I have a Rheem Criterion II gas furnace. Unit starts and runs okay most of the time. However it will stop firing up occasionally and will go into 1 hour lockout mode. In an hour it will fire up or when you open unit which automatically reset it. It will them fire up okay. What is causing unit to work properly sometimes and not others?

Does the unit have status LEDs? If so, they would likely be visible through an oval-shaped viewer in the blower access panel, and a service label would be found inside same panel.

First easy step to take is locating flame senser, remove, clean with sandpaper, steelwool, sandcloth, or any abrassive cloth,, then reinstall and see how it works. You have a rheem, this means you have two green lights and one orange light located on the control board inside furnace. There may or may not be a looking glass cover one the doors. If there is, and you can see the lights with doors on, then tell me if the orange light is on steady or is it blinking(once flame comes on). If there is no looking glass, then remove bottom door, then hold the door safety switch in and watch the lights as the furnace cycles. Let me know what you find.

If flame lights, and orange light is flashing, then you need to clean flame senser. I will walk you through this if this is the problem. Or I will help you locate other problems if possible.

Cleaning flame sensor

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Thanks for the pics hvac.

HI Brian.

The flame senser you are looking for looks like the one in Hvac's second pic. It is L shaped, but the mounting bracket is different. Don't confuse it for the igniter. It should be on the left hand side of burners, one 1/4" sheetmetal screw holds it in place. You will have to remove screw and twist senser to get it out. Some people try to clean it without removing it but it is very difficult to clean it well without removing. Please keep us posted.

Flame starts and then quits once the fan kicks in, we have replaced the flame sensor.