Navien Intake Air Problem

I have a CR-240 that is having an air intake issue when I put on the cover. Without the cover, the unit works fine, but once the cover goes on, I get the error that means the system is not betting enough combustion air. There are only 2 45's and a 90 in the 30' run, so we do not have too many turns or too much length.
Has this happened to anyone else?

Blocked outdoor intake screen
or
No screen on it and animal crawled in and died inside intake pipe
or
Sagging pipe install holding water
or
Reversed vent piping
or
Blocked intake screen built into intake collar on heater, above premix burner fan

The Navien is a learning unit. If you take the door off and then turn it on, it will learn that environment. What I have done in the past is unplug the unit, make sure your dip switch settings are correct and the power switch inside the unit is on. Install the door and wait 10 minutes for the unit to completely drain power. Plug the unit in and hit the power button on the controller. If you have an A model with recirc and buffer tank and you have a V2.0 control board you will need to turn the recirc on at the controller, not just by dip switch. Should take care of any intake air issues. I live in Denver, Colorado and with our altitude we have run into these issues alot. Let me know if that helps.

What exactly does the Navien learn and how?

Blocked intake screen built into intake collar on heater, above premix burner fan

I have the same model and the same error message came up twice in the time we moved into our new house since Sept 09. Both times it was the internal intake filter was dirty. The manual shows how to access and clean the filter. Works fine after its clean.