Got a Code PO 0135: P0135--Front HO2S Heater Circuit Fault (Sensor 1)
Where is this? Thanks

I found 1 before the catalytic converter, and 1 after the converter. -I unplugged 4 pin connector to check condition of pins, Didn't do voltage or resistance check, plugged back in, cleared code, MIL light did not come back on, Runs fine now?

This code would be the O2 sensor before the cat. O2 sensors produce a voltage signal that varies with oxygen concentration. They must be really hot in order to work properly, so they have internal heaters. In my experience, O2 sensors fail slowly over time. You'll probably find that the problem will clear and then come back. You'll get the MIL more frequently as time progresses. Most manufacturer's require them be replaced at ~60k miles (100k km)

The cat inlet O2 sensor is much more important than the cat outlet one. The inlet is used by your car for fuel trim (i.e. it will affect your mileage), while the outlet is just used to measure cat efficiency.

If you replace it now (or at least soon) you'll avoid future MIL's and probably notice improved mileage. It's a fairly easy job on most cars... and not too bad in pricing either.

Rtw, thanks, it did have 1 H20S before and 1 after the catylitic converter.
My girlfriends daughter had wheels rotated, and oil changed, when she got it back it had troulble light and code of the H20S. If it happens again, I will measure the voltage between A and B, then resistance of C and D. Thanks again.