Ah yes, trying to convince the sales man at a dealership who knows less about what he's trying to talk about than my dear old grandma...
It's incredibly frustrating, modern "mechanics" literally read a number off a screen and if the value has a green light next to it then there is nothing wrong at all. I spent a very long time trying to explain how a) sensors can go faulty and b) they have large tolerances themselves to allow for their inaccuracies recently with VW when trying to diagnose a hesitation on my Passat. They were having none of it, there was no fault code logged, ergo there was no problem, even when I recreated it for their "master tech" while out on a drive, I was told my foot probably slipped on the accelerator.
In the end I figured out the symptoms were very much like a MAF sensor failure, and figured that it could either be the MAF OR the air going in to the MAF. The former is much cheaper to fix, so I checked the air filter which hadn't been changed in 50,000 miles (full VWSH, this is actually correct, they dont change it until 5 years or 60k miles...), £15 later perfectly smooth engine.
All I can say is you are literally speaking to a brick wall if BMW are anything like VW, they are taught that if the completely fallible, mass produced, sensors in their cars say everything is OK then it is, even if the car is on fire in front of their eyes.
All I can suggest is you take it right to the top at BMW and hope they filter down to the dealership to stop being pants, or you bite the bullet and go to an independent and go back to BMW with their findings (they will ignore you and say there was never a problem).
It is incredibly frustrating to be a slightly knowledgeable customer these days...
I recently went back as I suspect the glow plugs are failing (takes 3 burns on a cold morning to get a stutter/smoke free start), this is perfectly normal, they normally go at around 60k I find, but of course they plugged it in, charged me £60 to give me a print-out saying "it's all OK". I even left it overnight and went back the next morning and started it for them, showing the chugging and plumes of black smoke, they just stared and said "yeah that's what they do when cold at that age", I could have literally screamed I was so frustrated. Unfortunately VW bury the plugs under the head cover so a (very) amateur mech like me cant get to them, they would charge £200 to replace all 4 (£80+ labour) and would offer me no guarantee on the work as "they have deemed it was unnecessary so the problem, if there was one, would be something else). Any simple backstreet mechanic would agree with my diagnosis as the more you burn the plugs the smoother the start, they wouldn't even do a simple check of the voltages for me (need to take the cam cover off so I can't do it). In the end I've given up and left it, the cost is too high for me to do it as long as I am trying to keep a full VWSH, I'll just wait till the damn thing finally gives me a glow-plug fault and forces me to limp to them like an idiot.
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