What was found
(CAN) DURING SELECTION OF GEAR DOWN ON APPROACH, THE GEAR FAILED TO EXTEND. THREE RED GEAR LIGHTS ILLUMINATED, AMBER WARNING LIGHT IN SELECTOR HANDLE ILLUMINATED, GEAR DOORS DID NOT OPEN. AIRCRAFT RETURNED TO ORIGINATING AIRPORT, ALTERNATE GEAR EXTENSION USED, GEAR EXTENDED AND AIRCRAFT LANDED NORMALLY. PSEU SHOWING FAULT CODES 53 & 55 (INTERNAL CARD FAILURES). LANDING GEAR PROXIMITY CONTROL BOX (PSEU) REPLACED. GEAR SWINGS CARRIED OUT . AIRCRAFT RETURNED TO SERVICE , NO FURTHER FAULTS.