Output Speed Sensor Circuit

You purchased a new what? Did anyone diagnose the cause of the problem?

There is a misunderstanding. You do not purchase an entire circuit; you diagnose it. I suspect you purchased a speed sensor. This is a real common mistake, and your mechanic is right. In fact, I do not read that very often. What he is referring to is diagnostic fault codes never say to replace a part or that one is bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. First we have to inspect the wiring for breaks or grounded wires, and the connector terminals to see if one is corroded or stretched. Next, voltage readings will help to identify a wiring problem, but that is much easier when you have a scanner to view live data. When you pop in a new part that is referenced in a fault code, there is only about a fifty percent chance that part will solve the problem.

By replacing the part yourself, you spent a little money on a good suspect and saved much more on the mechanic's diagnostic time. Unfortunately this time it did not pan out, but you are probably still money ahead in the long run.

That depends on the mechanic's experience and training. At an auto electrical shop you will find mechanics, who are good with electrical theory. At regular repair shops, you will find mechanics who typically do not like electrical work because they are not comfortable working on something they cannot see, but every shop has their specialists. For this circuit there are only going to be two or three wires, and it is not very complicated. You also have to remember there could be a mechanical problem related to the sensor. That is where you are better off with a dealership mechanic with factory training. Older speed sensors were driven by plastic gears that could break, but the sensors were pretty simply inside. Newer ones are magnetic and are triggered by a toothed steel wheel. Much less to go wrong mechanically, but there is quite a bit of circuitry inside those sensors, so they are prone to failure.

I would have a place that can clear the adaptation of the computer so the car can relearn to it's operation.

Only German cars have this option.

Best, Ken