00_s2k

I have a 2000 s2000 i never really had any problems with CEL's or anything before and the car is never beat on and is well maintained but for the last week or so my gas milage has been getting worse, then 2 days ago between gears i noticed a small backfire the BAM there it was a CEL. I figured it was an o2 sensor so i went into my work to run the code it was a Secondary Heated Oxygen Sensor Circuit High Voltage (Sensor 2) and Secondary Heated Oxygen Sensor Heater Circuit Malfunction, so today i replaced the rear sensor and reset the ECU. I drove the car for about 10-15miles with out the CEL coming back on so i thought I was in good shape and then there it was, back on again so whats the problem. As far as exhaust mods i do have a test pipe conected to a Kikamoto cat back exhaust, but like i said i never had any issues before and this has been my set up under the car for quite awhile. Please give me some advice!