Why is there two O2 sensors? My 96 Ranger 2.3 ltr has one O2 sensors in front of the Cat and one behind the Cat. I unpluged the one behind the Cat and can not tell any difference. No check engine lights or ill effects. I want to put on larger exhaust pipe and was wondering if I can ditch that second O2 sensor?? Do I need it? What does it do that the front one is not doing?

 

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I'm no expert but I think it must monitor the performance of the converter. I can't believe it didn't set off the check engine light. I'd drive it for a while to make sure it doesn't do anything before you go ahead with your exhaust plans.

 

The PCM infers conversion efficiency by comparing the ratio of the O2 sensor switch events before and after the cats. If the rear sensor is gone, you will get a CEL and trouble codes. It may take a couple or a few trips but it will happen.

 

I had a ford tech tell me one day, the people that design vehicles always put things there for a reason...something to think about.

 

The check engine light does work, but it's not come on since unplugging it and driving 15 miles.

 

It takes a minimum of two trips with the same problem for the CEL to light. In the case of the Catalyst Monitor, it could take several trips because of the strict entry conditions for running the test.

 

It will come on, the ecm is going to think the cat is no longer there and throw a code. rwenzing is right on the how it works part.

 

So what does it do? It apparently doesn't change the fuel setting like the front sensor. As a Cat goes bad and clogs what good is it going to do measuring the efficiency before and after the Cat.

 

No, the rear sensor does not affect fuel control at all. It is there to alert the driver and the state inspection station that the conversion efficiency of the cats has degraded to the point that it is no longer legal. Cat efficiency monitoring is mandated by the EPA.

 

That makes since. So as long as I have a CAT but no 2nd sensor it should not effect the efficient running engine. right? If I don't get a CEL by this Friday, driving it every day between now and Friday. Do you think I would be ok to start my weekend project of changing the exhaust?

 

You could still have a muffler shop wield in o2 bungs on the new pipe just so everything looks and works right. But like others have mentioned, rear o2 is just there to monitor how well the cat is working.

 

I really appreciate everyone's help and input. You've all given me good ideas and a lot to think about before I start my project.

 

You can turn the 02 off with a tuner.. you really dont need that 02 from what i have been told.

 

I have a code reader, but it doesn't change programing. Who makes the tuners that do this???

 

Of course today Friday the check engine light has come on. Will check the code when I get home, but it's probably something to do with the CAT sensor now being hooked up.

 

SCT and diablosport tuners had that option to turn off the rear 02's. Mine does. Im pretty sure the EPA made them take that option off though.

 

Wow those tuners are quite expensive! I cleared the CEL code which was the error code for the rear O2 sensor. I plugged the O2 sensor back up and tied it to the outside of the exhaust pipe so it would detect heat. Hopefully that will prevent the CEL, I'm to cheap to buy one of those expensive tuners.

 

Sorry, that won't work. The O2 sensors have to be in the exhaust flow inside the exhaust pipe.

 

No, no, no, no, no!~

If you want to run catless (Like I do in my Evo) then simply use 2 spark-plug defoulers to pull the sensor out of the flow so it doesn't smell all of the ****. I have been running this way since the car was 1 week old when I put my catless block-back exhaust on. 13500Miles, no CEL, no codes. Tuners have been doing this for years on turbo-cars.








You can turn off the o2 sensor right at the ECU, but I wouldn't buy a handheld "tuner" - All you need is the software for your ecu and laptop to put it on. Find out what hex address it is.

 

Thanks Jp7, I will give that a try. That looks like my answer without spending a fortune.

 

As you can see from the pictures I have 2 defoulers screwed together on there. I purchased them for about 4 or 5$.

You will need to use a drill press to open the first one up because the nose of your o2 sensor has a bigger diameter than the defouler. I used a 1/2" inch drill bit, and the o2 sensor had a .47" diameter, so it can just barely get any uncatalyzed exhaust.

 

Thanks for the info!!! You all are great! i'm so glad I found this forum.

Will report back with my results. This should do the trick. I picked up a sightly used Flowmaster muffler for $30 to go along with my changes.

 

I find it useful to join other forums for vehicles I do not own.

(for example, I joined the honda civic forum, just to find people to buy my LEDs)

You should join a Lancer Evo forum, or a Impreza STI forum if you are looking for ways to get around ECU/EPA stuff. This is how I learned to remove my converter and fool my ecu. Also disabled my EGR as well!

 

Back in the days before O2 sensors removing CATs resulted in better MPG's. Has that been your experience with your Evo??