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This warning occurs when the transmission senses a fault or is missing a needed input that is required for proper operation. This warning is not specific to any one thing and can occur for many, many reasons. Anything from a simple low fluid level or failed sensor to internal wear / slippage or complete transmission failure can cause this. Even a missing input from the ABS system or a failing alternator can cause this, etc, etc, etc.
The good news is that when this warning comes on, the transmission computer logs a record of the fault, which can be read out to shed additional light on the exact nature of the issue. Thus, the only way to determine why the transmission warning is on is to have the fault memory read out using a BMW compatible scan tool. Once the fault memory is read, the problem can be identified, and the issue fixed.
Without knowing what faults are stored, all a person can do is guess at the problem – which can get very expensive, very quickly.
You can always try to unhook the battery for an hour to see if the warning will go away, but even if it does, it will only be a matter of time until it comes back – it will keep coming back until the underlying fault is resolved.
You can also reset the transmission by turning the key to position 2 (all the way without starting the car or pressing the start/stop button twice with foot OFF the brake
) and pushing the accelerator pedal to the floor for 2min. Again, the fault will likely return though until the underlying issue is addressed.
At this point, you can try the above as temporary fixes, but ultimately you will need to take the vehicle to a shop that has the proper equipment to read the fault memory so that the issue can be determined.
There is no other way to resolve this issue – the fault memory must first be read out so that the issue can be identified. Any action taken without first knowing the stored fault logs is risky guess work. Once the issue is identified with the aid of the fault logs, the problem can be corrected, the warning switched off, and normal transmission operation restored.
Please let me know if you have further questions. Please do not forget to click a positive rating as this is the only way I receive credit for my answer, thank you. Best wishes with your vehicle