SI = Service indicators ie your little red, yellow and green led's on the instrument panel. Inside the instrument cluster is a circuit board that among other things keeps a tab on the cars service and has batteries to power this, in case you should disconnect the main car battery. Eventually these l...

Had same prob on my 325i. Was the SI board. Bought a replacement board with batteries already soldered in. Now works fine. Leaky batteries damage the SI board.

Thanks Tez
Come to think of it i've an idea there was no nut on the thread comin thru from the fuel guage (like there was on the temp guage!!!) Back out with the instrument cluster again

On the back of the instrument cluster? Why is this? Thanks

Mine too? I recently replaced the SI board and now on start up my fuel guage reads full then drops back to the correct reading after a few minutes. It didn't do this before the new SI board??? Don't know if connected or something wrong at fuel tank end? Help....