Subaru Repair: Reoccuring P0400, legacy outback, lab scope

Question
I have 1996 Legacy outback that has a p0400 EGR flow malfunction code that just keeps coming back.  I have tried replacing the egr valve, the transducer, the solenoid, I even cleaned the egr tube of carbon but it still comes back.  when I changed the battery it did go away for 5 days before returning. I am starting to wonder if the computer is going on the car.  It wasn't taken care of from the previous owner.  The price was great but had to replace the gas tank, front oil seals, and the oil pan gasket.  I did flush the motor when I first got the car and found the oil to look like coffee that had been brewing for three days.  I hope you can point me in the right direction for this.  The car otherwise runs fine and gets around 25 mpg

Answer
P0400 is a conditional code note a componet code, meaning that the conditions have been met to set p0400 but not that the EGR is faulty as they rarely fail on a Subaru.You have to look at what the ECM uses to determine if the EGR valve is open or not.In your case depending on wheter its auto or manual the ECM uses a combination of the upsteam O2 sensor,switching silenoid valve and pressure source sensor.When you disconnect the battery or clear the codes you also reset the emmissions systems monitoring devices and the vehicle has to go through a drive cycle before the EGR monitoring system comes back on line and the ECM can monnitor EGR function.I understand that you want to try and fix this yourself but conditional codes are tricky to diagnose.You have to determone first if EGR function is good if so you have to look at the entire monitoring system.The problem here is that unless you own a lab scope and know what the  wave form signal from the upstream o2 sensor looks like when the EGR valve opens you really are only going to throw parts at it.The EGR system in your car is designed to lower NOX a substance known to kill cells in plant life that would normally create oxygen.There is only so much that you can do yourself when it comes to diagnosing emmissions related issues.I hope this helps and please get it fixed