I have a Trane gas furnace with a White Rodgers 50A50 controller. With the thermostat in the OFF position (it is not calling for heat or AC), the air circulator fan comes on. The diag code either is in lockout (two flashes) or says flame sensed with gas valve off (five flashes). The flame rollout sensor appears fine, it is not melted and has continuity. I question the flame sensor. Should this be a short or open when there is no flame? If the flame sensor is functioning properly, then could it just be the controller that is hosed?

Odds are you need an new 50 a 50 control did you try sanding the flame sencor?

Didn't try sanding the sensor, but I can't see how that would help. The system is not calling for heat and the flame isn't on at all. The system is essentially in standby, with no call for heat or AC. It appears something mysteriously failed and the air circulator came on. The literature indicates that the air circulator will come on if it senses a flame rollout failure. I also think that the 50a50 controller might be bad, but don't want to spend $100+ unless I am reasonably sure first!

Well fist before you go buy a new control board, check all the limits, and role out switches. You may have up to 5 of them. One on eigh side of the burner. And possibly one in the blower compartment. And the main high limit.

http://bestbuyheatingandairconditioning.com/Merchant2/graphics/00000001/LIMD25303217.jpg roleouts need to be reset by pressing the red button.


High limt is supposed to self reset but may be stuck. Tap it with a screw driver.

http://midwestapplianceparts.com/images/CARRIER-HIGH-LIMIT-1.jpg

Thanks, was following most of the wires to find the sensors. I think it is the flame sensor, I am going to remove it and test it on the range.