What was found
(CAN) PILOTS REPORTED THAT ON HEAD START DEPARTURE OUT OF YVR, THE NR 1 ENGINE FIRE DETECT SYSTEM WOULD NOT TEST. MAINTENANCE WAS CALLED OUT TO THE AIRCRAFT AND FOUND THAT THE PROBLEM TO BE THE TEST SWITCH. THEY SPRAY CLEANED THE SWITCH & IT TESTED SERVICEABLE. AIRCRAFT RETURNED TO SERVICE HOWEVER ON THE OUTBOARD LEG THE FIRE DETECT SYSTEM WOULD NOT TEST ONCE AIRBORNE. CREW ELECTED TO RETURN TO YVR & LANDED WITHOUT INCIDENT. MAINTENANCE REPLACED THE NR 1 FIRE WARNING TEST SWITCH, P/N 3026218A9, & TESTED SERVICEABLE. AIRCRAFT RETURNED TO SERVICE WITH NO FURTHER INCIDENTS TO REPORT.