Hi, Welcome to Just Answer, I am Clayton. I can help you with this.
The Turbo speed sensor is located on the center section of the turbo, between the exhaust section and the compressor section. It is easy to change. Should just unscrew.
Are you getting a fault code, if so what is it?
If you are throwing a Turbo Overspeed fault (code 64), then the turbo is overspinning,due to a faulty sensor or could be due to a wastegate line off(if equipped), or there is a boost leak somewhere.
Let me know, the service should not have affected anything unless they didn't put oil in or something. Or cut an O-ring on the fuel filter.