As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CABIN WAS PRESSURIZING BUT PILOT REPORTED ERRATIC CABIN RATE OF CLIMB AND ELECTED TO RETURN TO BASE. MX CHECKED TAILCONE FOR PRESSURE LEAKS NONE FOUND. MX CHECKED PRESSURE CONTROLLER AND FOUND CONTAMINATION IN THE FILTER WHICH APPEARED TO BE AN INSECT. REPLACED FILTER AND GROUND TESTED IAW THE MM AND CHECKED OK. ACFT DEPARTED AND NO PROBLEMS OCCURRED.