As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FUMES BELIEVED TO COME FROM AVIONICS. VALVE LOCKED IN THE CLOSED POSITION NOT ENOUGH AIR IN AVIONICS COMPARTMENT BEING RECYCLED CAUSING FUMES TO RISE TO CABIN AREA. RETURNED TO DEPARTURE. REPLACED THE VENT AVIONICS OUTLET VALVE AND FILTER, OPS CHECKED OK.