The first thing is to have the diagnostic fault codes read and recorded. If there's a code for a missing wheel speed signal from one of the front wheels, the repair involves replacing the wheel bearing assembly. A little normal play develops in them, but since these wheel speed sensors don't develop much of a signal to begin with, that play lets the sensor move away from the toothed tone ring enough that a solid signal isn't seen by the computer. Since speed is a factor in the strength of a generated signal, at first the problem shows up as a loss of signal as you approach a stop. False activation is the result. As the problem gets worse, no signal is generated at higher speeds. That's when the fault code sets.
It's important to understand that this is a real common problem on GM front-wheel-drive cars and it will occur to the other front bearing too. There is always a long list of conditions that must be met to set a fault code, and one of those conditions is certain other codes aren't already set. That means if the second front sensor stops generating a signal, that will not be detected. The computer compares all four wheel speeds to each other to know when something is wrong. When one sensor sets a fault code, some self-tests that use the defective circuit as a reference are suspended. In this case, a problem with the second sensor won't be detected.
All the mechanic has to go on to give you an estimate for repair is that fault code. After replacing the bearing assembly and erasing the fault code, system self-tests resume and that's when the second problem gets detected. That could happen when the mechanic takes the car on a test drive, or right after you leave the shop. Naturally you assume, incorrectly, that the problem wasn't diagnosed properly.
This can be avoided by having the first problem repaired right away. The longer you wait, the better chance there is of that second problem developing.
Monday, July 27th, 2015 AT 5:38 PM