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P401 code - HELP!

1997 F150. 6Cyl, 4.2L, 139K miles. I am totally stumped and hoping someone can help me. The truck threw a P401 code(EGR Flow insufficient)and won’t pass emmisions here (due by the end of March). So far this is what I’ve done : Replaced EGR, replaced DPFE, replaced EVR solenoid, removed throttle body manifold under EGR and cleaned all ports (they weren’t even dirty), cleaned and checked all vacumn lines from the DPFE to the main EGR tube from exhaust. Also cleaned the main EGR exhaust tube. Everything is squeaky clean. All voltages going to the DPFE/EGR are spot on and the whole system seems OK. But the P401 code literally comes on almost immediately (less than 2 miles) after clearing the code (I hooked up my scanner to see how long). I read thru another forum that a bad coil could cause this code, but I never heard of tha ??. HELP !!!

Have you verified whether or not the EVR solenoid is actually being activated and is actually pulling vacuum? One vacuum connector on the EVR is attached to engine vacuum. The other goes to the EGR valve. All its doing when the solenoid is activated is allowing some amount of that engine vacuum through to the EGR. Its that application of vacuum that activates the valve allowing the flow.

You didn’t mention it specifically so I’ll just ask - have you verified that voltage is making it to the EVR solenoid? Then you want to verify that the EVR gets vacuum from the engine (perhaps that supply line/port is clogged, broken, etc). Then you want to verify that vacuum is making it thru to the EGR valve when the EVR is commanded on. Obviously you use a voltmeter to check for the voltage. An easy way to make sure all of the vacuum stuff is in order is to just tee a vacuum gauge in at the EGR valve. At idle the vacuum to the valve should be at or near 0 in. If the engine is warmed up and hits about 1500 rpm or above you should see the vacuum get applied (and probably even hear the EVR valve operate if your head is stuck under the hood).

I’ll go back and check the voltages you mentioned as I don’t think I checked those. Thanks.

For the P0401 I think the more likely issue would be the vacuum. The easiest way to check the lines is just to pull them off, plug one end (with anything, even just pressing your finger on it) and put vacuum on the other. I have a hand vacuum pump but for this you can just try to suck through it like its a straw. Obviously it should hold vacuum. To check the port, the best thing to do is put a vacuum gauge on where ever that supply line connects and you should get full engine vacuum there. If you don’t have a vacuum gauge you can just pull it off while the truck is running. If the port is open sufficiently the truck should start to run like crap (surging, maybe stalling).

It wouldn’t hurt to clean the MAF sensor.