Code 13 is not 02 sensor code..it is the lambda system code.

So, it is either lean or rich, meaning an intake air leak or improper fuel pressure.
So... you are back to looking at the FP regulator along with possible intake leaks. I would take a full FP test, specially the OFF hold testing.
There are two other possibles ..one is dirty injectors [ run some cleaner thru for a tank] and the other is an internal vac leak..these are hard to find and usually are the EGR is not seating well or the Purge Valve is leaking by...that one is common b/c they get full of crap and stick open.

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A Dalton