1997 Subaru Legacy Outback P0507 Code After Head Gasket Repair

I own a 1997 Subaru Legacy Outback with a 2.5L engine and over 134,000 miles. The vehicle was experiencing overheating, which we diagnosed as a head gasket failure. After replacing the head gasket and reassembling the engine, the car runs smoothly and performs better than in years. However, after the rebuild, the OBD2 fault code P0507 appeared — this code did not exist before the repair. The engine idles steadily at approximately 750 RPM and runs without noticeable issues. I've thoroughly inspected for vacuum leaks and other common causes, but no abnormalities have been found. Can anyone suggest possible reasons for this new P0507 code after the head gasket replacement? Any insights or diagnostic tips would be greatly appreciated.