03 grand cherokee p0135 BD code - hissing sound

My 03 grand cherokee just recently started making a hissing sound near the muffler. at the same time the check engine light came on and I checked the OBD code and it was P0135 - Oxygen sensor heater element malfunction. I'm assuming that because of the hissing sound, it is not just a faulty sensor. Does anyone know what might be causing it? Hose leak? Bad muffler? What wuld be the best way to check?

usually bad sensor, remove the 1/1 o2 sensor and check resistance of the 2 black wires on the 02 sensors at 60-70 deg should be somewhere around 7-15 ohms..