Q: I had p2096 and 2196 codes. I replaced the upstream o2 sensor. The p2196 went off for a while but the 2096 remained.

The two codes indicate that the air fuel ratio in the engine is off. In your circumstance, the short and long term fuel trim values should be read and evaluated using a real time scan tool as that will help narrow the diagnostic. Also, the actual output of the pre and post oxygen sensors should be graphed on the scan tool. Depending on the results, some vehicle systems, or potential malfunctions, that might need to be evaluated to get the codes resolved include: low fuel pressure caused by a clogged filter, failing fuel pump, failed fuel pressure regulator or clogged or leaking injectors, large vacuum leak that causes a massive amount of un-metered air to enter the intake manifold resulting in an overly lean mixture. Other possibilities are a faulty MAF (Mass Air Flow) sensor or a faulty ECT (Engine Coolant Temperature) sensor. To resolve this, the recommended service is a Check Engine Light diagnostic. If you request that service, the responding certified mechanic will come to your location, download the trouble codes, run the necessary tests and let you know exactly what has failed and why. If you authorize the indicated repairs, the mechanic will perform that as well, again right at your location. Once all necessary repairs are complete, the check engine light should stay off, at least so long as no new malfunctions crop up. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.