ABS diagnostic

Sometimes not, depending on how it failed. The ABS computer watches and compares all four sensors while you're driving. When one fails, there is nothing else to compare it to. It can't rely on the rear sensors because it knows they will be going at different speeds when you go around a corner. Once the first sensor is replaced, the computer will be able to detect the other sensor is not working. The mechanic has no way of knowing that until the first one is replaced. I'm not sure which type of sensor Ford is using but if it is built into the wheel bearing assembly, GM has been using that design and they are having a huge problem with the signals dropping out from the tiny normal play that develops in them. Their junk metal bins are full of those bearing assemblies. Funny thing is those bearings would work perfectly fine on cars without anti-lock brakes. To make it worse, the same exact bearing without the sensor costs almost twice as much as the bearing with the sensor.