Went to install a HYPERTECH POWER PROGRAMMER III on my truck and subject codes showed up.

The truck is a 2004 F-250 Super Duty with the 6.0 diesel. Curretnly DTCs P0404 and P0405 are showing up in the computer. The definitions for the codes furnished by John Patton on this site are: P0404 is "Exhaust Gas Recirculation Circuit range/Performance' P0405 is "Exhaust Gas RecirculationSensor A Circuit Low"

The local dealer worked on the truck about 6 weeks ago. He worked on the EGR and rebuilt the turbo.

I know less than nothing about the codes, etc. I would like a simple definition of the codes and what they mean as far as performance. Also I would like to know under normal circumstance would the dealer have worked on the truck's EGR and then cleared the codes or would he have left them in the computer. The only info I have is that the codes are in the computer. Can the dealers reader determine what date the codes first entered the computer?

With the HYPERTEK I can clear the codes from the computer. Should I do that and see if they show up again or would the best thing be to take it back to the dealer?

Thanks in advance.