P1188 — along with P1187, P1189 and P1190 — is a BMW fuel-mixture / oxygen-sensor adaptation code, not a transmission code, and that trips a lot of people up.
P1188 means the DME's mixture adaptation for bank 1 has run to the end of its range: the long-term fuel trim had to add or pull so much fuel to hold the target that it ran out of room
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