I logged my STFT (Short Term Fuel Trim) on the way home using a bluetooth OBDII reader and the Dash Command Android app (would have used Torque but couldn't figure out how to log the O2 sensor data).

From what I've read your STFT shouldn't go over +10% or -10%, if it does your O2 sensor may be on the way out.
With about a 20 minute drive and 1 second interval reads my STFT measured beyond +10% 13 times with a +15.6% high and measured beyond -10% 19 times with a -15.4% low.
Anyone else logged this data or has knowledge about this with MINIs?
Do you think the O2 sensor is on the way out? Most of the times it measured over was for several seconds beyond the 10% range. (such as -10% to -15% for 6 seconds).

Throttle response has definitely gone down from what I remember. I'm right on the edge of 120k miles. Mileage is still around 30mpg.

 

Since you have a pulley....you would EXPECT the fuel trims to be compensating.....
Remember...maps are used when hard on the gas, but light acceleration and cruise uses the o2 sensor to adjust the fuel....