Engine Light on And Better MPG

[Dakes55]

Now this is a really weird one, bank 1 lambda sensor has been playing up for a while now and putting the engine check light on about every 150 miles, I have the new Bosch sensors to put on when I get time as VCDS is showing that its past its sell by date and needs replacing, I have just been re setting it for now, the weird thing is that I find hard to believe is that im getting much better MPG when the engine light is on, I would have thought that the mpg would have been worse, the car is a 2.8 v6 AMX engine, any thoughts on this.