C425 PLT ON REPOSITION FLT FOR AIR AMBULANCE HAS MASTER CAUTION LIGHT ACTIVATE ALONG WITH A 'DUCT OVERHEAT' LIGHT. AN EMER IS DECLARED AND A DIVERT TO NEARBY ARPT FOR LNDG.

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ON REPOSITIONING FLT IN CESSNA 425G AIR AMBULANCE. DEPARTED SAC ON IFR FLT PLAN IN VMC; TALKING WITH SAC DEP; CLRED DIRECT TO REDDING (O85) AT 12000 FT. UPON PASSING APPROX 5000 FT MSL; MASTER WARNING ACTIVATED; ALONG WITH 'DUCT OVERHEAT' WARNING LIGHT ON CAUTION LIGHT PANEL. THIS WOULD NORMALLY INDICATE THAT THE ELECTRIC HEATER HAS OVERHEATED THE DUCT LEADING TO THE CABIN. EMER PROCS ADVISE TO SHUT OFF THE ELECTRIC HEATER AND LEAVE THE HEATER FAN ON HIGH. THE ELECTRIC HEATER HAD NOT BEEN ON. ADDITIONALLY; THE L ENG BLEED AIR SYS ADJACENT TO THE SONIC VENTURI AT THE L WING ROOT HAD BEEN WORKED ON EARLIER IN THE DAY; DUE TO A STUCK L BLEED AIR SHUTOFF VALVE. I REACHED DOWN AND SHUT OFF BOTH BLEED AIR SOURCES; DECLARED AN EMER TO SAC DEP; AND REQUESTED TO LAND AT SMF. I STARTED A RAPID DSCNT AND WAS HANDED OFF TO SMF LCL CTLR; WHO CLRED ME TO LAND ON RWY 16L. I CROSSED THE ARPT AND ENTERED A L DOWNWIND. ON BASE LEG; THE WARNING LIGHTS WENT OUT AND I LANDED WITHOUT FURTHER EVENT AND WAS FOLLOWED BY EMER EQUIP TO THE GA RAMP. ON THE GND I WAS UNABLE TO DETERMINE THE CAUSE OF THE PROB INITIALLY. THEN AFTER FURTHER EXAMINATION THE DIRECTOR OF OPS WHO FLEW IN TO PICK US UP FOUND A BLUE 'SICK SACK' ENVELOPE PLUGGING THE INTAKE SCOOP FOR THE HEATING DUCT. THIS HAD PREVENTED CABIN AIR FROM RECIRCULATING IN THE DUCT AND CAUSED THE OVERHEAT. BY SHUTTING OFF THE BLEED AIR SOURCES; I HAD REMOVED THE ONLY OTHER SOURCE OF HEAT TO THE DUCT; COOLING DOWN THE DUCT AND THUS SHUTTING OFF THE WARNING LIGHTS. THIS INTAKE SCOOP IS LOCATED AT THE BASE OF THE R SIDEWALL TO THE R OF THE COPLT'S R ANKLE. THE POH EMER SECTION/CHKLISTS DO NOT ADDRESS THIS PROB; HOWEVER RARE. THE ACFT WAS RETURNED TO HOME BASE WITHOUT FURTHER INCIDENT.