my first ford, car has been sitting for 4 years, getting a code 1151, don't know much about mustangs and where things are, any help would be appreciated; manuals, videos, etc..
The "possible" list of causes for this DTC code are very long. O2 sensor 21 is bank 2 (driver's side) sensor 1 (before cat).
Since the list is soooo long the usual way is to start with any "clues" given by the car's maintenance history. One thing to consider is "stale" gas and gummed up fuel injectors. If a fuel injector problem is suspected consider an injector cleaning and flow test service such as injectorRX.com. Once cleaned and tested they are as good as new. Cheaper than new.
It's also possible to use the car's fuel pump to empty the gas tank.
If interested in getting a full set of Ford service manuals and wiring diagrams for your car I maybe able to help. PM if interested.
>>>From Ford service manual P1151 - Lack of HO2S-21 Switch, Sensor Indicates Lean
A HEGO sensor indicating lean at the end of a test is trying to correct for an over-rich condition. The test fails when fuel control system no longer detects switching for a calibrated amount of time.