instead of speculating.

✓ The per-printer health check — run once now, and after any hardware change
Tick as you go

Three of those items are the next three lessons. Do the rest now, and leave the last three unticked — you'll come back.

4Check yourself

From memory. Question 3 reaches back to Phase 0.

Question 1 of 3
Your selftest passes but prints still show ghosting. What has the pass actually told you?
Question 2 of 3
Why does the MK4-family selftest include a loadcell step that the MK3S has no equivalent for?
Question 3 of 3
Why does the Core One's door position belong in the health check at all?
✎ Your task · do this with your tutor

Run the health check on one printer

Pick one machine — whichever you intend to calibrate first. Work down the checklist above as far as "Belt tension", which is next lesson. Then write into tuning-log.md:

  1. Printer, firmware version, date.
  2. Selftest result — and if anything failed, exactly which step and what the message said.
  3. MK3S: the skew value from Support → XYZ cal. details.
    MK4S / Core One: confirmation that the loadcell step completed, plus (Core One) the door and vent state you've decided on.
  4. Nozzle: size, and your honest estimate of how much filament has gone through it.
  5. One line: "The thing about this machine I am least confident in is ___."

→ Paste it back to me. Item 5 is the one I care about most — it usually predicts where the calibration will go wrong, and it's cheaper to investigate now than in Phase 2.

What you can now say

  1. The selftest is the floor of the calibration order — free, ten minutes, and everything downstream assumes it passed.
  2. The path differs by firmware family: Calibration → Selftest on the MK3S, Control → Calibrations & tests on the MK4S and Core One.
  3. The MK3S selftest measures belts; the Buddy machines don't — which is why Lesson 6 looks completely different on each.
  4. A pass proves connection and motion, not adjustment. Tension, squareness, first layer and thermal stability all remain open.
  5. Re-run it after every hardware change, and log the result so there's a baseline to compare against later.
📖
Primary source · read this next
Calibration — Prusa Knowledge Base
The per-model calibration hub. Find your exact printer in the model list before reading anything — the KB splits these articles by model family, and following an MK4S article on an MK3S will give you a menu path that doesn't exist.
💬 I'm your tutor — ask me anything. If a selftest step fails, paste me the exact message and the machine. Prusa's failure articles are good but terse, and half the work is deciding whether you're looking at a wiring fault, a tension problem, or a genuinely dead part. Confused beats stuck.
Linked: Glossary → selftest, skew, loadcell, PINDA, Input Shaper, CoreXY, bedslinger, glass transition temperature, calibration order, tuning log. Keep it close with → The Calibration Card.

§References

  1. Prusa Research. Selftest (MK3/MK3S/MK3S+). The Calibration → Selftest path, the seven checks in order, and that the selftest is the first part of the Calibration Wizard. Link.
  2. Prusa Research. XYZ calibration details. Detailed XYZ calibration results are readable from Support → XYZ cal. details from firmware 3.0.12. Link.
  3. Prusa Research. Check Axis Length X/Y/Z (MK3S). Axis-length errors are commonly caused by belt tension being slightly too tight or too loose. Link.
  4. Prusa Research. Selftest failed (CORE One/+). After initial calibration, each selftest step can be started from Control → Calibrations & tests. Link.
  5. Prusa Research. Loadcell (MK4/S, MK3.9/S, XL). The only loadcell adjustment needed is during the selftest; afterwards first layer calibration happens automatically during mesh bed levelling. Link.
  6. Prusa Research. Input Shaper. Calibration is usually unnecessary on a stock printer, and belt adjustment, nozzle inspection and debris removal are recommended beforehand. Link.

Lesson L5 of the prusa-profile-tuning course · all lessons · The Calibration Card · glossary