What is the ECM mean?

sounds like Electronic Control Module~ surely stores the trouble codes~ if it's grounded properly will remember everything~ as far as the fuel guage...this is a low technology affair as you've got a float and potentiometer...this mechanical system will fail...they all do~ hardly think a lazer pointer would be appropriate under these circumstances~ may be a co-incidence this battery change/incorrect level...as it takes two devices...the float in the tank...and the variable resistor...both mechanical devices~ if this technology has left me behind, I've a lot to learn~ so yes probably in the tank~ so sorry for the gas fumes that your mechanic will be huffin~ perhaps the fuel pump/float assembly can be removed with one ring above the tank...most are like this...easily remove the back seat and turn the keeper ring~ you may end up doin' that~

Try disconnecting the battery and reconnecting. It may or may not fix it.