P135 OBD code - what is it and how do i correct it.

[tiso_us]

OK, so i have this tracker that is reading this P135 OBD code, as far as i got was an "upstream O2 heater failure", i can reset the OBD code and the Engine runs smoother, when the "check engine" light comes on, it idles and drives rough.

There are 2 O2 sensors, one located at the donut on the exhaust, the other located right after the catalyst, by the way, it has no catalyst, someone put a pipe on instead.

I replaced the first O2 sensor with another O2 sensor, still same problem. Am i mistaken about this O2 sensor or does "upstream O2 heater failure" means something else?.

Is there a work-a-round to this problem?.