Oxygen Sensor in Loop Mode

Oxygen sensor in closed loop mode.

Oxygen sensors have a feature called closed loop, which basically does not send the ECU a signal, the ECU will richen the air-fuel mixture, speeding up the warming up process of the engine.  I actually tested the closed loop mode by accident, while I was testing out my lambda tester on my car.  I back probed the oxy sensor terminal to find the signal wires.  At first I was getting a constant 14V (Hans thought this was strange) and as the engine was running, it suddenly changed to a signal that changed by itself.

I recommend that when back probing the oxy sensor, follow the cable to a terminal that connects to another terminal, and back probe it there.  You might notice that the wires stick out of the oxy sensor and look quite bare compared to the rest of the cable, do not cover it with shrink wrap or tape as this bareness is also a part of the oxy sensor.  It actually works by comparing the oxygen levels in the exhaust gases to the oxygen levels on the outside.  (I actually feel that the oxy sensor should have a separate sensor that runs to the air filter to sense oxygen levels there instead of beside the exhaust piping where it's hot and stuffy).

So the loop-mode is basically just a feature to heat up the engine, the oxy sensor (in some cases) and the cat converter.