The O2 sensor fine-tunes the fuel management system and keeps the emissions low. Keep in mind that the PCM does not use the sensor under the following conditions: cold engine, acceleration, or deceleration. It is basically used during constant speed. If it is bad your fuel economy will decrease.
The O2 sensor will not perform correctly unless it is hot. Therefore the engine must be at operating temperature before the PCM/ECU (computer uses it). The heater element helps to get it hot quicker which helps emissions. If the heater element, which is an integral part of the sensor, does not work (because it is broken) then the PCM stores a code.
Simply have it replaced with an OEM type only.
Oxygen sensors are probably the least reliable sensor on any vehicle. The volt output should ave around 0.45 volts
94 Black Laredo 4-Liter with QuadraTrac (42RE trans & NP249 TC) 200,000 + miles purchased new
88 Cherokee 2-door with 5speed manual 139K miles(deceased) purchased new