I don't have a link handy but here is the text from the original link I copied some time back (sorry he refers to Pics but I only copied the text when I saved it)
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 <pics/eng53-diode.jpg>
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
<pics/eng53.harness.jpg>. 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!
Originally Posted by jschick
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