The fans in the Mistralton City gym of Black 2 will probably cause much grief
for Twitch Plays Pokemon.

The fans have three speeds:
- Slow: runs for 120 frames (2 seconds)
- Medium: louder, runs for 120 frames (another 2 seconds)
- Fast: sends unprotected player flying back
Thus the unmodified fan interval is 240 frames (4 seconds).

There is a counter which increments once every 2 frames, located at 0231B3F0.
It is 16 bit. That memory location does appear to be used for other things.

Here is the function which determines fan speed:
021E:F184  2878  CMP R0, #78
021E:F186  D301  BCC #21EF18C
021E:F188  2002  MOV R0, #2
021E:F18A  4770  BX LR
021E:F18C  283C  CMP R0, #3C
021E:F18E  D301  BCC #21EF194
021E:F190  2001  MOV R0, #1
021E:F192  4770  BX LR
021E:F194  2000  MOV R0, #0
021E:F196  4770  BX LR

This function returns 0 (corresponds to low fan speed), 1 (corresponds
to medium fan speed) or 2 (corresponds to high fan speed) depending on its
argument. The thresholds are 0x3c (60) and 0x78 (120).

The time spent at each speed can be modified by adjusting the thresholds.
Unfourtunately, Thumb-mode instruction encoding doesn't allow immediate
operands to be higher than 0xff (8.5 seconds), which may not be enough.
Here is a replacement function I came up with (note: function must be less
than 20 bytes):

021E:F184  XXXX  LSR R0, R0, #YY
021E:F186  2802  CMP R0, #2
021E:F188  D300  BCC #21EF18C
021E:F18A  2002  MOV R0, #2
021E:F18C  4770  BX LR

This uses shifts to divide, and clamps the result to 2. The values
for XXXX and YY are:
- XXXX = 0940, YY = 06: fan interval = 128 frames (2.13 seconds)
- XXXX = 0980, YY = 07: fan interval = 256 frames (4.27 seconds ~ original)
- XXXX = 09C0, YY = 08: fan interval = 512 frames (8.53 seconds)
- XXXX = 0A00, YY = 09: fan interval = 1024 frames (17.1 seconds)
- XXXX = 0A40, YY = 10: fan interval = 2048 frames (34.1 seconds)
- XXXX = 0A80, YY = 11: fan interval = 4096 frames (68.3 seconds)
- XXXX = 0AC0, YY = 12: fan interval = 8192 frames (137 seconds)

Here is an action replay code for this:

521EF174 63206B60
121EF184 0000XXXX
121EF186 00002802
121EF188 0000D300
121EF18A 00002002
121EF18C 00004770
D0000000 00000000

The "521EF174 63206B60" and "D0000000 00000000" make this code conditional.
This is required because Pokemon Black 2 uses dynamically-loaded code
overlays, and unconditional writing will cause the game to freeze in battles.
The "63206B60" is unrelated code from the same overlay.