MK4S/MMU3: false filament sensor triggers caused by PTFE routing / buffer geometry (repro + fix)
I’d like to share a detailed write-up of an issue I’ve been troubleshooting for a while on my MK4S + MMU3. After multiple contacts we kept circling around “filament sensor acting up”, but the root cause (and fix) turned out to be a “silent” mechanical interaction between PTFE routing/buffer placement and the filament sensor behavior during repeated toolchanges.
I’m sending this both as a resolution for my case and as documentation that might help other users, because it’s easy to misdiagnose as a faulty sensor.
Hardware / setup:
Printer: Prusa MK4S
Multi-material: MMU3
Filament: PLA (also observed with other PLA spools), multi-color prints
Change made before issue started: I changed my filament/buffer setup to a different buffer and a different PTFE routing path (“flow to PTFE” changed)
Symptom
During a multi-material print, the printer intermittently behaves as if a filament sensor event occurs (e.g., “filament missing” / unexpected filament change behavior). It typically happens after the printer has already performed 2–3 toolchanges successfully.
Key observation:
- The first 1–2 toolchanges often work fine.
- The problem appears on the 2nd/3rd (or shortly after), and then becomes repeatable.
- This strongly suggests a cumulative mechanical effect rather than a random electronic failure.
When it happens (pattern)
- The issue primarily occurs during repeated unload/reload cycles (toolchanges), not necessarily during steady-state printing.
- It becomes much more likely once several toolchanges have occurred in the same print.
Root cause (what actually caused it)
- The problem was caused by the geometry of the PTFE path between spools → buffer → MMU3 → printer:
- With my “window/high placement” buffer setup, the filament path included significant height differences and multiple up/down bends (S-shaped routing).
- This created an “elastic / spring-like” behavior in the PTFE system: when MMU3 pulled filament, part of the motion was absorbed by the PTFE bends straightening out rather than translating immediately as clean filament movement at the sensor.
- Under repeated toolchanges, this effect becomes more pronounced (hysteresis / accumulated slack and friction), and the motion seen by the filament sensor becomes intermittent or delayed.
- The filament sensor/firmware then interprets this as an abnormal movement condition (appearing like a “filament missing” situation), which triggers the unwanted behavior.
Important detail:
The PTFE tube length did NOT change. Only the routing/shape (height changes and bends) changed. This implies the issue is not “too long PTFE” but “too elastic geometry (bends + height)”.
Fix (what resolved it)
- I re-arranged the buffer and PTFE routing to minimize height differences and remove the “spring” geometry:
- Moved the buffer down from a high position (window) to roughly the same height plane as the printer input (side-by-side with the printer).
- Kept the PTFE routing as direct and planar as possible:
- Avoided “down then up then down” routing.
- Reduced S-bends and large hanging loops.
- Ensured the feed into the MMU3 is straighter (less side load).
- Result: the “elastic” effect was removed and filament movement became immediate and consistent at the sensor.
Verification (evidence it’s fixed)
- After changing only the buffer placement / PTFE routing geometry, I ran a Prusa multi-material PLA test print with frequent toolchanges:
- Before: failures typically occurred around 2–3 toolchanges.
- After: print reached ~75% completion with >20 toolchanges (and continued) without the issue reproducing.
This strongly indicates the filament sensor itself was not defective; it was reacting to a mechanical feed path that caused delayed/irregular movement during toolchanges.
Why this is a “silent” failure mode
From the user’s perspective, it looks exactly like:
- a flaky filament sensor, or
- MMU3 unreliability
But the real cause is PTFE routing geometry (height changes, bends, and induced hysteresis) interacting with sensor thresholds during repeated unload/reload operations.
Users with similar “vertical” spool/buffer layouts (especially where filament travels down and back up) may hit this, while users with planar routing may never see it, which makes it hard to spot.
Suggestion (optional)
If helpful for documentation/support scripts, consider adding a troubleshooting note such as:
“When experiencing false filament sensor triggers or unexpected MMU behavior during repeated toolchanges (especially after the first few toolchanges), verify the PTFE routing geometry and buffer placement. Large height differences and S-shaped up/down routing can create an elastic ‘spring’ effect that delays filament movement at the sensor.”
If you want, I can also provide before/after photos of the filament routing to illustrate the difference.
Thanks for your help so far, and I hope this write-up is useful for future cases.
Best regards,Bjarne