I have a Goodman model GMNT 080 4B. The furnace start like normal but when the temperature reaches about 64 degrees I receive an error light. The fan continues to run but the burners do not. I have already tried cleaning the flame sensor but the problem continued.

The error indicator light is flashing 3 blinks. According to the service panel this indicates 'Pressure Switch Failure to Close". Check Venter, Pressure Switch, Vent Blockage.

What does the pressure switch look like? and what does it mean by 'Venter'? I have already check to make sure the vents were open. Thanks in advance.

pressue is a black thingy that has 2 wires and a vac hose going to it,,,,,check from start to end and make sure vent isn`t stopped up,,,birds nest,,or anything in way of venting

You may find a picture of the pressure switch in the sticky at http://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html
The vent it refers to is not the one your heat comes out of, but the one the combustion products from the furnace exits, a form of chimney.

The problem sounds to me more like a high limit switch. Reading through the section of the sticky on sequence of operations will help you undestand everything.

Check to make sure the vent pipe on the furnace is clear(hot gas exhaust). Check the small rubber tubes going to the pressure switch to make sure they have no holes. you can blow and suck lightly on the tube connected to the pressure switch to see if it is stuck(listen for a click when doing this).

I had the 3 blink problem on my GMNT080-4b. After fighting it for a couple of years and (according to the installer) having the goodman rep look at it, the problem turned out quite simple. At least that is how I would describe the installer! The spec says this unit requires the 3" pvc while the installer installed 2". Why didn't the rep find this? This unit requires 3" regardless of line length! I changed to 3' and so far the problem is gone!

the original thread was posted 2 years ago,,,I hope the problem has been resolved by now

This is posted 2 years too late for the OP, but I have similar 3 blinking light lockout on my Goodman GMS8 model and here's what the problem was in case someone else has similar problem. Disconnect the rubber hose at Pressure switch end leading into inducer motor. Blow in rubber hose and see if the nipple from inducer motor is blocked up with crud. You should be able to blow freely into the nipple of the inducer motor. My was blocked so I cleaned out by inserting a small drill and rotating by hand to remove crud.