Have 2003 bmw 525i,replaced battery now anti-theft wont let me start car..what do i do? how do i reset the code to start car

SOURCE: how do i reset the

Hello

Disconnect the positive cable from the car's battery. Hold down the brake pedal to drain the car's electrical system. This resets the car's central computer.

Put keys in ignition and turn to first position then reconnect battery. Hopefully then, you will have reset the alarm or at least be able to re sync it.

SOURCE: How do you reset the anti theft radio code in 94

First make sure that the code has not been noted somewhere within the car (underneath the ash tray, glove box , rear of sun visor etc) 
 
To see if your code is available here  for free then the radio will have to be removed from the dashboard (and if you are lucky, someone may have written the code on the casing ) .
 
If not then POST ANOTHER NEW  QUESTION


quoting the radio MAKE+MODEL plus ALL the LETTERS+NUMBERS on ANY LABELS ,BARCODES or STAMPED IN THE METAL CASING
 
 
note: Unfortunately not all radios can be decoded using the Serial Number
 
Otherwise try your Main Dealer ...........or take it into a Decode Service who will plug in a laptop to read/recode the memory chip (EEPROM). This takes a few minutes and shouldn`t be too expensive

SOURCE: I installed a new ecu in my bmw 316i, its now

it will probably have to go to a bmw dealer to be coded. your new ECU is not recognising the code in your ignition key, so it wont allow the engine to be started. you will require to produce registration docs and photo proof of ID.