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Anyway, the weird thing now is that after erasing the codes, the car seems to be getting normal gas mileage again. It may be a bit early to tell, so I'm going to watch it over the next day or two and see. Maybe there was a stuck setting and the ECU reset cleared it somehow?
Possible. You may want to reschedule your mechanic, to give it a few weeks or a few tankfuls, or whatever.
For some logical thinking behind the "stuck codes", I recall how our van from that era would work. We'd have a dead battery occasionally, the kids were small and sometimes would not get the door shut all the way, and we wouldn't notice the dome lights were on, and that would drain the battery.
Well every time that happened, the van would run just awful, it would take about 5 miles of driving for things to settle in, and maybe another 20~50 for it to be back to normal. It was so bad, sometimes dying at a stop sign, that DW always wanted me to take it out for her and get it running right. What happens, is it loses the settings that it had 'learned' that keep it running well, and it takes a while to re-learn those.
Perhaps your sensors were acting up, and your car 'learned' a setting that worked with a faulty sensor, so it seems to run OK, but with bad gas mileage? And be disconnecting the battery, the settings are back to factory default, which is maybe closer to 'normal' than the learned bad settings?
If that's the case, that sensor is probably going to go bad again. But it might be worth waiting, your mechanic will have trouble finding an intermittent problem. Though, just shotgunning it and replacing the sensor might not be a bad idea, depending on the cost to do that, and if you have a good idea which sensor.
-ERD50
