Q: So I have a 2006 Subaru Impreza 2.5i and my Check Engine light has turned on , gave me code P0456 small leak detection evap system

You did not indicate if the new gas cap resolved the P0456 trouble code. If it didn’t, the appropriate diagnostic is a check engine light diagnostic. With regard to gas caps, if a cap is faulty the best practice is to install only a genuine OEM cap from, in your case, a Subaru dealer. Aftermarket caps are not necessarily an exact duplicate of what the OEM manufacturer designed and consequently use of aftermarket caps can create or prolong diagnostic problems. If the check light is off but you have a new issue, namely loss of power, or lack of response to accelerator pedal input, the appropriate diagnostic is an engine loss of power diagnostic. Loss of power can be due to plugged fuel injectors, faulty oxygen sensors, fuel pump and/or filter, vacuum leaks, ignition system malfunctions, bad valve timing if the timing belt or chain is worn or skipped, blocked exhaust (e.g., catalytic converter fails or become clogged), a faulty mass airflow sensor or throttle position sensor, and so forth. If you request the recommended diagnostic, the certified mechanic will get this resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.