The ECM will go into Open Loop eventually if you do that, but only because it's sitting there parked/idling and the coolant temperature drops below 45 degrees Celsius. I cannot see how a single voltage signal over one wire from the O2 sensor to the ECM is possibly going to advise the ECM to go into Open Loop even if there is (say) such code in that ECM that I'm not noticing.

Good question about the ECM and learning. It does NOT learn anything. There is not a single byte of Non-volatile memory in it. The sensors must be calibrated appropriately; for example, the TPS must be about half a volt with ignition on/engine off or you'll have all sorts of issues near or at wide open throttle (not to mention TCC lockup, but that's not ECM controlled so I digress).

The ECM barely keeps any memory of the sensor readings between its loop cycles even. There are a couple of bytes for knock counts and related timing retard for the past couple of cycles, two previous MAP sensor readings, two previous VSS readings, four previous TPS readings, not much more than that. It's a pretty simple computer and is operating for the most part on current state readings.

The ECM does remember a few things between starts. There is a counter for how many times the car has been started since a trouble code was thrown; it discards trouble codes after it hasn't seen one for a certain number of starts (IIRC it's 20 but I'd have to check that again). There is a hot restart timer that kicks it into closed loop quickly after a momentary stop/restart. Maybe a couple of other things, not much. Everything is "forgotten" if the ECM is unplugged or you pull a battery cable.

As for how far downstream an unheated O2 sensor can be mounted, that's where I'm way out of my league. I totally agree that it's possible for the ECM to decide it's closed loop time from the coolant temperature and run time but the O2 sensor is mounted somewhere that it's not getting hot enough to respond with a proper reading.

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