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I assume by "reset" he meant that he disconnected the battery for several minutes, then reconnected the battery. As I understand it, the ECU has an adaptive learning function. This function helps maintain the proper fuel and timing settings for mainly idle conditions. When you disconnect the ECU, it loses the information on driving habits, altitude, color of the driver's socks, and other vital information.
When the battery is reconnected and the car driven again, the ECU must relearn the needed information. I've seen recommendations from Volvo to its technicians to avoid "driveability" problems for the customer by giving several short stop and start runs. The information I read said that it may take 12 to 15 start engine, drive and stop engine cycles to allow the ECU to relearn.
I suppose it is possible that this resetting process might affect the gas mileage but I've never heard of it before. According to my Volvo manual, the adaptive learning function serves to adjust idle speed and mixture to make up for normal aging of the O2 sensor, the injectors, and maybe other sensors. The fact that Volvo says driveability is affected makes me think that more than the idle and mixture settings are affected.
As an experiment, you could disconnect the battery, reconnect it and see if mileage and driveability are changed. Let us know what you find out. Of course, over a year's time, many other things could act to lower your gas mileage.
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