Repair an EGR with Device Trouble Code P0405
My 1998 Trooper Check Engine Light came on. I read out the DTC and it indicated P0405. This happened
about three years ago and I repaired it as described below. The problem came back again a few months ago
and I did the same repair.
Background:
The computer (Powertrain Control Module- PCM) commands the position of the EGR and detects whether
the EGR responded correctly. The EGR pintle (the thing that opens and closes in the valve) is connected to
the wiper of a potentiometer. The PCM measures the voltage on this wiper to detect if the pintle is
positioned correctly. See the figure below.
Terminals A and E carry the current to the coil in the EGR that moves the pintle. Terminals B and D are
the ends of the pot. Terminal C is the pot wiper. Physically, the connector is the one mounted on top of the
EGR. Since the terminals A, B, C, D, E are in a line across the connector, there is no problem guessing
which is which. The outer two are the coil, the next inner two are the ends of the pot and the center one is
the wiper.
The Problem:
Both times that I got the DTC P0405, the connection to one of the ends of the pot was broken. The break
was deep inside the EGR.
The trouble shooting information basically guides you to determine if there is a break in the pot circuit and
where it is, inside the EGR (so you buy a new one) or in the cabling. I would start by measuring the
resistance between terminals B and D. The manual says it should be between 5 and 5.5 K ohms. I believe
that should actually be around 10 K ohms. Of course since the PCM is reading the voltage on the wiper,
any resistance value would work. Any value other than infinity (open circuit) that is. Infinity is what mine
measured when it failed. Of course you need to check the resistance from the wiper, terminal C and
terminal B or D in case the break is to terminal C..
The pot is formed by depositing conductive strips on a ceramic substrate about the size of a postage stamp.
The figure below shows the pot.
Note how the right most conductive strip has deteriorated. Somewhere along the right-hand strip there is a
break.
Opening the EGR
Opening up the EGR took some doing. In fact, I made some mistakes the first time which could confuse
this description but I will try to explain as I go along.
To open the EGR I cut a vertical slit in it as shown below. I used my Dremel Tool but a hacksaw should
work also.
To close it up later, I just put a hose clamp around it.
After you finish the cut, separate the can a little and slide the lower part including the flange down out of
the can. The coil will come out with it. When the coil comes out, terminals A and E slide out of the upper
part and stay with the coil. Be careful. Below is a picture of the two halves with terminals A and E labeled.
They are just prongs that you are viewing almost straight on.
The part on the right above comes apart as shown below. I have labeled a few things. “Glob” is a glob of
silicone sealant from the last time I repaired the EGR. “A” and “E” are where terminals A and E were
fitted through two slots. “X” is a part that I had to add. The first time I took this apart I didn’t know how it
was made so I cut out a rectangle in the plastic around terminals B, C, and D. I should not have done that
so I had to fabricate a spacer to replace what I had cut out. That is part X. Instead of cutting out the
rectangle, I should have just pealed back a lip on the “Ring”. The actual lip is just under the front “leg” of
the letter “R”. It goes all around, through the missing rectangle and back to “R”. It isn’t really needed
because when you reassemble the unit, the metal plate (with “GLOB” on it) will keep everything together.
Once you peal back the lip and pull it apart you can see the ceramic substrate for the pot, terminals B, C,
and D, and on the other piece, the wiper made up of those two silver/white prongs.
Now you can gently pull out the substrate like below.
This view is the backside showing terminals B, C, and D.
The Repair:
On the front side, I cleaned it off a little of the gook and most of the right strip flaked off. I repaired the
strip using a special Circuit Writer pen that deposits a silver trace. The figure below shows that pen and the
cleaned ceramic.
I got the circuit writer from RadioShack but I don’t think they carry them anymore. I have seen them on
Amazon and other places. They cost about $10-$15. The figure below shows the repair.
Frankly I was doubtful this would work the first time because I am replacing a restive material with a silver
trace that has basically zero resistance. I gather the PCM is somewhat forgiving about what it will accept
for a reading. I guess as long as the voltage changes when the pintle moves, that is good enough.
Use an ohmmeter to verify that you made the connection. Then, assembly is the reverse of the process.
(Don’t you just love it when the manuals say that?) As I said, I put a hose clamp around the whole unit to
keep it from opening where the can was cut.
I hope this is of use to someone. It got me three years more out of the EGR and another three I hope.