V6 Check Engine Light Dilemma

[Dan2002]

After reading every thread there is about the dreaded P0401 CEL ive been getting on my 96 V6, I've come to a dead end. I've replaced the EGR, EGR Modulator, and the VSV is good. The temp. sensor tests good as well. I've taken the intake manifold apart and cleaned out the carbon, as well as replacing all of the vacuum lines. Finally I took a 4.7K resistor and put it in place of the temp sensor, reset the computer, and the code never returned. My question is what is the failure point? Could the temp. sensor be bad and still test good?

[new echo owner]

The Temp sensor hardly ever goes bad.

How did yo do your test on the EGR system?

N.E.O.

[Dan2002]

I did the tests exactly as they were in the repair manual, and I've also done a few tests that were on alldata as well. They have all come back successful which is why im puzzled.

[new echo owner]

Did you apply vacuum directly to the EGR valve with the IAC disconnected to see if it will stall the engine or make the engine run rough?

N.E.O.

[Dan2002]

Yep, I did the test twice actually to make sure I wasnt losing my mind. It didnt stall the engine completely but it did run pretty rough.

[new echo owner]

Did you replace the VSV or just test it? How often does the code set? BTW, which repair manual were you using for the test?

N.E.O.

[Dan2002]

Both actually. The one that was original tested bad, I junked it and picked up a few from a pull-a-part. The rest test good. The blowing in the port test, the continuity test, and applying battery power, they all pass. I was using the 1MZFE engine manual found here http://camrystuff.com/index.php?page=Gen3_Manuals and an alldata pro subscription.

[new echo owner]

So I can assume that you have also check the vacuum ports at the throttle body and check the readings at the ECM too?

N.E.O.

[Dan2002]

I have, the ports are clear, and heres one thing I have noticed. The EGR Gas temp. never reaches anything above about 120F, and according to what i've read for the EGR system to be functional the gasses need to be quite a bit higher, which is what that resistor is telling the computer.

[new echo owner]

How did you get the 120F reading? Does you car have after market exhaust or any kind of mod?

The ECM is looking for a 72F difference between the sensor temp and the ambient temp, if it doesn't see that under the test condition it will flagged it and then turn on the CEL on the second trip.

N.E.O.

[Dan2002]

I have a portable OBDII computer I use for that kind of thing, thats how I saw the 120F reading. No its all stock, no aftermarket parts or mods.

So what could cause a 72F difference not to happen?

[new echo owner]

Assumming the EGR valve is getting the plenty of exhuast flow and the sensor is functioning correctly, afew things come to mind: poor or corroded connections, weak pin tension, or high resistances in the wiring. If I remember correctly, as the sensor heats up its resistance goes down, so if any one of the above is true, it will increase the resistance that the ECM is seeing.

What you can do is compare the resistance at the sensor connector and then again at the pins at the ECM to see if there is any differences between the two readings.

N.E.O.

[Dan2002]

Originally Posted by new echo owner

Assumming the EGR valve is getting the plenty of exhuast flow and the sensor is functioning correctly, afew things come to mind: poor or corroded connections, weak pin tension, or high resistances in the wiring. If I remember correctly, as the sensor heats up its resistance goes down, so if any one of the above is true, it will increase the resistance that the ECM is seeing.

What you can do is compare the resistance at the sensor connector and then again at the pins at the ECM to see if there is any differences between the two readings.

N.E.O.

I'm positive its getting plenty of flow, but I will have to check out the connections when I get a chance, that sounds like a good possibility.

[new echo owner]

Will await your finding!

N.E.O.

[Dan2002]

Ok, my mind is officially blown. Did the test, everything checked out fine. Even got another temp sensor and, as soon as I took the resistor off and installed the new temp sensor and started the car up bam CEL back on right away. At this point I have no idea what else it could be.