P0173 is a fuel adaptation code which has nothing to do with the gas cap. A gas cap code would be P0455: large leak in evap system or P0442 small leak in evap system.
An adaptation code can only be evaluated by reading the adaptation numbers. The code will probably still be there unless the battery has been disconnected. The lights shouldn't go out but they do. It should take 40 key cycles with no fault to reset the light but some continuously monitored values will come and go but the code stays there in any case, till erased by scanner or battery removal.
A P0173 code will only apply to one bank so the numbers are especially important. There are two sets of numbers that will set the light. These are the partial load multiplicative adaptation numbers and the CTP (closed throttle position) additive adaptation numbers. One of the numbers will be at the limit or real close. Knowing which will be the mechanism for diagnosis. I wrote a little about this in an article that appeared in last months "Import Car" a trade publication:
http://www.babcox.com/editorial/ic/ic60442.htm