I'm not sure that spraying WD40 will do much, except for the sensor connections which may have some corrosion on them.
I replaced the rear sensor on my Z3 for a grand total of £13.86 and it did the trick after a reset and short drive.
I was able to scope it and measure resistance to ensure it definitely was U/S and then purchased a replacement....didn't want to spend even this small amount unless I was sure it definitely was faulty.
If you can measure your sensors then if open circuit, replace it.