Q: do i need to replace both sensors

Only oxygen sensors that are DIRECTLY tested and fail need to be replaced. Please keep in mind that OBD trouble codes do not conclusively diagnose failed parts, nor could they. Consequently, the only way to condemn an oxygen sensor (or any part) is to test it directly. Just to give you an illustration, if the wire to a brand new oxygen sensor is loose, the check engine light will come on and an "oxygen sensor code" will be saved. You might read the code and think, "I need an oxygen sensor." But, there is nothing wrong with the oxygen sensor in that scenario because the sensor is brand new. Instead, the code has set because a wire to the sensor is loose and so the computer cannot read any data from the sensor. The point is a real mechanic will almost never replace a part on a car based on a code. Instead, only replace parts that are properly, and directly tested, and once tested do not meet the required specifications. That strategy will save you a lot of money, too, plus you’ll gain a better understanding of exactly what vehicle technology can and cannot do.

With regard to oxygen sensors, the ONLY technique that can be used to determine if an oxygen sensor is actually bad, and thus must be replaced, is to view it’s operation in real time on an appropriate scanner and/or hook it up to an oscilloscope. If the oxygen sensor is functioning properly, at 2,500 RPM, on a scope you will see a rough sine wave with a trough at .2, a peak at .8 volts and a frequency of about 5-7 hertz. If you see that waveform, you are good to go. If you don’t see that waveform, replace the oxygen sensor. If you see that waveform and yet the check engine light remains on and an oxygen sensor related code is still there, the problem is NOT the oxygen sensor but rather a different vehicle component. If a mechanic tells you an oxygen sensor is bad, ask him exactly what he saw on the scope. If he didn’t view the sensor on a scope, then he or she is just guessing (and wasting your time and money) as to the status of the sensor or the cause of the check engine light. To get your issue promptly and professionally resolved for you, please request a check engine light diagnostic - oxygen sensor waveform check and the responding certified mechanic will come to your location and determine what, if anything, is required to be repaired on your vehicle. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.