My car stalls when put into gear and sometimes while starting to drive

It really sounds like a vacuum leak. How did you check for a vacuum leak?

Code 44 is a "lean exhaust indication" meaning either the o2 sensor is stuck lean or there is a vacuum leak causing the fuel mixture to be too lean or in other words too much air to fuel ratio. One way I check for vacuum leaks is while the engine is running spray starting fluid on all the intake gaskets. Meaning the throttle body gasket, intake gasket and spray briefly listening for an increase in engine rpm. Another way is with a smoker tool. This tool puts out smoke and when you seal it off on the throttle body it fills the intake with smoke and any smoke leaking out of a gasket or vacuum hose will be visible.
Inspect all the vacuum hoses and gaskets for issues. Spraying with starting fluid (aerosol can) is probably your best bet.