Looking for some expert feedback here so I can be best-informed before I bring the car back into the dealership - appreciate any of your time reading this (long):
I brought my 2012 Legacy GT in for its 90k mile service yesterday and also had them replace the timing belt while it was in there (ouch to the wallet). I've been running an AccessPort Stage 1 tune for over 7 years now without any issue whatsoever. I picked up the vehicle this afternoon, giving it to them for the multi-day job due to not having the belt in stock.
When I drove the car home today I noticed that it seemed to stumble on gear shifts, as if the throttle was coming on slow or the clutch was engaging late, causing my body to slightly lean forward from unsmooth/rookie-like shifting. I thought it might be a good idea to reset the ECU when I got home to clear any no-longer-applicable learnings, and so I did, but afterward I found that the car had a hard time turning over on the first start, which I seem to recall as a common symptom anytime the ECU is reset. However, while stationary I fluttered the throttle until it hit about 4-5k RPM, and on the way back down it almost stalled out, passing below the 1k mark, bouncing that way and repeatedly almost stalling.
I decided to try re-flashing the ECU to my Stage 1 tune (normal, not high nor low waste gate), but I found that the AccessPort repeatedly asked me to turn the key to the OFF and then back ON before pressing OK. I repeated this process several times to no avail, eventually cancelling out of the routine.
After attempting the reflash I left the AccessPort plugged in and navigated it to the Boost monitor. Surprisingly, it read -0.2 PSI at idle, and did not move when giving the car throttle. Usually I would expect some negative PSI at idle, and a bounce of some kind when giving it throttle while stationary. I decided to take the car around the block to monitor the boost levels, but found that the AccessPort read -0.2 PSI the entire way, even when I got hard on the throttle. When I did get hard on the throttle the car "felt" correct, the turbo seemed to be operating, but the throttle response still felt slow.
Pulling into the driveway and sitting idle again after going around the block, the car seemed to idle fine. However, while stationary if I gave it enough throttle to push up to 4-5k RPM and let it come back down on its own, the car would go back into almost stalling out when the needle came back down, bouncing and almost stalling again and again.
As a double check, I have an aftermarket radio that has an OBD2 connection for showing car information, including the boost level. Coincidentally, this reading also showed zero, despite feathering the throttle.
So to me it seems that either my car's boost sensor has become disconnected/damaged, or the OBD2 port has become damaged and prevents both my aftermarket radio and my AccessPort from reading it correctly. But given the weird gear shift behavior and the stumbling while idle behavior, it seems that the ECU is potentially in a kind of limp mode because it cannot receive and involve boost levels. I do not have any CELs though.
Does this seem like a reasonable assessment? Is there anything easy for me to check (MAF/MAP, etc.)?