1998 Honda Civic ECM

I recently went to get my emissions test and some problem codes came up two saying the primary O2 sensor was bad one saying the catalytic converter was bad and the last one saying the car was running lean. So I took it to the mechanic and he cleared the codes and for some reason it was saying not ready on the computer that they use to test your car.

So he looked at the computer and there were no volts going to the primary sensor. So he pulled the primary O2 sensor and it turns out the rear one was in the front one and vice versa. And so the rear one and front one both worked when they were plugged into the primary spot. So I know its not the O2 sensor that is bad so then the mechanic looked and he said it went to a closed system and would not go back to an open system and that the volts stay at zero after the car starts up after a few seconds.

I checked the fuse and it is not burnt out or anything. He told me that its probably part of the ECM burned out that grounded where the primary sensor would go.

The other thing that is weird is that I have driven my car for about 400 miles since the codes were reset and none of them have come back up and all the systems still say not ready.

something else that is random that I have no idea why but the car before the mechanic switched the O2 sensors positions I was getting 27 mpg now I just filled up today and now im getting 34 mpg.

So do you think I need a new ECM?