Bambu Lab AMS: high humidity (level A-E or percentage), spent desiccant and wet filament

Symptom

The AMS shows high humidity (level C, D or E on the first-generation AMS, or a high percentage on the AMS 2 Pro or AMS HT), humidity rises again after drying, or the filament coming from the AMS strings, bubbles or crackles while printing.

Likely causes, from most to least likely

1. Spent desiccant: the orange indicator beads turn dark green when they no longer absorb.
2. AMS covers or seals not closing properly: a loose seal on the lower cover or a deformed upper cover.
3. Temperature changes: as it cools after drying, or with big day-night swings, relative humidity rises even though no water got in.
4. Highly hygroscopic filaments (PA, PAHT, PET-CF, TPU) stored without drying.
5. On the first-generation AMS the sensor sits on the bottom board and its heat affects the reading: that is why it shows levels A to E instead of a percentage.

Checks to tell which one it is

• Check the desiccant colour: yellow is fresh; dark green is spent.
• Check that the lower cover seal fits and the upper cover closes fully without deforming.
• If humidity rises right after drying, wait for the AMS to cool down before worrying.
• If the filament strings or crackles while printing, dry it even if the AMS shows low humidity.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Replace the desiccant

   Use Bambu Lab's official silica gel and replace it when the orange beads have changed colour. Do not use calcium chloride: once saturated it releases water and can damage the electronics.

   Check: With the AMS closed, humidity drops within a few hours.

   If that does not fix it: Check how the AMS closes.

2. Check the seals

   Seat the lower cover seal properly and check that the upper cover closes fully; if it is deformed, it will not seal.

   Check: Humidity stays low for days.

   If that does not fix it: Dry the filament.

3. Dry the filament

   In the AMS 2 Pro or AMS HT: insert the spool, choose the filament type and start drying (it cannot dry while auto-feeding). The AMS 2 Pro suits filaments dried at 65 °C or less; the AMS HT, all of them. With an X1, P1 or A2L, every AMS 2 Pro needs its own power adapter to dry, and the AMS HT always needs its own cable. Without a drying AMS, use the bed of an enclosed printer with the spool covered, or a dryer.

   Check: The filament stops crackling and stringing.

   If that does not fix it: Decide by filament type (next step).

4. Decide by filament, not by the number

   The AMS humidity tells you how the filament is stored and whether the AMS seals well; it is not the signal to dry. Highly hygroscopic filaments (PA, PAHT, PET-CF, TPU) are always dried before printing; well-stored PLA and PETG rarely need it.

   Check: You print without strings, bubbles or crackling.

   If that does not fix it: If the AMS shows a humidity sensor warning, look up that HMS code in the Bambu Lab wiki.

Prevention

• Keep unused filament in the AMS or a closed box with desiccant.
• On the first-generation AMS, from level C replace the desiccant and check the seals.
• In humid regions humidity rises quickly even after drying: this is normal.

Still not fixed?

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Printers it applies to

Bambu Lab's FDM printers: A1, A1 mini, A2L, H2C, H2D, H2D Pro, H2S, P1P, P1S, P2S, X1, X1 Carbon, X1E and X2D.

Official sources

This guide summarises the official documentation linked here:

• AMS Humidity Detection Function · Bambu Lab
• Filament Drying Recommendations · Bambu Lab