Engine Light on And Better MPG
 

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.

I guess the warning should translate to:
WARNING, your engine is not using enough fuel for an A8!!!

This is a premium car, and should have premium fuel consumption to match :ROFL:

Better by how much, and is that measured or simply what the computer is reporting?

If the O2 sensor is over-reading that may cause one bank to run lean and therefore use less fuel. Or, it may simply throw off the consumption signal being sent to the instruments.

its just what the computer is showing on the dash, before this happened the fuel gauge used to go down pretty quick, but now its not moving much at all, I didn't even think about that it would be running lean, I think I need to change the lambda sensors and see what happens

How many miles on the clock? Original O2's with high mileage (over 100,000) means they are most likely off with their readings anyway.


My take on your observations is this:

With a MAF based tune, the ECU uses MAF to estimate fuel required, then checks this against the O2 sensors as a comparison for fine tuning the trim and base marking the MAF readings. This requires that both sensors are in good condition or at least within spec.

As the O2's go bad, they can give poor O2 readings and throw the mix out which may cause increased fuel consumption at that point.
Once the ECU faults the sensor, it stops using its reading in fuel mix calculations, and might very well reset the MAF values to factory. That would explain why your fuel economy has improved.

If I'm right, once you replace the sensors with new ones, the improved fuel economy should remain, although this will be dependant on how good the MAF is, and whether it leaned out the fuel trim once the O2 sensor was faulted.