Oxygen sensor (off car)

oxygen sensor

this sensor is one on the most important sensors of the modern engine, it tells the ecu if the engine it tells the ecu if the engine is running ritch or lean.
the sensor is not being used during warm up because the engine is in open loop. this means that the injectors are squirting a specific amount of fuel into the combustion chamber during engine warm up.
once the engine reaches its ideal running temperature (depending on manufacture, around 300°C) this is when the oxygen sensor is warmed up and starts sending signals to the ecu (closed loop)

this sensor may have 1 to 4 wires depending on manufacture:
signal wire
ground wire
heater positive
heater ground

if the engine is running to lean the output to the ecu will be around 0.2 volts and if the engine is running too ritch the output to the ecu will be around 0.8 volts (depending on manufacture)