You may go for somethink like this:
(c)koda Felicia - praktyczne porady użytkownika
This is in Polsh so if you have any trouble with google translate it just let me know.
Here is a schematics for this unit (fairy simple)
The operation of this is simple 3 LEDs are signaling Lean, Normal and Rich mode. So I think it suits the need to know when engine is working in lean burn mode best.