P0440 OBD2 Code in VW Passat: Causes, Symptoms, and Gas Mileage Impact

Hi everyone, I recently discovered the P0440 diagnostic code on my VW Passat after the check engine light came back on. Although I've seen this code before, I'm not entirely sure what it means or how it affects fuel efficiency. After using an OBD2 code reader to confirm the P0440 fault, I noticed a significant drop in my gas mileage—this has me concerned. My question is: if there's an underlying issue (other than a loose gas cap), would the check engine light come back on? Is the P0440 code directly linked to reduced fuel economy? Any real-world experiences or technical insights from VW Passat owners would be greatly appreciated.