Code p0401

Hello,

In the diagrams down below I have included the factory diagnostic troubleshooting guide for your vehicle for DTC P0401. Please go through this guide and get back to us with what you are able to find out.

Thanks,
Alex
2CarPros

Hello Alex,

Thank you. I went through the diagrams guide you provided. I don't currently have any MAP specific DTC's. I could test the MAP sensor when I get the chance although it's likely that it's working correctly. All ports are clear of blockages and from what I can tell, free of leaks.

I took some more voltage readings while driving yesterday on the EGR position signal wire (C). When the computer should be commanding EGR my .79v should increase. Instead, I noticed that it's dropping down to a .6 to .7 range (it's not a steady .79 as I previously thought). A couple times it managed to drop to .5v. Does this sound like a ground fault? And where would I start? Battery terminals are tight. Could it be corrosion/loose connection on the wires feeding the PCM?

Something I just noticed is that pin E only shows 12v when it's not plugged in. If I plug in the connector it drops to .02v. For whatever reason I don't remember it being like this. Unplugged the connector and used my test light on it and I don't have enough current going through this circuit to light the test light. Would this further indicate a ground issue? Perhaps corrosion on the ground wire the PCM is using for this 12v supply?... I don't have a wire diagram for this car. All the wires that go to the PCM go into the side of 2 plugs (as opposed to straight back where you can see which wire goes to its corresponding pin).

Hello again,

Pin E on your vehicle's EGR Valve connector is for the EGR Valve Control from the PCM and it is going to a coil inside the EGR Valve.(See the attached wiring diagram in the diagrams down below). If the voltage is dropping down to 0.2V, that is the PCM controlling the voltage output. Also, the previous guide for DTC P0401 is from Chevrolet for this DTC so follow the guide, and you will fix the problem. In it's description it states that it is not only the EGR valve, but the relationship of the MAP sensor to the EGR Valve on deceleration that is the problem. Please go through the guide and get back to us with what you are able to find out.

Thanks,
Alex
2CarPros

Alex,

Sorry for the delay. I've actually been sick. I plan on working on the Impala here soon when I can. I plan on looking at the MAP thoroughly hopefully near the end of this week. Just thought I'd give an update since it's been a while.

Hello again,

Thank you. No worries. We are here for you whenever you get around to it. We are not going anywhere, and will be here when you are ready.

Thanks,
Alex
2CarPros

Update:
Okay, so I tested the MAP. All passages for the MAP are free of obstructions and there are no leaks. The MAP sensor itself tested out good as well. I checked signal voltage while applying vacuum. Here are my results:
0" Hg: 4.7v
5" Hg: 3.8v
10" Hg: 2.9v
15" Hg: 1.97v
20" Hg: 1.04v

Also, I have some new DTC's as well as some old ones. p0401, p0403 (EGR control circuit), p0405 (EGR sensor "A" circuit low), p0440 (evap control system), and p1404.

It's likely that I pulled the EGR wire harness with key on since the last time I've cleared the codes which could have triggered the 403 and 405 codes. The hose from the PCV valve to the evap solenoid was in bad shape so that probably had something to do with the p0440.

As for the p1404 I'm not sure why that one came up. It seems relevant but with the other 4 codes I'm concerned it may be a false positive. I plan on clearing the codes tomorrow, doing more tests, and driving it around for a bit to see what comes back.

Diagnostic steps taken:
1) Yes, scanned for faults
2) No MAP DTCs
3) Exhaust system inspected. No modifications or leaks.
4) EGR valve gasket is new (no leaks)
5) EGR valve was replaced due to faulty potentiometer.
6) EGR inlet pipe removed and inspected. Ports and pipe are clean and free of damage.
7) Cleared DTCs. Check engine light came back on after less than 5 miles.
8) Only DTC present is p0401.

Wire harnesses appear to be in good condition. No DTCs triggered when moving or tapping wire harnesses.

I don't know if it's a fault of my reader or what but I'm not able to access EGR desired % and EGR error data. I'm using an OBDLink LX. I have that data as an option, however the Impala doesn't seem to be reporting it. What IS known is that the signal voltage for the EGR pos is not increasing during decel as it ought.

I don't know what the fail count is for the p1404 but I plan on leaving the p0401 alone for now to see if any other DTCs pop up. A p1404 seems like it would be appropriate for this problem. According to the info on this site (under "p1404 Chevrolet Meaning") I should expect the p1404 to come back: https://www.engine-codes.com/p1404_chevrolet.html
But again, there's a possibility the p1404 could have been triggered while testing with the key on.

Hello again,

Sorry for the delay. In the diagrams down below I have included the factory diagnostic troubleshooting guides for DTC's P0403, P0405, P0440, and P1404. Please go through them and get back to us with what you are able to find out.

Thanks,
Alex
2CarPros

Hello Alex,

So because my scanner doesn't have bi-directional control we took it to a shop. Mechanic was able to confirm that the brand new EGR I installed actually had a bad potentiometer as well (although it failed in a different way than the previous one). I then removed the new valve and exchanged it for another. But before installing it I thought I should do at least one last inspection of the ports. I managed to find a plug in the air intake side that I had previously overlooked (I should've known). The EGR side of the port was clear but the port goes down and then comes back up at a steep (perhaps 60 degree) angle. Apparently a perfect angle for collecting deposit! Had to remove the throttle to properly see and remove the clog. After that, the Impala was able to pass its emissions tests.

Just want to say thank you very much for your time and expertise! I don't know if there's a way I can compensate you a little, but if it's possible please let me know.

SCWICKEDSTANG is one of our best! We don't expect donations for our work currently. The only think we ask is that you come back to let us know what happens to fix the car. Please let us know what you find. We are interested to see what it is.