The shop manual just says to clear the DTC with the scan tool then start the vehicle and force a scan to confirm the repair. This relearn may also be necessary after cleaning.

A good first order test on the MAP sensor is key on engine off and compare the baro pressure to the local weather data. ALso check the quality of the 5v ref voltages.

Exhaust back pressure is with a gauge in an upstream O2 sensor port.

I did not list the causes in any particular order and I dont work on enough trucks to give you a perspective.

George

 

At this point I'm just ordering an egr valve. I'd sure like to end up with a definite conclusion to the P0404 saga. The problem is widely reported everywhere but nobody has come back to say what fixes it. If the new valve doesn't fix it, we can move on to the tedious pcm testing....

 

Some problems are hard for the diy mechanic because of the lack of a bidirectional scanner that can command various testing conditions and monitor live status, a cornerstone of diagnosing problems like these. This is compounded by some signals being essentially AC and much harder to measure/monitor. Then of course there is the involvement of an engine computer with class 2 serial communication data, etc. Most of the repair guidance by GM assumes the existence of such a factory scanner.

Keep us posted.


George

 

Ended up ordering a cheap EGR online since it was mainly for testing. Installed it this afternoon then went through 4 different start and run cycles over about 4 hours...my usual trips to the farm, store, etc. With old EGR that would have put P0404 in pending faults during first run, and turned on the check engine light during 2nd or 3rd run. With new EGR the light never came on and there are no pending fault codes. So I'm going to say that a new EGR fixed my P0404.

Now for details on test results, if you're interested....

Confirmed the red wire will test 12v until its plugged into the EGR, then it goes to 0. Normal.

Ground and 5v reference tests passed same as old EGR (good).

Where the fault seemed to be was that pintle position signal (sensor?) on the brown (center) wire. Old EGR was .67v when closed to 4.9v fully open. New EGR is .77v when closed to 4.6v fully open. According to what you shared from your friend, this could clearly cause the "out of range by 10% or more" setting the P0404.

So maybe the key to testing the P0404 is really that pintle position signal. Hopefully this will help somebody else. Going to run it a while to make sure code doesn't come back, but think this problem is solved by replacing the EGR. I do need to buy a better quality one, I seriously bought the cheapest of the cheap lol.

 

Thanks for the update. Glad your fixed.

George