No toe stepping assumed, there are 2 kilobytes of code in the ECM which is only partially (and sometimes incorrectly) commented. Perhaps I'm missing something, though I am pretty much firm in my stand that the loop status is decided entirely from coolant temperature, there is no temperature gauge at the O2 sensor.

I agree that the ECM will drop back to Open Loop, I've seen it myself with my scanner. The event happens when the coolant temperature drops below 45 degrees Celsius. Typically this will happen if you throw it in park after just barely warming it up then go and try to tune the carb or something like that. There is nothing about the way the O2 sensor is reacting that will cause the loop to change from Closed to Open in my review of the ECM code.

Agreed on the Rich/Lean trouble codes - if the O2 sensor sticks on either a very low or very high voltage for a concerning amount of time then trouble code 45/44 are thrown. Parameters are in the PROM here;

31B4:   2E                          FCB     $2E ;	MAX O2 CODE 44
31B5:   19                          FCB     $19 ;	MIN TIME CODE 44
31B6:   3C                          FCB     $3C ;	MIN TIME CODE 45
31B7:   AD                          FCB     $AD ;	MIN O2 CODE 45

Translating that: 25 seconds of less than 18% of whatever the maximum voltage is for the O2 sensor triggers Code 44 ($19 = 25 seconds, $2E = 46, out of 255 maximum that's about 18%). 60 seconds of more than 68% of whatever the maximum voltage is for the O2 sensor triggers code 45 ($3C = 60 seconds, $AD = 173, out of 255 maximum that's about 68%).

My view continues to be - A heated O2 sensor will only help the 4th Gen Monte Carlo SS if some PROM parameters are tweaked to change the coolant temperature thresholds. I watched it tonight, at 52 degrees celsius, ECM switched to Closed Loop.

Last edited by Bitflipper; 02/03/16 06:13 AM.

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