Takagi T-Kjr flashing 4 code

I have a Takagi T-Kjr with a 4 flash code after the water runs and stops, I have to hit reset button to get hot water again and it will stay running as long as I don't shut the hot water off but as soon as I do I get a 4 code after the fan purges.
Steps I've taken:
1. replaced pressure switch.
2. cleaned burner and flu according to Takagi tech support. (done it twice)
3. changed switch #6 to on (outside) even though it's inside.
4. cleaned fan

Relevant info:
its a 2 year old LP heater and was working flawlessly until i decided to take out the pressure switch to put in to an identical unit for my floor heat, that is when i started to get the 4 code, even after swapping back the pressure switch and putting a new one in, I still get the 4 code. Any help would be great.

Update:
I tried blowing off the dust from the fan blades again and the heater worked as it was suppose to for about 5 or 6 times then it stopped working and started flashing a 4 code again. Is there any type of a sensor that I am missing that needs to be cleaned ?

Update:
after many days of waiting I finally got an email from a service tech and the suggestion he gave solved it. Make sure the tube for the pressure switch is in the L for low pressure and not the H for high pressure. Bam !!! it's been working fantastic for months.