Hi Stephen,
I am pleased to hear it is sorted for you now. Sorry I could not help you read the codes in the first instance.
People can sometimes be quick to slag off BMW dealers and will have had better experiences with XYZ, myself included. The issue is that BMW dealers and the technicians are often somewhat restricted by their own diagnostic systems and procedures. The dealer will have been using a BMW system called ISTA/D or possibly DIS which will be talking to the car and telling the technician what to do through a "test plan". The systems look quite flash but are quite restrictive and prescriptive. They will take the technician down a set path and then tell him to "replace XYZ".
It is all GUI driven and would probably run something like:
Select model > E85/E86
Perform Quick Diagnosis and Read Fault codes? > YES
DME fault found Oxygen Sensor Perform Test Plan S54 O2 Sensors? > YES
Start engine > OK
Press throttle > OK
Test Plan Completed Replace Pre Cat O2 Sensor Number 2 (shows picture with location and shows technician torque settings, etc).
When Replacement is Complete, Please Press Continue.
and so it goes on . . .
The system will not allow any diagnosis to continue and will therefore not allow the technician to proceed until he has changed the component it wants out. I guess these systems are good for trying to achieve consistency across the dealer network and helping technicians of differing experiences and knowledge work to a standard plan and script and therefore achieve consistent outcomes.
Sometimes however, perhaps in cases such as yours you want the technician to do a bit more of the thinking and diagnosis than simply following what the machine says. This is not so easy for them to do because of the restrictive system. After months and years the technicians get in the habit of just following the steps the system says for faults such as engine management / electrical. Most of the time, the system gets it right and it is all nice and easy for everyone.
Perhaps in your case, you have been a little unlucky and the car only really needed the pre cat sensor which was finally changed. If this was the case, the other one pre car sensor was likely nearly busted if not already anyway.
Perhaps the dealer should have performed a decent test drive after the first replacement but when you look at the steps I put above, they probably ran the car up at the end of the Test Plan and the system said "All OK Now".
In any case, it sounds like they have tried to look after you and at the end of the day, they fixed the issue, the car is now sorted and both the pre cat sensors are now new
It is not always easy to diagnose and find faults such as this, especially when diagnostic systems interpret information and simply say "Change X Out" without giving the technician an opportunity to really get his brain going.
