Hi, dont go reaching for the main dealer just yet. My 155 does this every time I disconnect the battery. Firstly do you have the master key ( brownish red colour - if not you could have probs in the future)
disconnect the battery again, make sure if it fully charged, leave off car for a while to allow ecu to forget what is in its memory. before you put battery back on touch the 2 battey leads together this apparently clears the memory on ecu. NOTE DO THIS BEFORE YOU PUT THE BATTERY ON. YOU ONLY TOUCH THE BATTERY LEADS TOGETHER NOT THE BATTERY TERMINALS.
Stick battery back on, make sure the terminals are clean and tight, give em a rub with sand paper before fiting leads, give all the relays around the battery a tap. The cross fingers and try to start the car. Seems like an old wive tail but works for me.