the flywheel sensor measures RPM and engine timing. If it is not working then the ecu does not know where the engine is in its cycle and the ignition will probably not work at all. Im not sure if the engine would even run although JB may have tried this out!
Its is located on the block near the flywheel end of course. The manual says to remove the first section of the exaust to get to it so I guess it is under there. The picture shows ot quite low down, near the bottom of the block - only to be expected I suppose!
Part number is 46411135, seems to be the same for most cars but I`ve forgotten your car type - sorry!