93 BMW 325 i gear light

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This car has an automatic transmission. ABout 3 years ago the “gear” light on the dash board started coming on intermittantly and the transmission would go into “limp home” mode (one gear only, no shifting). We had it checked, but no one could diagnose it and we were referred to the dealer. The dealer sold us a new transmission (which is probably worth twice what the car is). Shortly after, the light came on again, and the transmission went into limp home mode. We immediately took it back to the dealer (the light was on all the way in). When the car was safely parked in the dealer’s garage, we turned off the motor. When it was turned back on, the gear light was no longer on, and the transmission worked normally. The dealer was not able to find a problem and said that the codes are “reset” when the engine starts. The light has continued to come on, and the transmission continues to go into “limp home” mode. But, it always fixes itself after some period of time. I don’t think it’s the transmission itself since it did the same thing before and after replacing it. Rather, I’m suspecting some electronic control - like a transmission control module, but I’m having trouble finding out how to confirm that, order one, or replace it. Any ideas?