Problem solved! Yep, it was the fuel tank breather. I replaced it yesterday (3 minute job) and Volia! No more Short Term Fuel Trimming (once the O2 Lambda sensor kicks in). If I had had to bring it to BMW to fix, they would have charged a fortune just to open the bonnet!

I first tried the Air Flow Meter. No change. Luckily I was able to return it. If the breather didn't solve it, I was going to change out the Lambda sensor..... That would have been a bigger job, but no need now. I'll return that component now too.

Andrew.

Thanks for that. Definatly on the radar. I ran the engine with the fuel cap open to see if that helped. Didn't.  I am thinking I may have a faulty Lamda sensor as the high Trim condition only kicks in when it heats up and the system goes into closed loop feedback.... Still analysing the data.

We have seen a defective fuel tank breather valve cause this error. Can't guarantee that this is the case with your MINI, but might be worth a look.

Anyone have experience with this Engine Error Code? 

P0171

System Too Lean (Bank 1)

After engine start, all normal until I start driving and then stop. Engine idles high around 1.5k RPM. Starting and stopping the engine sometimes gets it back to normal idle ~500RPM. I am thinking it might be a faulty sensor? If not, anyone any ideas?

Engine is Cooper S Petrol, 2016.

Thanks,

Andrew.