My 2015 1.4 octavia with around 35k miles is running very rich, lambda 0.85. The engine is throwing an engine too lean code and The car has been into the garage 2 times now. The first time he diagnosed it as a faulty lambda sensor and once replaced this seemed to fix it for a few hundred miles then it suddenly got worse again so I took it back this time he said the engine software needed updated for the new sensor. This also seemed to fix the problem for around 500 or so miles and suddenly started running rich again around 38mpg from the usual 55mpg. I don't know what to do now. The car has also had a smoke test and passed that with no leaks. I've currently unplugged the pre cat sensor and it runs better with no stuttering in open loop mode but still not great. Already spent £600 so far so don't really want to take it back to the garage. 

Could be that the new sensor is duff, stranger things have happened. Makes sense that the car is running rich if it thinks its running lean

 

 

Worth a look

 

I had the same problem as described there, got the code and light on dashboard from time to time. Noticed this thread and  decided to throw money to my car without further investigation. B1S2 (160€) was relatively cheap compared to B1S1(450€ and already changed) so it was worth to try it for me. No more code and light and seems that I had the same problem as described in the linked thread. 

Dudes who changed my B1S2 insisted that tester showed it was working fine and it must be air leak, but changed it anyway because I told them to do so.

I tested for air leaks and found none.

However I did not notice so significant MPG change as you are describing. Because values were mostly correct and sometimes drifted. It is possible though that B1S2 is much more fubared than mine was and still give errors to B1S1.

You should test it before throwing money to it blindly. I got lucky.

Edited 20 January, 20206 yr by TonisT

I was thinking about the post cat sensor but it deosnt explain why when I unplug the pre cat sensor the car runs relativly normally, around 50 rather than 55mpg. It still doesn't make sense to me how my pre cat sensor could have failed at only 35k miles. I was also getting a too lean fault code rather than too rich code.