troubleshooting the Concord 80+ gas furnace

05:14 ______________________________________________________________________ I have a 1998 Concord 80+ gas furnace which is igniting, but will only stay on 10-20 seconds then shut off. It will normal operate 1 full cycle, but when the home cools, the furnace will re ignite, then shut down. After a day or two, the furnace will again heat home one full cycle then next cycle, shuts down. It has the blinking light trouble shooter on the furnace, 1- bright and dim flash indicates normal operation. When the furnace malfunctions, the light will be blinking. 2 flashes of the green light indicates the pressure switch stuck closed. 3 flashes indicate pressure switch or auxilary limits stuck open. 4 flashes indicate open roll out switch or high limits. 5 flashes, flame sensor out of sequence, and 6 flashes indicate system lock out. Mine is flashing 6, my furnace man has no idea what the ‘ system lock out ‘ means, can anyone help? Thank you in advance , Karla Karla Papple Replies: hvacmgr “Re(1):troubleshooting the Concord 80+ gas fur” , posted Fri 8 Oct 16:06: ______________________________________________________________________ If the board is a White Rodgers 50A50-XXX (3 numbers) board then the 6 flash code means that there is either a problem with the flame sensor (may just be dirty) or there is a ground problem in the flame sensor circuit [this message was edited by hvacmgr on Fri 8 Oct 16:07]