I have a Rheem Classic 90 gas furnace. Lately I have noticed that when heat is required, the furnace turns on normally but shut down after 10 minutes prior to reaching the required temperature. Within a minute the cycle starts all over again and shuts down after 10 minutes.

I have noticed that when operating normally the 3 led lights are on (yellow, green and green). Then the yellow led turns off and the next led (green) flashes in sequences of 3 times, pause, 3 times pause...

What do you think is wrong and can you tell me if I can fix it?

Somewhere on the furnace, usually inside the blower compartment door or on the board itself is a key to the blink code. If you can find it, please tell us the meaning of a 3 code.

Hi Mr. White,

It says 3 blips equals the limit switch is open.

It could be a faulty limit switch but not likely. The most probable causes are: dirty or highly restrictive air filter, closed or blocked vents (either supply or return), dirty blower wheel, dirty A/C coil (if you have A/C connected to the furnace). There are some common problems listed, more or less, in order of easy to cure.