These idiots worked on a 2000 ford focus about 3 years ago that had a check engine code for downdraft O2 sensor. They replaced the O2 sensor, and it did not correct the problem. After $300, their "diagnosis" was that the computer was bad. (Forget the fact that the car started and RAN!)
I just found that someone had pulled the PCV valve out of the valve cover and TAPED IT OFF. They also pulled the REF port hose from the EGR Pressure Sensor off the EGR valve and left it hanging.
Now, I don't know if THEY did that, but they certainly should have NOTICED it.
After $850 worth of parts (new catalytic convertor, 3 O2 sensors, spark plugs and wires) I reconnected these hoses and valves and low and behold, no check engine code.
Stay far away from these morons