well as vince to said it generally better to change both but I dont really think you have a solenoid problem , I think you have valve body or presure manifold sensor problem ,
The code you mentioned sets when the presure manifold sensor doe not get the hydraulic signal expected from the solenoid , The other possibility is the presure manifold sensor is bad and just not sending the right signal , This type stuff is anoying to diagnose sometimes in a shop , typically you replace all the electrical possibilites first try , Manifold sensor,solenoids in a single operation , but realizing cost is a concern i can under stand one thing at a time .
Next I would do two things , I would pull the pan and replace the manifold sensor but while I was in here i would also remove that solenoid again and use a small screw driver to stroke the valve behind it and mak sure it will move ,
Im sorry your having a problem but I will be here if you have any questions all weekend

 

well, here's the deal. im going to go ahead an put it in my buddies shop an let him fukkk with it, i also have a rear main seal leakin oil everywhere, so while he's in the i guess im going to go ahead an start moding the motor, probably just cam an pushrods internally for now, then externally probably get headers an a catback exhaust. he says this is the 3rd a4 hes had the same prob on f-bodies within the last 6 months i cant believe this is that common of a problem. so guess its just time to open up the check book, pull out some credit cards, an visit a few atm's......