Q: i have a code that says it could be an oxygen sensor (THERE IS 4) HOW CAN U TELL WHICH ONE IS BAD?

Oxygen sensor function can be tested and verified using an oscilloscope, automotive scope, or a vehicle specific, manufacturer scan tool. A faulty O2 sensor, if it exists can be found. However, the Diagnostic Trouble Code you are referring to, namely P0301, has many, many underlying causes other than the oxygen sensor. In fact, any of the below listed faults can cause that Code to set:

Faulty spark plug in cylinder 1
No compression in cylinder 1
Damage or excessive wear of spark plug wires and/or coils
Damage or excessive wear of distributor cap (not applicable to all vehicles)
Damage or excessive wear of rotor button (not applicable to all vehicles)
Faulty fuel injector
Clogged EGR valves or tubes
Burned valves
Inaccurate ignition timing
Vacuum leaks
Low fuel (rare)
Leaking from head gasket
Cracked distributor cap
Faulty camshaft sensor
Faulty crankshaft sensor
Faulty mass air flow sensor
Faulty oxygen sensor
Faulty throttle position sensor
Faulty catalytic converter
Faulty powertrain control module (rare)

As a consequence of the large number of possibilities, in order to get your vehicle properly diagnosed and repaired I recommend that you request a check engine light diagnostic during which a certified mechanic from YourMechanic would use a code scanner to confirm the specific diagnostic trouble code(s) from your car’s PCM that have caused the check engine light to illuminate. Using those codes, the appropriate individual vehicle components or sub-systems (often a sensor, circuit, and the like) are then carefully tested, based on specifications set forth in your car’s Factory Service Manual, to pinpoint the cause of the fault or the reason why the check engine light has illuminated. Once the faulty part or component is identified, it is explained to your satisfaction and the mechanic will let you know of the cost to repair. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.