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!

Comments? Email me.