Oh, ok, now I understand you.. The sensor you are referring to is actually a "Scan Tool" or "Code Reader". I thought that he had already told you what was wrong and that you had already replaced a faulty "Sensor" on the vehicle. At this point you have not had it repaired, correct? Did the mechanic at least give you a list of the codes retrieved from the vehicle when he connected the scan tool?
At this point, I will not be able to help much without these codes to go from . Like the mechanic stated, there are way too many possibilities that it is not something that we can just guess at. The codes as retrieved from the vehicle will go a long way in helping us get this figured out.
Let me explain the traction control system to you so that you can understand how the system works. Basically, the traction control system is there in case the vehicle is in a scenario where acceleration traction can be lost such as in slippery road conditions such as snow, rain, ice or sand. If you happen to be accelerating over these road conditions and the drive wheels lose traction or start spinning out, the traction control will intervene and lower engine RPM and apply brakes as necessary to gain traction again. Once traction is gained, the vehicle will accelerate normally. When the light is on, it means that there has been a system malfunction that could disable the traction control in one of these traction loss events. The system is diagnosed with a scanner to help find the cause of the malfunction. The light being on only refers to a system malfunction and NOT to tell you that there is no traction between the tires and the road. Traction is still there and it is still ok to drive, it just means that if the wheels spin on slippery road conditions that the Traction Control System will not intervene to help you gain traction. In this instance, it is up to you to manually let off the accelerator and press the brake pedal to gain traction....
If you are able to obtain the codes from the Traction Control System (ABS Module) post them here and I will do my best to help you figure out what needs to be done to fix this..
Keep me posted...