P0401

Here's what the repair manual shows as potential causes.



It's pretty much what you just worked on. Sometimes people don't realize they need to clean the EGR cooler and they stop at the EGR valve and the EGR pipe. Sometimes people clean the intake manifold but miss the tiny EGR openings that feed each cylinder. You pretty sure you got those?

 

Here's another thread where a mystifying P0401 finally got tracked down.

In that case, it was a problem in the valve, which I think you have eliminated by replacing the valve. So I don't suggest that you will find the same ultimate cause as in that other thread, but you might find some use in the techniques used to chase it down.

 

Did you do much cleaning when you replaced your EGR? Seems like you didn't do enough cleaning?

 

When you replaced the EGR valve was it a newer part revision / kit? Or where did you get the "new" valve from? I believe there was a newer part and it also needed a software update to go with it. This video mentions that as well.

 

I spent days cleaning my EGR cooler and intake manifold. They are clean. I still get P0401 (along with other codes). When cleaning, I removed the top of the valve motor and pushed on the rod of the valve. It did not seem to be sticking.
However, I might have damaged the MAP sensor because I did not think about it being an electronics component. I sprayed brake cleaner and carb cleaner into the intake manifold. It is likely some spray reached the MAP sensor. Do you think that I damaged the sensor? Is there a test that I can do to test it? I have a VOM tester.
Also, one time when starting the engine, it made a clicking, shaking for 30 seconds. P0401 indicates a lack of EGR flow and that would not cause a rough idle.

 

Actually you can have a sticking open egr valve and get a P0401.

“The ECM monitors the pressure inside the intake manifold while opening and closing the EGR valve during fuel cut operation. If there is no change in the manifold absolute pressure sensor value, the ECM interprets this as a malfunction in the EGR valve assembly, illuminates the MIL and stores the DTC (2 trip detection logic).”

There is an egr step test in Techstream that can tell if the valve is sealing or opening by monitoring the map pressure when the egr opens or closes.

 

If you're worried about your MAP sensor, you can start by looking at that reading while the engine is stopped, and seeing how close it is to your local weather station's barometer.

 

Here is the revised egr tsb directly addressing sticking egr valves causing P0401.

 

I “suspect” the MAP sensors are pretty durable; they regularly endure being oil coated, due to PCV inlet in the vicinity. I think I sprayed mine with brake cleaner first time I had the intake manifold off. Did switch to MAF sensor cleaner after that.