Mark is right, the symbol is the Greek letter 'lambda' and refers to the oxygen sensor. I don't know about the 740 turbos but some older 240s have a reminder light which comes on when it is time to change the oxygen sensor (maybe 30,000 miles?). You should check the operation of that sensor and maybe replace it. The warning light can be reset by pushing a button on a small box attached to the back of the speedometer. Remove the cover and press the white button to reset.

The above is for the older 240s. I thought that the newer ones used the heated, 3-wire, oxygen sensor which isn't replaced on a fixed schedule but only when it failed to work. Have you looked at the 700/900 FAQs (see pull down menu at the top)?