Q: engine light (02 sensor) on 1999 Ford Contour

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I have replaced all three 02 sensors and my engine light came on again and the test shows it to be the 02 sensor. What is most likely reason and what is best thing to do now?
(1) Answer
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It would be really useful to know the diagnostic codes exactly and which sensor in particular, which I would know from the diagnostic code. It's possible that the code you're getting is because one or more of the sensors is reading out of range due to another problem, like an air leak into the intake or exhaust, a faulty fuel injector or a failing catalytic converter.
Here's an explanation of the check engine light function:
The check engine light indicates that the computer has detected a malfunction in the engine/emission control system and has diagnostic codes and data stored which would be essential in diagnosing the problem.
A technician with a scanner can access the diagnostic codes and data and then be able to give you a diagnosis.
If you want to find a Ford specialist, here's our directory link for you:
The code is 1131, which is the 02 sensor. I have used one of the hand held 'cheap' home diagnosis models. The gentleman at Advance Auto parts said it was bank one sensor one and then after that I just figured the next two in line were bad, so I changed them. I can reset the computer but after about 30 miles the light comes back on. I just don't feel like paying a 'professional' $75 to hook it up to their machine, but if that needs to be done then so be it, just trying the most obvious options, which may be wrong but things I can check myself