I got a CEL and the code was for bad oxygen sensors.
I took the car to the mechanic and they changed all the two sensors.
After they replace them, the up stream sensor's reading is bad. the voltage was too low.
I replace the bad sensor and put the new one in, but still have CEL on.
I don't know what cause the problem.
My question is that which sensor goes whree.
there are two sensors that one for the up stream and the other one for the down stream.
they put the down stream one right behind the second CAT and the up stream one behind the header's Y pipe.
Please help me what cause the CEL on.
Thank you.

 

have you resetted ECU?

 

I reset the ECU 5 times and after 20 mins CEL come on everytime I reset the ECU.

 

are you sure they put the right o2 in the right spot?

 

How did they wire them? Did they clip off the plugs, or extend any wires?

 

Did you ever extend the wires to install a header or aftermarket exhaust? If so, you may want to check over the wires again.