Test emissions failed

The diagnostic fault codes do not "reset" on their own, and you don't want them to. They are the only thing the mechanic has to go on when figuring out where to start the diagnosis. That is even more important when you have an intermittent problem. The codes tell him which circuit or system needs that further diagnosis, or the unacceptable operating condition.

"Too much oxygen getting into the tank" doesn't make sense. As the gas is used up, there had better be air going in to replace it, otherwise the tank would get pumped into a vacuum and would collapse. I have a feeling he said there was too much unburned oxygen in the exhaust gas, which is a "lean" operating condition. That would be determined by the fault code, but the cause of that condition still needs to be diagnosed.

Be aware there are multiple types of excessive emissions. The most common problems relate to improper burning of the fuel and air in the engine. The majority of those emissions are cleaned up by the catalytic converter(s), then what goes out the tail pipe is measured at the testing station. A totally different source of emissions is the "fuel vapor recovery system". Fuel in the tank expands on a hot day and gets pushed into the charcoal canister where it is stored. Later, the purge valve is opened to allow those vapors to be drawn out and burned in the engine. Since the '96 models, that vapor recovery system has been monitored to be sure there are no leaks. A leak in that system would allow gas to evaporate out of the gas tank, even while the car is parked. The problem is the parts in the system cause as many problems as they solve. Typical emissions testing stations aren't equipped to test for fuel vapor emissions. They only have the diagnostic fault codes to go by to tell them there is a problem. If the problem is intermittent, it may be not acting up during the testing, but the fault code in memory would be the reason the testing failed.