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P401 Code - What's left to check ?!

1997 Ford F150. 6 cyl, 4.2L, 138K miles. (Still) keep getting back the P401 code. I’ve checked (and re-checked) and cleaned all the ports and checked connections on the EGR valve, DPFE sensor and EVR solenoid. Everything seems to be working OK. Have replaced the EGR valve, DPFE sensor and EVR solenoid and (still) after clearing the code and driving ~30 miles, I keep getting the CEL light pop back on and P401 code popping up. Is their anything else to check ??

You probably need to replace the DPFE sensor if you keep getting this code. Simply checking the connections is not enough.

I’ve replaced the EGR valve, DPFE Sensor and the EVR solenoid ( all 3 parts). To my knowledge, there’s no component left to replace - is there ??

At this point I think it needs to go to a shop that can properly diagnose this. Either one of the parts you already replaced is faulty, you have a wiring problem, a vacuum supply problem, or an obstruction in an EGR passage somewhere. These things are all easily tested and verified as good or bad with the right equipment. What do you have to test at home?

I know, it sounds like I’m not helping you at all, right?

With a vacuum gauge, test for good vacuum–at least 18"–at the EVR.

With a voltmeter, with the engine running at idle, test for 12-14 volts at both wires at the EVR. If you can, drive the car and test for loss of your 12 volts at one of the wires, the other should stay constant. Better yet, drive the car with a vacuum gauge attached to the EGR valve hose and check for vacuum while you’re driving.

With a voltmeter, engine idling, test for about 5 volts at one wire of DPFE, less than .10 volts at one wire, and something in between at the other. Make sure this voltage is steady and does not fluctuate when the engine is idling. Now with a vacuum pump open the EGR valve and see what this voltage does. It should change significantly.

If it doesn’t change significantly or doesn’t make the engine run quite poorly at idle you have an obstruction somewhere, possibly in the EGR pipe.

Actually you’ve helped alot. I think the easiest (and cheapest) thing to do at this point is to disconnect the main EGR pipe and try to clean it as far down as possible. This was really the only thing I’m suspecting since it was difficult to clean the first go around and this is sounding like it’s a likely suspect for obstruction further down the pipe than I may have gone the first time.

You’re going to have to remove it, there’s an orifice in there–a small hole–that is designed to create a pressure differential in the pipe that the computer uses to detect the amount of flow through the pipe. Hence the sensor is a DPFE “Differential (or delta)Pressure Feedback EGR”. Anyway, I’ve seen those plug up with carbon. I usually just replace them, but then again I have to warranty my work.

Good luck.