Is there a DIY open-loop indicator?

[Occasionally6]

Originally Posted by Focus-Ak

Apparently the reason it can be inaccurate (and probably is on at least some [most?] cars) is that it assumes a certain O2 sensor reading necessarily means the ECU is putting the engine into open-loop operation. As the article explains, however, the ECU is most likely considering other factors besides the O2 sensor output before it decides whether to enter (or leave) open-loop. That seems to me to make sense. Does anyone know differently?

The exact mid point of the O2 sensor output isn't super critical to find. What the Closed Loop Monitor - indeed the LED light gauge - indicates is that there are rapid transitions across the mid point (stoichiometry). If that is occurring, it's in closed loop.

To summarise, the CLM LED will be off when lean, on when rich and flashing in closed loop.

The LED gauge type provides exactly the same information with some (vague) indication of the magnitude of the rich or lean, at twice the cost (both are inexpensive).