Which oxygen sensor to replace???

I have a 96 1.9 LX with the check engine light turned on. I got the codes checked and it had something to do with the gas cap. I replaces the cap and the codes were cleared for about a week and now its back. Someone said it could be the oxgen sensor. Is it the one under the car connected to the header or the one in front of the car near the radiator????

Thanks in advance!!!

Did you have someone check the codes again? Since this is a 96 vehicle it is supposed to have the OBD-II compliant codes. I believe there are 2 different codes that would indicate which sensor is producing the fault.

I know places like Autozone will check the codes for free if you can get the vehicle there.