2013 Mustang GT O2 sensor question

[George_San_Jose1]

It sound like you are on the right track, but the problem-causer is hiding. Be sure to check the PCV system and that the brake booster holds vacuum. fyi, a similarly hard to resolve O2 sensor problem was discussed here a few months ago:

Water-cooled VW Beetle perplexing problem

I found this perplexing problem described in a classic-car magazine (Classics World, May, 2024). VW Beetle (new type , water-cooled), don’t know model year, but OBD II era . OBD II codes say “too lean”. But no other obvious symptoms. Starts and runs well. Owner thinks there may be a vacuum leak in the valve cover area , so replaces the oil filler cap, thinking the cap’s seal may be leaking, allowing air into the engine. After replacing the cap, the engine won’t run. I can’…