On a 80% Rheem upflow furnace... The AC works fine but when switched to heat nothing happens. "OK" LED is on solid, second LED is blinking 3 times, "FLAME" LED is not on at all. What can be the problem?

I would contact Rheem if it is new or an authorized contractor for advice.

An LED that blinks four times followed by a two-second pause is signaling the pressure switch is closed when it should be open. The switch itself might be stuck, or the tubing that attaches the switch to the furnace's draft inducer might be blocked

You didn't mention what thermostat(s) you have but most simply require you to go from cool to heat and then check the temp setting(s0 to make sure they are in your comfort zone.

62-24084-02 LED CODES
POWER LED - GREEN
ON - Normal
OFF - No power to board
OK LED - GREEN
One Blink - One hour retry delay
Two Blinks - Pressure switch failure (remains open)
Three Blinks - Limit switch fault
Four Blinks - Pressure switch failure (remains closed)
Five Blinks - Twinning fault
FLAME LED - YELLOW
ON - Good flame
Rapid Blink - Flame present with gas valve de-energizing
Slow Blink - Marginal flame

did the original installer show you that the system operated properly in heat mode back in May ????

You will need to reset the furnace by turning the power off to it. Then as the furnace tries to light, watch the LED for blinking. Hopefully it will give a code other than 1. Try checking the air filter for blockage, make sure all hoses are free of moisture and any drains are not blocked. Check the air intake and exhaust pipe for sags where water could block it.

The sequence of operation is this:
Thermostat calls for heat, the LED may blink telling you it registers the call. The inducer (exhaust) blower starts closing contacts in the pressure switch (small disc shaped thing), the igniter glows orange (seen thru a small window high up on furnace front), gas valve opens allowing main burner to light, flame travels to all other burners, flame rod senses flame within a few seconds and keeps gas on. Room air blower starts within 90 seconds.

Any deviation or delay in this sequence and it goes into lock-out.

I have this same POS model. It is not the thermostat. this model is just garbage and cant wait to get something that works right. i have been fighting this thing for 4+ years now.

the pressure switch is closed before the draft motor closes it.the board is telling you that this is not right.pull the wire(either) off of the pressure switch to see if furnace tries to light.if it does,you should replace the pressure switch.