06 Lacrosse CX problems downshifting from 2nd to 1st


New Member
Thanks in advance for the help.

Bought an 06 Lacrosse CX in Feb 2022 with ~61k miles. Initial diagnostic revealed a "#1 solenoid" code. Not truly recognizable, I continued to drive. A few months later, after driving 15-20 minutes there began a progressive delay in downshift from 2nd to 1st and problems on highway downshifting from OD to 3rd. After 30 minutes, from a complete stop the car only starts in 2nd gear, even when manually shifting to 1st.

Brought to a local transmission repair shop. After diagnosing, they replaced all solenoids and sensors, upgraded a few OEM gears, installed a TransGo shift kit, new fluid and reset the PCM. The problem persisted but repair shop recommended keep driving for the computer to relearn driving habits. About 6,000 miles later, it continued and I brought back. They replaced the shift sensors (2nd set) added a few other things like a transmission manifold (?) but it didn't solve the problem.

A couple of months ago, after replacing the PCM, diagnostics began throwing codes pointing to the throttle body. Throttle body on 3.8L is electronic with fuel injection (built-in throttle position sensor). They replaced the throttle body with a reman unit, test drove for roughly 60 miles with no issue. Got the car back, no issues and feeling really good at this point. THEN, a few days ago and after about 400 miles, started seeing AGAIN the delayed downshift 2nd to 1st which if driven long enough won't drop down to 1st and starts in 2nd.

I love my Buick but this is killing my enjoyment of driving. Only things left I can think of are to replace the gas pedal (w/accelerator sensor) and maybe the ECM? Am I missing something else? Again, thanks and I appreciate any and all responses on this.