I have an A4 1.8T with major upgrades and Giac software. I just removed the catalyst and installed a full racing downpipe.Since I use the car for the track only, I removed the catalyst. After installing the downpipe my check engine light comes on due to the faulty reading of the second oxygen sensor. When the car is running without the check engine light it runs beautiful, but once it turns on I notice a significant performance loss. I'd like to know if there is something related between the check engine light coming on because of the faulty O2 sensor reading and the performance loss experienced.

 

The ECU goes into limp mode when the check engine light comes on. (5psi of boost max)

 

Thanks Leszek, that was what I thought could be happening cause the performance loss is very noticeable. Now that I confirmed it, is there a way to trick the ECU or bypass that O2 sensor reading, to not make the OBDII trigger the check engine light?

 

You need to get an O2 sensor bypass which will trick the ECU into thinking that your cat is OK.

 

if you can find the spec for the output as in .4v
just hook it up to a constant scource for that power value and it will think everything is okay

 

the ecu to let it know that the mixture is optimal. So I guess that I can do what you suggested and tap into a constant source which I will then pair it with the black cable from the second oxygen sensor which is usually the cable that carries the output. But my question will be, how am I able to bring down the voltage from my constant source down to .4 volts. Do I need to use a transformer or something between the source cable and the output cable of the sensor? Because I am sure that if I hook up up a cable with .4 constant voltage to the output cable on the sensor I will be able to trick the computer into thinking that the mix is optimal. But the downside is that I don't know how to be able to bring down the voltage of a live cable. Thanks again and would appreciate any further help...

 

he is the local audi electrical guru
im shure he could help you there

 

Vout = Vin *(R2 /(R1+R2))

R1 and R2 are your cheapo radio shack resistors. Start with the values in tens of kiloohms so that you don't drain a battery fast - I'll explain later. R1 and R2 are connected in series, one end connects to the other. The other end of R2 connects to ground. The other end of R1 connects to a constant source - like a 1.5 volt battery. This is also your Vin. The oxygen sensor tap of the ECU or Vout taps in between R1 and R2.

You can also use your car voltage as your Vin, but remember to target Vin at 14.5V (not 12V). So as an example, If I use a steady 1.5V battery source (actually, when brand new they supply about 1.6, when getting older around 1.3V), then I start with an R2 of like 10kohm, Vout = 0.4V, I calculate R1 to be 27.5kohm, so I use the closest value of 27kohm (not all resistor values are avaliable). Now why kohm? because you want the least drain from the 1.5 battery so it will last long. This example will give you a current = 1.5 / (27k+10kohm) = 40.4uA, so your battery will outlas the one in your wallclock. You don't have to worry about an off switch too, because an O2 sensor is a small battery always driving out a voltage. It drops when there is no exhaust there though, but it is always on.

Now before you get fancy with voltage dividers, note that you can do this in this project because the ECU doesn't suck any current from an O2 sensor. The ECU input is in the 100 megaohm range. Do not test an O2 sensor with a cheap ammeter less than 10mohm. The O2 sensor will get damaged. The voltage divider is good for taps that do not suck any significant current. Good luck