Definitely have a good look at all your vacuum lines. Vacuum leaks will throw this code because the PCM as stated, commands the valve open on deceleration while running the diagnostic, and expects to see a loss of vacuum as reported by the MAP sensor. If you are already leaking vacuum somewhere else, opening the EGR won't cause enough of a change and you'll get a P0401.

Vacuum leaks will cause a lot of diagnostic issues on modern automobiles. Been there, done that... the hard way.

Since the car is running good it has been low priority but I plan on looking at the lines around the map sensor this weekend.

Ok, Got it fixed. Turned out it was one of the vacuum lines I replaced when I did the LIM. All the connectors on the lines were brittle when I did the LIM and I could only find a universal T for the three lines that connect in front of the SC. One of the lines had slipped off one ridge on the connector and must have been leaking only slightly. So after jamming it very hard over the ridge of the connector the code went away! I may go back to the factory connectors as it is a cleaner setup but all is well now. Focusing on the MAP sensor and working back through the lines did the trick. I would recommend looking at the lines connected to the MAP sensor on the P0401 code troubleshooting section (https://www.gmforum.com/obd-ii-trouble-shooting-153/p0401-284082/) and also, the 9 volt battery trick to check if the EGR valve is working (http://www.wellsmfgcorp.com/video_egr_hq.html) and also put the link http://gearchatter.com/viewtopic9016...f22f51cf703f80 that Bill Boost provided in the troubleshooting section also. Thanks for all your help!

Glad you found it. As I suspected, a vacuum leak. I just had to replace about 3 fittings on my Bonny. The tee from the fuel pressure regulator, the adapter at the SC boost solenoid and the tee in front of the SC where yours was leaking. BTW, I've also had those hard line sections get brittle and crack.

Vacuum is oh so important nowadays. I suggest hooking up a vacuum gauge when you know it'* tight so that you can compare it further down the line to see if you have a leak somewhere. It will immediately tell you if you have a true leak or a MAP sensor that'* lying to the PCM.

GregB

Hi Greg, Did you use the factory fittings when you replaced the lines? I had went to the dealer but they couldn't find the part numbers for the factory fittings. I looked at Rock Auto and couldn't find them and the couple of parts stores I went to had limited supplies of fittings.

The big thing on the info I posted is that...even though you see EGR in the code description, it'* not necessarily an EGR issue.

Good point Bill. When I did search the posts and also found the info on P0401 I didn't find a lot of info on specific troubleshooting steps or specific causes so I was thinking it might be helpful to add more to the doc. I think the site is wonderful and has helped me greatly.

I didn't have to replace the hard lines. They broke right at the rubber fitting and there was enough slack to push them securely into a new rubber fitting. I got the replacement vacuum fittings at the local Auto Zone, but they rotted within a year and a half, and I just replaced them again. I have seen that small hard line in the auto parts store even though it'* not custom bent. You could probably form it using a heat gun if you really wanted to. I've done that with other kinds of plastic tubing.

GregB