Cannot fix P0401 / P0402 codes on 96 Sport 4.0 OHV | Ford Explorer Forums - Serious Explorations

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Cannot fix P0401 / P0402 codes on 96 Sport 4.0 OHV


June 15, 2014
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City, State
Chandler, AZ
Year, Model & Trim Level
1996 Explorer Sport 4x4
I recently purchased a '96 Ford Explorer Sport 4x4, w\233,000 miles on it.
Took it to emissions in order to register and title it, and it had 4 trouble codes,

P0401 insufficient EGR flow
P0402 EGR flow excessive at idle
P0340 camshaft position sensor circuit malfunction
P0420 catalytic converter efficiency below threshold

plus the MIL (Malfunction Indicator Light) would not turn on.

They gave me 2 months to complete the repairs and retest without paying another $27 to Arizona DOT.

Researched on the forum, and did the following:

Replaced DPFE sensor (mine was the older aluminum type, got a plastic one on eBay, $26) for the P0401/P0402 errors. Still got them after installing it, went on to the next codes.

Replaced camshaft position sensor and synchronizer, got an aftermarket CMP sensor and Motorcraft camshaft synchronizer (normally my quest for cheap parts would have led me to a $35 aftermarket synchronizer on eBay, but read too many problems with the cheaper aftermarket ones, so went with the Motorcraft one DA-2099 - $105 on eBay). Quite a job with some friends getting that done, the truck moved on us slightly while putting in the synchroniser, moved about 2-3 degrees past TDC on the crankshaft, tried to guestimate the slight forward motion of the synchroniser flag and seated it - and it started right up after completing the job, no P0340 code anymore.
BTW, the CMP sensor metal was completely missing on the old CMP sensor, and the camshaft synchronizer flag was twisted and bent badly on the old synchronizer.

Replaced both upstream 02 sensors (2 SMP brand for $26 on eBay) and ran a gallon of pure lacquer thinner (before the sensor replacement) per Scotty Kilmer, a youtube mechanic guru. The thinner didn't fix the P0420 code, but thought it was worth a try. Replacing the upstream O2 sensors DID fix it.

Replaced the check engine light in the instrument panel, by swapping it with the annoying ABS light that was on constantly. While I was at it, decided to swap the instrument panel backlights with some new LED lights. Found some great articles for this on explorerforum, chose blue 4-LED lights - looks AWESOME.:D

After all this was done, all the codes went away, except for P0401 and P0402 codes.
Looked for damaged hoses going up the to DPFE sensor, they appear OK.
Checked the EGR valve, it would kill the engine if we applied vacuum to it.
tested the amount of vacuum that was getting put to the EGR valve from the EVR / EGR solenoid, and it was very minimal, would only occasionally flicker off the zero vacuum line. A mechanic friend recommended replacing the EGR solenoid before doing a whole lot of electrical testing, might be a quick fix - did that, $30 at autozone, but still getting P0402, P0401 codes.

Drove it for about 25 miles after replacing the EGR solenoid, it only threw the P0402 code that night, but later next day, it showed both P0402 and P0401.

Not sure what to try next, is getting a bit exasperating.

If I don't get it fixed soon, (maybe too late already . . :(), I will have to pay ADOT another $27, but at this point, with $250 into the repairs already, I guess thats minor . . .

FYI, the engine seems to run good, to my untrained ear. During the camshaft synchroniser replacement, put in new sparks plugs and wires (HUGE wear gap on the old spark plugs). Idles at around 900 RPM.

Truly am in awe of the knowledge and experience on here. As a mechanic wannabe, this forum has given me the courage to tackle jobs I would normally have considered beyond my skill level. Thanks for any help you can give!!

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Did you check that the hoses going to the DPFE are not clogged and not, by some chance, reversed?
BTW: I wouldn't read too much into not seeing any vacuum applied to the EGR valve. The valve is only commanded to open under certain conditions, which you probably wont be able to duplicate by just idling in the garage.

Replaced DPFE sensor (mine was the older aluminum type, got a plastic one on eBay, $26) for the P0401/P0402 errors. Still got them after installing it, went on to the next codes.

Drove it for about 25 miles after replacing the EGR solenoid, it only threw the P0402 code that night, but later next day, it showed both P0402 and P0401.

Its possible the EGR is sticking so it takes excessive vacuum to open.

If sticking, it will be delayed in opening tripping the inadequate flow error and then slam wide open and trip the excessive flow.

Try pulling the valve and cleaning it.

Thx 1998exp, I took off the hoses connecting to DPFE, they both look in good order and were not clogged. They weren't brittle either - maybe recently replaced?
They are connected to the plastic DPFE in the same order that they come off the EGR exhaust pipe, is that the correct order?

SoHK Alumni - will try removing the EGR valve today, already have a replacement. Sprayed with PB Blaster last weekend and this morning, hopefully can get the nut to move on the EGR exhaust pipe!

Got EGR valve replaced what an ordeal!

Got the EGR valve replaced finally. Tried on my own wrenching it, couldn't get it to budge. Took it over to a mechanic friend of mine, he ending up using a die grinder and an air chisel, but we finally got it off!! :)
He cleaned up my EGR-to-plenum hose, put on an additional rubber grommet, some electrical tape, and some black RTV to try and seal the EGR supply tube to the intake plenum.
He also recommended replacing the EGR inlet tube coming up from the exhaust, dorman part # 598-138. Said it was only a matter of time before the new EGR valve seized on the old tube.

Cleared the trouble codes, will now test - will post results after driving tomorrow. Thx again for all your input!

toypaseo - they look like standard black hoses - if the codes come back today, will look at replacing those too.

Well, the codes came back today after replacing the EGR valve. Did a live data OBD2 scan with the Forscan program on the DPFE voltage, it was fluctuating between .68v and 1.16v, I think thats in the normal range. Still at a loss what to do next, my mechanic friend thinks we should look for possible issue with the throttle position sensor or a bad ground in the system.

P0401, P0402 codes still coming back

Still getting P0401 code after driving for awhile, P0402 comes later.
Ran live data scan tool, it shows the EGR working for a time, then faulting.
If I turn it off and turn it on, it will work for a time, EGR fluctuating normally, then faulting after a few minutes. Stays faulted for awhile, but will sometimes go back to normal operation. When cold, it takes longer to fault than when warm. Wanted to see what realtime data I should monitor to find a connection to what is causing these insufficient & excessive+ EGR flow codes. Currently I am trying to monitor

EGR duty cycle
EGR fault status
DPFE voltage
EGR vacuum regulator (EVR solenoid?) fault
Throtte Position rate
Hard acceleration signal (ARC)

If anyone has any other ideas on what to try, would love to hear them. I may have to take it to a mechanic if all else fails - Thanks in advance!

Found a great website for testing components of the EGR system,
Tested the new DPFE sensor I installed, and was not able to get it to go above 1 volt on the top #1 wire on the DPFE connector when applying vacuum by mouth to the EGR valve.
Well, I guess the $25 DPFE sensor I bought on eBay may be no good. Will pickup one from autoparts store tomorrow and test.

No more codes!!!

The replacement DPFE sensor from Autozone works fine, no more P0401/P0402 codes! I guess I couldn't believe a new sensor would not work correctly. Lesson of the day : Don't buy the $25 DPFE sensors from eBay, go with the $60+ DPFE sensors for best quality. :D
Went and passed emissions with no problems, now on to registration and title!