Furnace runs OK when the vent pipe is removed. As soon as I attach the vent pipe the unit shuts off. I checked the pipe it's not clogged.

What size is the vent, 2 or 3 inch? How have you checked the vent pipe? Do you mean you have cut and visually inspected every inch of the vent? What is the error code on the control board flashing? The more info you can give me, the better I can try to help. Is the vent sloped back to the furnace, or is there a sag in the vent that could allow condensation to build? Lots of option here. Lets take it one step at a time, okay?

The vent pipe is 2" and yes I cut the pipe and did a visual check.
The error code is 2 blinks which I guess indicates a pressure valve issue.
I will be replacing the valve tomorrow.

Okay, hope that's the fix. Its called a pressure switch. One thing to check is the condensate drain system on the furnace. Remove and clear all drain hoses and clear the ports they hook to. Check the hose from the pressure switch to the draft inducer for any leakage, as well as any other pressure switch hoses and connections. If the new pressure switch doesn't do the trick, It may be time to call in a tech, as you don't want to just start throwing money into replacement parts when a tech can test to find exactly what needs to be done. To many times I have seen homeowners replace nearly every component in the furnace, then call me just to find it was a simple ten minute fix with no new parts required. Money wasted. Your choice.

Most of the time, I find by installing a 3" vent the problem goes away & stays away. This is especially true if the furnace is 75 kbtu or greater, the vent is horizontal, or getting close to the manufacturer's max for a 2" vent.

I completely agree, Grady.

The problem turned out to be the intake pipe was clogged not the exhaust pipe.
After clearing the clogged pipe the furnace works fine.

Thanks for the update. Glad you got it fixed. And sorry, I should have been more specific when asking if you checked the vent. I should have said vent exhaust AND combustion air intake pipes.