The code 1151 on a 1998 Mustang indicates an issue with the oxygen sensor circuit in bank 1 sensor 1 which could be caused by faulty oxygen sensor, damaged or disconnected oxygen sensor wiring, vacuum leaks, exhaust leaks ,faulty fuel injector or issues with Engine Control Module (ECM).
It's advisable to have a qualified mechanic to investigate and resolve the issue as it might affect fuel efficiency and emissions.
