The button in the shifter engages/diengages the fourth gear of the transmission. When the little arrowe light is on, it means that the transmission will work through only the first three grears. Will not ruin yout transmission or engine. Do note that if you're doing alot of highway driving at high speeds you will be driving at a high RPM, ... and this is never good on an engine. The manual for the 240 states that upon a restart of the car the switch is supposed to automatically release, ie: every time you start the car it should be in 4 gear mode (no light on dash). If the light remains, and/or the switch is not engaging disengaging, ... probably an electrical thing to look into. Sorry, I can't be of any help on the actual repair. In a nutshell driving around town, under 55 mph, ... will be fine in the 3 gear mode until you an get the tranny switch rectified.