o2 sensor mods

[steven.brewer]

I've been looking for a good way to deal with the 02 sensor besides
wrapping with tin foil. I found the following and it is interesting.
I'm thinking of trying this. Anyone have any comments on whether it
sounds like it might work or not?

Sensor Modification Keman 2004 Drugs for your oxygen sensor. Well, not
really.

This modification very cheaply fools your secondary (post pre-cat) O2
Sensors into thinking that there is actually a lot less oxygen present
than there really is. Why would you want to do this modification?
1. You're running rich
2. Your oxygen sensors are on the fritz.
3. You gutted your precats, took out your main cats while you were at
it, didn't want to buy a chip and got tired of that dumb check engine
light staring you in the face.

(Not that I'd ever do something like #3. I'm a law abiding citizen and
millions of whales would die if you removed your cats. Shame on you!
Hehe)

Disclaimer: While I am an Audi technician the modifications listed
here are not endorsed by Audi. I provide these instructions based on
my own findings and experiments conducted outside of Audi of America,
using my own tools in my own garage. If your car blows up after you
perform this modification, don't go running to my dealership expecting
me to fix it for free. They won't, and I will refuse to acknowledge
that anything posted here was created by me.

SO.. You removed your precats and got the dreaded "Precatalyst
efficiency below threshold" ... well this will fix it. Chances are
you're reading this because you're too cheap to buy a chip that would
eliminate this problem (like me), and also too cheap to have purchased
WAY overpriced downpipes that have secondary O2 sensor bungs that are
behind the main cats. Or maybe you did buy those expensive downpipes
and got rid of the main cats too.

The first mistake people often make is trying to trick the ECM by
using a resistor inline on the O2 sensor. After all, the "post
catalytic" O2 sensors are now outputting 0.95 volts instead of an
expected 0.1 to 0.8. A resistor of value X, where value X is whatever
your friends brothers son's nephew runs on his Honda with 100%
success. The problem is our ECM uses a mega-ohm measurement device,
meaning just about no matter what kind of resistance you put in, the
full amount of voltage will still be seen. Remember, resistance does
not drop voltage, it limits current. If the ECM were to apply a decent
load, then resistance would inadvertently drop the voltage it saw. So
this is the right idea, but the wrong method.

Next mistake I've seen is a complicated circuit that takes the O2
sensor signals, processes them, and re-transmits them at a lower
value. Geeze!!! Talk about overkill. While that will work, how about
the most simple and cheap design possible.

1 Standard Diode from radio shack, P/N: 276-1141

The average rectifier diode is a one-way valve. It has a voltage drop
when current passes through it, which for the sake of discussion is
usually around 0.5 volts. There are millions of different diodes that
can greatly alter that figure, but for our purposes we will use that
figure because it's exactly what we need. Your precats aren't burning
off any of those unburned hydrocarbons when they're laying in a
million pieces in the trashcan, so there is plenty of oxygen present
as far as the downstream O2 sensors are concerned. They're putting out
0.95 volts. Pass through this lovely $1.29 incarnation, and whee! It's
0.45 volts. What does the ECU expect? 0.45 volts is within perfect
specifications.

Common misconception:

- The ECM measures the difference between upstream and downstream and
calculates mixture based on that.

Bzzt! Wrong. Maybe on some cars, but not on ours. Those downstream
sensors serve only one purpose: Measuring the efficiency of the
precats. They honestly don't even measure the main catalysts! Cats age
and lose some efficiency over time. If the precats aged and the air/
fuel ratio was based off of this, you'd eventually run pig rich or
your engine would lean out and predetonate itself to pieces under
boost. That would be bad, and stupid. The primary O2 sensors do a
perfectly fine job measuring the A/F ratio results.. And that's what
they do. We're not going to touch them.

What we are going to do, is touch the post pre-cat O2 sensors. I have
to call them that, because they're BEFORE the main cats, but AFTER the
little pre-catalysts. Specifically, we're going to modify the wiring
harness of the sensors themselves. That way if you decide you want to
change back to completely stock, you can just put a new O2 sensor in
and the wiring comes with it.

Audi was nice and put the harnesses on the passenger firewall HERE.
Green and Brown are your friends. Don't touch the black connectors,
those are your good O2 sensors and that's where the real magic of the
engine is contained.

Which connector is what?? The side you want to be modifying has pins
in sticking out of the connector. If you see flat pins, you're cutting
into the sensor side of the harness. If you see 4 flat slots for pins
to go into, you're cutting into the ECM side. Bad!

As you can see in the picture, I slid back the rubber boot and cut the
BLACK wire. This is the O2 sensor (+) output. Grey is reference
voltage, and both whites are the heater circuit. Cut the black wire,
crimp in the diode with the silver bar facing the ECM side (towards
the connector). If it's backwards, your ECM won't see any voltage at
all. So if that happened, now you know why.

Make sure you cut enough of the wire to make up for the length of the
conglomeration. It's about 1.5" if you use two crimp connectors and
cut the diode leads to about 0.5" long. Bonus points if you heat-
shrink this connection, but with the rubber boot slid back around it I
really don't think you'll have any corrosion problems here.

Remember, silver bar on the edge of the diode is on the ECM side, not
the sensor side. This allows positive voltage to flow from the sensor
through the diode to the ECM.

Repeat for both the brown and green connectors (left and right post
pre-cat O2 sensors)

And when you're done, the voltage drop occurs and all is good. Clear
those codes and enjoy!