PROM = Programmable Read Only Memory
*type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration tables or device configuration.
*Failure modes...
There are two limitations of stored information; endurance, and data retention.
During rewrites, the gate oxide in the floating-gate transistors gradually accumulates trapped electrons. The electric field of the trapped electrons adds to the electrons in the floating gate, lowering the window between threshold voltages for zeros vs ones. After sufficient number of rewrite cycles, the difference becomes too small to be recognizable, the cell is stuck in programmed state, and endurance failure occurs. The manufacturers usually specify minimal number of rewrites being 106 or more.
During storage, the electrons injected into the floating gate may drift through the insulator, especially at increased temperature, and cause charge loss, reverting the cell into erased state. The manufacturers usually guarantee data retention of 10 years or more.
thats all i know for now...try resetting the OBC, step 21:
http://www.bmwclubmalaysia.com/forums/showthread.php?t=20484
good luck...
