So I got a truck with 290k on the clock about 2.5 years ago and have had a stubborn P0401 code. I finally found and fixed the last issue with the system.
I had two clogs and one restrictive VSV. Identifying them was a bit of a challenge and thought I'd share this experience.
The first clog was with the transition tube that runs through the upper intake allowing the VSV to be linked to the EGR valve. This is a pic from the topside
On this one I used a 1/16" drill bit and just hogged it out. This was the first clog so I thought I was done. At this point I attached a section of hose and drew air through it. It passed air but was restricted so I swapped in a VSV which proved to flow air better.
Still no love and the P401 came back on.
After a few sessions of testing the control side of things I decided to check the vacuum lines going to the modulator from the throttle body. The middles line was clogged solid. This next pic shows the top view and the view from inside the throttle body.
After two hours of trying to unclog that line I gave up and swapped in another throttle body. I cleared the code and took it for a two drive cycle run and the error hasn't returned. This is a route that I've used several times to test the system and until today the code always returned.
Both of those spots are still clogged on mine. Ended up disabling EGR..
When I did try to drill out that transfer pipe the bits I had wouldn't go all the way through the manifold, and it wasn't worth pulling the mani just to get the EGR working.
I had exact same blockages. Took a fine 90 deg pick and lots of throttle body clear to get the throttle body fixed, and ended up breaking off a bit in the pass thru and just bypassed around with Vaccuum hose. Have been p401 free now for 5000mi
The P0401 code sucks. My rig had the code for two years then wouldnt pass smog. I did the diagnostic checks on every piece and ended up replacing all the hoses, the egr valve, modulator and vsv. I used some wire to clean all the small passage ways out.
I took a guitar string (.032") and fed it down that hole in the top of the manifold and it stopped at the bottom. How much farther should it go? When it stopped at the approximate bottom of the passage way I assumed that it might be getting jammed in the elbow at the bottom. Now I don't know... But I've had the damn 401 code since I've owned the truck, four years. All EGR parts have been replaced.
Not to hijack...just to add..... Today, I found another area that was clogged. The "filter" hose I think someone said in another thread. It sits right beside the PCV hose where it connects to the intake plenum. I used a 15/16 socket to remove it then used a stiff wire to break loose the gunk. I then sprayed cleaner until it was coming out other side clean and clear. I also sprayed lots of throttle body cleaner all thru the plenum as far as I could thread the small red straw from the spray can. I have been having the PO401 code show up each day but nothing showed after driving about 40 miles after the above cleaning. Well see what happens tomorrow.
I took a guitar string (.032") and fed it down that hole in the top of the manifold and it stopped at the bottom. How much farther should it go? When it stopped at the approximate bottom of the passage way I assumed that it might be getting jammed in the elbow at the bottom. Now I don't know... But I've had the damn 401 code since I've owned the truck, four years. All EGR parts have been replaced.
I just came across this post amidst all the p401 code threads. I had mine start setting off the code right after adding Techron fuel system cleaner for the first time (as several others he had happen with fuel cleaners of multiple brands). Due to the timing it seemed like either it would of either moved some deposits to create a blockage or fouled a sensor. Going through some of the common fixes (before finding this thread) I tried the charcoal canister carbon refill and the Dorman solenoid with no improvement. After reading this I found that cleaning out the middle port on top of the throttle body which would not pass air as posted seems to have fixed my problem (I tried the port by the air temp sensor first with no changes). I just went from Cincinnati to Gatlinburg, four wheeling on the "Hurricane Creek" trail up to Max Patch NC, drove around the city and mountains all week and only had the code come on once just after arriving on the original trip down. Reset it, did all the driving and back home with no other issues (so I might need to clean it a lititle more). This is the first time I've not had the code come on for more then 20 miles since it started so very certain this is the problem area for my issue. I just used a short length of rigid single strand stainless steel wire we use for chimney liners and a needle nose pliers kept close to the opening to force it into the port and get it to break through. I might try adding some intake cleaner into the port to try and clear it out better as well after the initial result.
