This is a common mistake but not usually with mechanics. Diagnostic fault codes never say to replace parts or that they're defective. They only indicate the circuit or system that needs further diagnosis. The sensor referenced in the code is the last thing to suspect after all the other possibilities have been eliminated. It is even more likely two different sensors didn't fail at the same time. Rather, something that's in common with both of them is the first suspect. That is the wiring. Your mechanic should be doing electrical tests, visual inspections, and watching the response of the sensors on a scanner.
To add to the confusion, there are dozens of fault codes related to the oxygen sensors and they get pretty specific as to the type of failure the Engine Computer detected. They can indicate power and signal wires shorted together, one sensor is reading "lean" too long, it is just responding too slowly, there's break in the heater wire, etc, and they indicate which sensor has the problem. Some of these codes refer to an electrical problem and some refer to a performance problem that usually has nothing to do with the sensor itself. The sensor is just reporting the undesired condition it is seeing.
We need to know the exact code number to know what to diagnose. Without knowing that code, replacing a sensor is likely to solve the condition that set that code perhaps 20 percent of the time.
The Engine Computer should also be the last thing to suspect but it's the first thing mechanics jump on when they don't know what else to do. The computer is just detecting the problem and reporting it to let the mechanic know which circuit has the problem so he doesn't have to manually test each one.
Thursday, October 10th, 2013 AT 12:04 PM