Thread: Need help fixing EGR code P0401!!!

HI! I'm new to the forum but I usually read around to find some helpful tips to work on my car. And now I'm in need of some advice to get rid of a nagging problem I'm having.

I have a 2001 Grand Prix GTP, and it ran great until I started noticing coolant pooling around the fuel injectors. Seeing as how I work part time at a mechanics shop, we replaced the LIM gaskets and injector o-rings, and problem fixed. That was until a few days later when the SES light came on and I pulled P0401 EGR INSUFFICIENT FLOW DETECTED. I was sort of confused because I didn't even mess with the EGR valve, in fact, I cleaned out the passages. Trying to find the root of the problem, I discovered a small kink in the tube coming from the EGR valve going into the lower manifold underneath the throttle body and replaced it with a new one and cleared the code. All is fine until I get the same code again! And so begins the hunt...

After a new EGR valve + gasket, a new MAP sensor, new vacuum lines and a PCV valve (i figured it would be a vacuum leak), supercharger gasket, throttle body gasket... I still keep getting the code! So I figured it was time to bring in the heavy stuff.

I checked the manifold vacuum with a gauge: approximately 17-18 "Hg at idle. I thought it was sort of low but the head mechanic said it was fine. I checked vacuum at all points where a vacuum line connected and they all measured around the same amount of vacuum. I sprayed carb cleaner on all the exposed vacuum lines as well as around the LIM gaskets and the upper gasket. So there is no vacuum leak or restriction. Next we checked the exhaust backpressure and it's fine also. Reading the data from the MODIS scanner, the MAP sensor showed a BARO pressure of 26.6 "Hg, and thats about right for the altitude here so I can conclude that the sensor is reading correctly. I tested the linear EGR % and went all the way to 100% and the engine stumbled but never died. The MAP sensor reading at idle was showing 8.6 "Hg and a voltage around .44-.47 volts. I thought this was low also as I had read in a service manual that the MAP voltage at idle should be anywhere between 1-1.5 V. Seeing as how the voltage should be higher than what it currently shows, that indicates that the vacuum is actually HIGHER no: Lower output voltage= higher manifold vacuum, higher output voltage= lower manifold vacuum.

So I checked the voltage at the output wire on the harness with a multi-meter and it also reads around .44-.47 V and a healthy 4.98 V supply. I know now that it's not a wiring problem. I then switched the MAP sensor with one that I know works to rule out a defective sensor and it showed the same pressure and voltages. Whats even more weird is that the readiness monitors show READY for the EGR even though the code is still present!

I'm not completely throwing out the vacuum leak possibility but I don't even know where else to check for a leak. The engine runs just as smooth as it did before the fixes to the gaskets and stuff and the gas mileage is still great (about 26.6 mpg). I'm really getting frustrated with this and also worried because my state inspection is due and it will not pass with the SES light on (even though the readiness monitors all show READY). I appreciate any input you all would have on this and hope that someone has another idea of what to look for or check. Thanks in advance!!!

did you ever figure this out? i just did a lim on my car and 400 miles later i have the same issue. also, possibly related the car will sometimes not start right away, it runs a second, dies and restarts, sometimes i go through this 3 times. once it is running, it's fine, but this is driving me nuts.

i did actually figure this out. What it ended up being was the bypass solenoid on the vacuum line in-line with the fuel pressure regulator and the line that goes to the rear of the engine to the MAP sensor. The valve i guess is supposed to vent some vacuum but it kept it high. I replaced the valve and cleared the codes and it never came back again. What was happening to throw the code is that when the EGR valve opens, the PCM is supposed to see a certain drop in manifold vacuum because of the vacuum leak the open EGR valve caused. When the PCM doesn't see this drop, it set the P0401 code EGR INSUFFICIENT FLOW DETECTED because it assumes that there isn't enough flow since the vacuum detected by the MAP sensor wasn't sufficient.

Ok. I'm having the same issue on my 01 GT. Replaced the EGR valve over the summer and the LIM gaskets in the winter.