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How To Properly Adjust Your Exhaust's O2 Sensor so as to not throw a CEL when running Catless

By: Mike (aka: twistedsymphony)

Notice: I claim no responsibility for any thing that results from you following these instructions. It simply outlines how I accomplished something. Proceed at your own risk and responsibility.

I reciently installed an HKS downpipe in my WRX. As many of you know that particular down pipe is catless. When you go catless you run the risk of throwing a CEL due to the O2 sensor after the cats no longer reading within the accecptable range.

If you happen to be lucky enough to have some sort of engine managment system I would recomend NOT doing this mod but rather increase the accecptable O2 range. This works fine for tricking the factory ECU but if you're looking at readouts from an aftermarket ECU they will be incorrect values.

Installing this simple circuit you can easily bump the O2 sensor signal back into the accecptable range and "trick" the ECU into thinking that there are indeed cats still in the exhaust system.

Click to Enlarge

Some notes about the circuit:

The resister is a 1Mohm resister which means the color strips should be Brown Black Green Gold. I would opt for a Metal Oxide Resister as they are better suited for high temp applications. Resisters are not Directional so there is no forward or backward direction.

The capacitor does have direction; tantalum capacitors should indicate on the body which side is positive (denoted by a +) also the longer leg of the capacitor is the positive leg.

To install:

1. If performing this mod when you upgrade your down pipe I'd recomend doing this at the same time.
Before you start be sure to unplug the negative terminal of your battery. This is a good safety habbit and serves to keep you from getting shocked. Also you'll want to reset the ECU when installing the mod so if you leave it off while you build the circuit it sould be good and cleared by the time you're done.

2. There is a grey pulg. to separate you'll need to push back the rubber suround and use a flat screw driver to push the tab on the side away from the plug. mine came apart VERY easy once this tab was lifted.

Next there are 2 wire ties... I just clipped these off of the car (keep them attached to the wire for now)

I opted to remove the downpipe before taking the sensor all the way off because I couldn't get enough leverage under the car. I ended up using a big adjustable wrench (that sucker was on there tight).

3. Once you have the 02 sensor and wire off you'll notice that the wire is essentially divided into 3rds by where the wire ties are attached. I added my circuit in the section closest to the plug. I did it here for 3 reasons. First this section doesn't get bent much, second it's gets mounted above a structual part so it doesn't get as much road grime, and 3rd also because it's above a structural part you can't see it unless you check for it.

You can use your favorite method to add the circuit into place. I soldered and I'm not sure if I was just having a bad day but I found the wires VERY difficult to solder.

Becarefull not to strip and cut more wire than you need to, this wire is tight to begin with so taking away more slack isn't the best thing to do.

4. Once you're done splicing in the cirucit be sure to seal it up good I used some water raited heat shrink tubing. if you dont have heat shrink tubing use some electrical tape and be sure it's sealed well. Also now is a good time to clip off the old wire ties if you havn't yet.

5. Put it back on in the oposite way you took it off. The O2 sensor first (tighten it down TIGHT don't want any leaks), lay the wire back in the orientation it was originaly in and plug it in. Then use some fresh wire ties to tie it to the chassy so it doesn't flap around or get caught on anything.

Once that's done and you're new downpipe is installed put the negative terminal back on the battery and fire it up... and enjoy your CEL free instrument panel!