03 Cadillac CTS Stalling

I have had my 2003 Cadillac CTS for just over a year, and I’m at around 115k miles. Suddenly the check engine light came on & it began stalling randomly a few days ago. One thing of note: last time it stalled on me, right before it stalled, I had been doing a steady 35mph and suddenly felt a hard downshift (it’s an automatic), then lost all power to the accelerator for a few seconds.

Today I had the crankshaft position sensor replaced, which according to my research (aka Googling) is a common problem. That didn’t cure the stalling issue, and the check engine light is still on.

A local mechanic tells me there is an O2 sensor shorted out, and shift solenoid C is stuck. Friends tell me to have the fuel system checked out.

Ideas before I spend a wad of hard-earned cash on nothing? Thank you!