First check all four with ohmmeter, resistance must be around 1-1,5kom. If all four has normal resistance, so all four is fine. If all four is fine, then there is two ways how to find the one [or two or whatever
] which gives too weak signal - first - poor man's way - by removing hubs and checking distance, second - lazy man's way - with oscilloscope.
My A90q had all four sensors in working condition, but only one provided correct signal and one weak signal, I don't know how it is possible because on early cars they are secured with bolts
