The ECU continually tests emission components as the car is driven under certain conditions. If you simply erase codes the ECU will eventually get around to testing those components again and the same codes will come back. (if the problems still exist) Catalyst and Evaporative system seem to always be the last two items to be completed when attempting to finish a drive cycle. A drive cycle can be completed in less than about 30 minutes for most cars using the correct driving techniques. (Usually 1800-2000 rpm steady throttle driving with periods of idling)
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