Rough idle

My vehicle idles around 500 which is causing it to stall at lights. I have tried
cleaning the throttle body and resetting ECU and this did nothing still idles at
500. I have also just noticed the water sensor may not be working properly as it
keeps dropping off to 0 when driving (brought a new one and installing it in a
few days). I am not sure if this will solve my problem but it is becoming very
annoying. I had a friend connect it up to a vag-com and he set the idle from
500 to 700 and I was getting no errors where as before it would come up as
misfiring, but it would not store the setting for some reason. Just wondering if
anyone has more of an insight to this? Would be really grateful.