Q: My remote won't open the driver side door.Besides batteries, what could be the problem??

I open the trunk or front passenger door to lock & unlock the car. Just replaced the remote key batteries, still doesn't work. Noticed that the drivers side door-sensor (small square) seems to have a dusty appeatance. I squatched offf some whitish powder with my fingernail. Perhaps it is corroded? The car is at the beach all the time

Mechanic Q: What test determines if there is a problem with the wheel speed sensor? ?

And what is the next step if it is not the problem?

Mechanic Q: Dash says "El Traction Support". Is this an expensive repair?.?

I'm just fix what need to be fixed to sell the car. I also have "ABS" round red light on the dash.