I had the error light (3 blinks) on my Goodman gmt090-3b furnace. The vent van would start up but the gas would not turn on and the furnace would not fire up. I was able to fix it by cleaning out the little nipple that the vacuum hose runs into on the vent fan from the pressure switch (2" round thing with 2 yellow wires and the vacuum hose running out of it). I figured out it was clogged by taking the vacuum hose off the pressure switch and blowing into it (with the other end still attached to the vent fan). I couldn't get any air through it. So I removed the other end of the hose (now the hose is completely off) and stuck a little screwdriver down the hole and cleaned out the little hole. I put the hose back on (at the vent fan end) and was able to blow through it. I hooked the other end back to the pressure switch and it fired right up. It now works great. Thanks for the info above-- it really helped and saved me a lot of $.

I had the same problem 3 blinks the unit would come up and run ,burners fire up but then shut down. After reading this thread decided to get on the Roof and check for a birds nest possibly blocking the ventor pipe.
I live in Northwest Florida, there was no birds nest but could see something behind the plastic screen,after pulling the screen cap off 2 huge wasp nest fell out. Problem solved.

Thanks to everyone. I just disconnected the rubber hose from both ends and blew through it in both directions. There was definitely an obstruction. I used a wire clothes hanger to clear some guck and now everything works fine.

I've tried all of the above. Cut 3" vent to even check for a clog, nothing. Checked hose going into pressure switch blew both ways and was clear. Furnace will run for 15 minutes then give me 3 blinking lights. Take hose off blow again works for 15 minutes goes out. Bad pressure switch? I don't know what to try next.

I'm actually getting a lot of moisture when I pull the exhaust vent out of the fan. Could it not be draining properly?

Just had similar trouble. Tried blowing in the tube but that didn't do the trick. Then I used a compressed air canister to blow it out and now is working. So perhaps try something a bit more forceful than your breath? Not sure but worked for me.
Thanks everyone for your input!

Thanks for the info on how to fix the 3 blinking light issue it was very helpful. I had that same problem. I found the motor and the tube going from the fan motor to a device which I assume was the pressure sensor. I was able to stick a 3/32 drill into the hole in the motor for about 1 inch. I tried a 1/16" drill and after turning it around, it broke loose some stuff and went in further. But the drill was not long enough to go further and it seemed that there was still more stuff inside. So I got a piece of 18 gauge steel wire (I didn't have larger wire) and pushed it through and scraped it on the sides of the hole. It seemed to clear out more stuff. Once it was reassembled, the furnace burner came on. So this solved the problem with no parts and no expense for a furnace repair person.