I just switched my program from the 91 Performance tune to the fuel economy tune. The first drive after the change was this morning to work. After about 15 miles, the engine light came on. I ran the diagnostic scan and got an error code of PO405, EGR Position Sensor Circuit Low. I deleted the message and re-installed the Economy tune. Hopefully this will solve the problem, but I am probably dreaming. I can not drive my Jeep for another 5 hours, so I can not find out if that worked or not. Is the error message a coincidence to the program change, or could the program change have caused the code?

 

Here is a list of possible causes for the code you have. Most deal with wire issues.

Most people have found the EGR valve has gone bad.

The last item, the PCM, is unlikely unless, like you said, playing with the program triggered it.

Possible Causes
(F856) 5-VOLT SUPPLY CIRCUIT SHORTED TO GROUND
(F856) 5-VOLT SUPPLY CIRCUIT OPEN
(K34) EGR SIGNAL CIRCUIT SHORTED TO GROUND
(K34) EGR SIGNAL CIRCUIT SHORTED TO THE (K900) SENSOR GROUND CIRCUIT
(K34) EGR SIGNAL CIRCUIT OPEN
EGR SOLENOID ASSEMBLY
POWERTRAIN CONTROL MODULE (PCM)

 

I reset the code and reinstalled the economy setting with my Superchips. No code thrown on the way home, so I am hoping it was an adjustment thing from switching programs.