What causes engine to start but not idle?

Hello,

Few parts can make the vehicle idle poorly.
It could be a faulty Idle air control valve. If the IAC valve is dirty it can impact the idling.
The IAC valve is located on the back of the throttle body.
You might be able to clean it out.

https://www.2carpros.com/articles/how-a-idle-air-control-valve-works
https://www.2carpros.com/articles/idle-air-control-valve-service

Also you might want to clean out the throttle body as well.

Another possibility is the throttle position sensor. A faulty TPS can make the engine to have low/poor idle. The TPS is located on the throttle body.

I have attached diagrams for your reference.

Please let me know of any questions.

Thank you.