Renault Megane Automatic Box Problem

Hi, I have a 2002 Renault Megane automatic. The problem I'm having is that when I drive on a long highway, a sensor activates. It's a lever that says "automatic." I took it to the mechanic, and they said it's for oil pressure regulation. Is it a sensor, or what? Where is it located? Another possibility is old oil, or maybe a clogged filter. But as far as I know, the transmission is sealed, so if that's the case, where is the filter located? Please help! I forgot to mention that when this sensor activates, if I turn the car off and on again, it goes away.

It could be two things: the solenoids in the automatic transmission
or the oil
. When the solenoids aren't working properly, they cause a loss of power
and the warning light comes on.
But first, try changing the oil.