Hello, I have been to a service.

P0503 †Vehicle Speed Sensor Intermittent/Erratic

There is nothing a mechanic can do with a scanner to cause a fault code. He can read them, and he can view live data to try to see what is triggering a code. Most scanners are also bidirectional, meaning they can talk back to the computers and command them to perform some of their functions. That allows you to test for proper operation and to make diagnostics checks.

Fault codes never say to replace parts. They only indicate the circuit or system that needs further diagnosis. The part referenced in the code could be defective but about half the time there is a different cause such as a corroded terminal in an electrical connector, a corroded splice, or a cut or grounded wire.

If you are not having any problems with the speedometer and no warning lights are on, the code might have been set accidentally if something was disconnected momentarily during other recent service. Under those conditions most codes erase automatically after a period of time. The computers compare a number of things to each other all the time to determine when one of them doesn't respond as expected. During testing the mechanic often causes a sensor to produce an output that is detected by a computer when the other expected signals do not arrive and that can set a code. A perfect example of that is spinning one wheel by hand while looking for bearing, brake, and tire problems. If the car has anti-lock brakes, the computer expects all four wheels to turn at the same time and same speed. That can set a code that can be ignored since it was set during an abnormal condition.