Technician replaced control board, motor and thermistor and I still get an f40 error code

Dryer starts and then shuts down and at times it does work

We need the brand and model number to look anything up. Did he clean the dryer? It sounds like a dirt problem. If it's dirty the thermistors will not sense properly or will be erratic and they are sensitive to dirt buildup which puts a layer like concrete inside the dryer.