CEL Code P401


1997 Ford F150. 6cyl, 4.2L 138K. Have CEL on and code reads: “P401 - EGR Flow insufficient”. I replaced the EGR valve and checked the ports leading into them and they seem clean. There is one additional component called “EGR feedback sensor” which I really don’t want to replace unless I can determine that this is faulty. Any ideas ?


The ports “seem” clean doesn’t sound good to me, but if you do make sure of that then yes I’d move to the rest of the system. One simple test of the EGR is just to put a little vacuum on it with the truck at idle. You can actually just do this with your mouth (but briefly and don’t breath it in). If you put as much as about 2in of vacuum on it it will start to open and the idle will falter. Go to about 8-10 inches and it should stall the truck.

Then there are actually two additional components. One is the DPFE sensor and the other is the EVR solenoid. The DPFE sensor feeds exhaust pressure info to the PCM which it uses, along with other stuff like engine temp & rpm info, tells the solenoid when to fire up.

If you have a voltmeter you can easily check the DPFE. I forget the actual specs, but they are all over the internet. Checking the EVR valve is fairly simple. Its hooked to engine vacuum on one port and to the EGR valve on the other (follow that vacuum line off of the EGR). It should apply approximately zero vacuum at idle. It should apply some if the engine is warm & hits about 1500rpm. If you put 12V to the solenoid it should go to full vacuum - do it with the truck running & the solenoid hooked to the EGR and it will stall the truck - or at least it will stumble very badly.

This is a very useful link: http://www.engine-light-help.com/egr-valve.html