I appreciate any help here.

I've been getting a Check Engine Light for several months.

Originally I was pulling OBD-II codes of P0133 O2S SLOW 11. As this indicated the front Oxygen I was waiting to replace it.

However after driving for around 2months this code was replaced with P0172 RICH 1.

My understanding is that this code typically is caused by a vacum leak. However if I reset the OBD-II computer this code does not immediately return. Sometimes it returns after a day and other times after a week or more (often after it will return after driving the car fairly hard).

Here is where the mystery is. When using the car code software I can make the following observations:

#1 $03 Fuel System 1 and 2 Status is System 1 Closed loop but fault with at least one oxygen sensor - may be using single oxygen sensor for fuel control

#2 $14 O2S11 SHRTFT11 does show voltage adjustments varied with short term fuel trim that appear to change as expected with idle

My question: Doesn't #2 indicate that my oxygen sensors are functioning ? Should I be tracking down my vacuum leak or replacing the O2 sensor ?

1997 850 T-5 148K miles

Thanks in advance.