Q: Why does my 2003 Buick LaSabre stall when I put it in gear.

Hello. It sounds like you have an issue with the torque converter clutch being locked up. If the clutch doe not release then putting the vehicle in gear is like popping the clutch on a manual transmission which will cause it to stall. It can possibly be a computer or sensor issue, but that is not as common. If the converter is the issue then the torque converter solenoid needs to be check. If the circuit is fine then the converter itself may be failing. If you are going to have this repaired, consider YourMechanic, as a certified mechanic can come to you to [diagnose why the vehicle stalls] https://www.yourmechanic.com/services/car-is-stalling-inspection

Hi there. The symptoms lead to a dirty or failing Idle Air Control valve (IAC). The IAC regulates airflow when the throttle plate is closed and if it was to be clogged up, you will have to step on the throttle to keep the engine from stalling out. The IAC may function when the engine is cold, however, when the engine is reaching operation temperature, then valve can cause the engine to stall out. If you need further assistance with your engine stalling out in gear, then seek out a professional, such as one from Your Mechanic, to help you.