Some help please. I have a 97 1500 4x4 with 300,000 km. I get a persistent check engine light that comes up P0401B-EGR flow insufficient. This has been going on for ~9 mo. I have replaced the vacumn pump, the waste gate solenoid, both egr solenoids, the map sensor, had the egr valve checked and the wiring checked for shorts.
I have seen the light come on at highway speeds without any perceptible change in power, rpm etc. The mileage and power remain the same. The light will come on,stay on for ~2 days, go out and then come back on again. I hope to sell the beast soon and get a 6.6 version 1 so I'd like to solve this problem.
Anyone had a similar experiece? Your input is appreciated.
Thanks
Alex

i would pull the egr valve and check the valve and mainly the upper intake plenum for soot clogging. the nature of the egr is exhaust being re-routed into the intake. when i replaced my intake manifold at about 170k mi. (now at 243k) with a "f" intake that doesn't have an egr. the origional one was so clogged that i wondered how it could flow anything. the ports were clogged up to about 1/8th of thier origional size. if you have checked everything else externally thats what i would check next. kennedy has some info on re-grinding the upper plenum on his website that may help if you want to do more than just clean it out.

Sounds bunged up with oily soot and gunk to me. Should take less than 15 min's to pull the upper intake off and have a look, no parts required, if you peel a gasket in proccess some RTV will fix that up.

RTV won't seal center, need to get the metal doughnut gasket from GM or you will leak between upper and lower intake manifold.

Have you checked the vacuum to the egr solenoid. The lack of return signal to the ECM seems like the only source of error as there is no component actually measuring egr flow. It should be getting that error code from the solenoid not signaling an open or closed position when expected.

Did you measure voltage on the plug that goes to the EGR solenoid? There should be 12V at idle between the two pins/hoses in the plug. Your sympthoms can be a broken wire contacting intermittendly.