on my 97 legacy L 167k miles automatic

 

hi,

i recently had the chance to see one of the two cats on my car and i noticed that there were spacers on both of the o2 sensors..

 

as i am not the first owner and dont know the history of this car.. i was wondering if someone had done this as a quick fix. i thought that if someone had previously gotten a check engine light, that they would have only put a spacer on the first o2 sensor and not both.. so if anyone could give some insight, id appreciate it.

 

thanks.

Does it have a header or stock exhaust?

stock exhaust. stock everything really.

The 'trick' is usually applied to the rear sensor only, to get rid of a real or bogus 'cat efficiency' code.

 

 

Dave

No car comes from the factory with anti-foulers on the O2 sensors. Somebody put them there. Why they put them on both? Beyond me. Does the CEL illuminate during the bulb test when you turn the ignition key on?

yes. the cel lights up during accesory mode. and the cel works for other codes. for example the throttle position sensor was unplugged and the cel came on.. but went away when it was plugged back in after a few cycles.

 

 

so i really cant understand why theyre there..

 

also. why would the trick be applied to the rear sensor only?

isnt the temperature supposed to go up after the cat does its job. and by spacing the rear o2 sensor, that would most likely give it a lower temp reading right?

Remove the one from the front sensor - it's going to make it run really rich with that on it. The front sensor is only for mixture adjustment. Someone didn't fully understand what they were doing.

 

It is applied to the rear sensor so that the computer see's a difference between the two despite the cat being bad or missing. This prevents the ECU from throwing a code indicating the cat is bad - which keeps your wallet nice and fat since you don't have to replace the cat at the cost of half your next paycheck.

 

GD

Remove the one from the front sensor - it's going to make it run really rich with that on it. The front sensor is only for mixture adjustment. Someone didn't fully understand what they were doing.

 

GD

 

 

 

okay. well that might explain why the car runs like crap to begin with..(no power + ~14 mpg around town driving) especialyl since i cant see the a/f ratio... without a gauge and i dont have a computer program to check..

 

and with both those spacers on, wouldnt that cause a CEL?

It is applied to the rear sensor so that the computer see's a difference between the two despite the cat being bad or missing. This prevents the ECU from throwing a code indicating the cat is bad - which keeps your wallet nice and fat since you don't have to replace the cat at the cost of half your next paycheck.

 

GD

 

okay. well then in the same logic, couldnt u just apply it to only the front sensor and have a difference between the two as well thus creating the same advantage?

what causes this not to be possible?

okay. well that might explain why the car runs like crap to begin with..(no power + ~14 mpg around town driving) especialyl since i cant see the a/f ratio... without a gauge and i dont have a computer program to check..

 

and with both those spacers on, wouldnt that cause a CEL?

 

Probably not. The computer isn't that smart. It only knows what it see's from the O2 sensors and if that is wrong then it's just going to run like crap. If the sensor's aren't bad then it just has to go with what they are telling it - no matter how rediculous that reading may be.

 

GD

okay. well then in the same logic, couldnt u just apply it to only the front sensor and have a difference between the two as well thus creating the same advantage?

what causes this not to be possible?

 

The difference is that the two sensors are used for ENTIRELY different functions. The front is used for fuel injection mixture control, while the second is used only to check the efficiency of the cat. If you space the front then the mixture control will read a lean signal and dump excess fuel into the engine. If you *don't* space the rear then the cat will look bad. Spacer = lean signal. The lean signal has to occur in the right place or the computer isn't fooled *in the right way*. It's still fooled I suppose - but the joke is on your wallet.

 

GD

Yeah, pull the front anti-fouler out, leave the rear one in. That should set it up good enough for now. Might want to replace the front 02 sensor while you have it out. Your gas millage should go up significantly.

Yeah, pull the front anti-fouler out, leave the rear one in. That should set it up good enough for now. Might want to replace the front 02 sensor while you have it out. Your gas millage should go up significantly.

 

 

 

okay! will do. im glad to have a new project. and no can do on replacing that front one. the cars about to die id say.. and if not, the registration is up in about a month and its not worth it to re register it. so im looking for a newer subaru.

 

ill post how it goes.

also. why would the trick be applied to the rear sensor only?

isnt the temperature supposed to go up after the cat does its job. and by spacing the rear o2 sensor, that would most likely give it a lower temp reading right?

Temperature has nothing to do with it. What the sensor is reading is oxygen content. It does this by measuring a reaction created by the catalyst of the sensor with gases in the exhaust stream. This creates an electrical pulse which the computer measures and translates into high or low oxygen content (lean vs rich). The only thing temperature has to do with it is the sensor has to be about 600°F before it will create the signal, until the ECU sees a signal from the sensor it runs in "open loop" mode. Basically a preset group of values that will allow the engine to run regardless of sensory input, though far from optimal performance.

 

 

The trick is used only for the rear sensor because that is the "check" sensor. The rear sensor only measures the difference in oxygen content after the catalytic converter has done its job. The rear O2 sensor is there only for the computer to determine when or if there is a problem with the catalytic converter. If the change in signal voltage from the front O2 sensor compares to the rear O2 sensor is inside a certain range the computer sees that as evidence of failure of the catalytic converter and sets a trouble code so that you will go get it fixed. Several reasons to do that, the least of which is environmental impact.

By placing the rear sensor in an anti-fouler, it pulls the sensor out of the main exhaust stream which gives the impression of a higher oxygen content which makes catalyst efficiency codes go away.

 

The front O2 sensor is used for adjusting how much fuel is injected into the engine. A high oxygen content will tell the computer the engine is running lean, and it will then inject more fuel to correct the condition. But the same deal applies when it is placed in an anti fouler. It pulls the sensor out of the main stream, which can make it read a higher oxygen content. The computer will then enrich the A/F mixture driving down performance and fuel economy, while at the same time driving up the work load on the catalytic converter which will eventually lead to failure. Then that catalyst efficiency code will actually be set for a good reason.

Which is why you don't put anti-foulers on the front O2 sensor.

Make sense?

any suggestions for the easiest way to find the leak??

Temperature has nothing to do with it. What the sensor is reading is oxygen content. It does this by measuring a reaction created by the catalyst of the sensor with gases in the exhaust stream. This creates an electrical pulse which the computer measures and translates into high or low oxygen content (lean vs rich). The only thing temperature has to do with it is the sensor has to be about 600°F before it will create the signal, until the ECU sees a signal from the sensor it runs in "open loop" mode. Basically a preset group of values that will allow the engine to run regardless of sensory input, though far from optimal performance.

 

 

The trick is used only for the rear sensor because that is the "check" sensor. The rear sensor only measures the difference in oxygen content after the catalytic converter has done its job. The rear O2 sensor is there only for the computer to determine when or if there is a problem with the catalytic converter. If the change in signal voltage from the front O2 sensor compares to the rear O2 sensor is inside a certain range the computer sees that as evidence of failure of the catalytic converter and sets a trouble code so that you will go get it fixed. Several reasons to do that, the least of which is environmental impact.

By placing the rear sensor in an anti-fouler, it pulls the sensor out of the main exhaust stream which gives the impression of a higher oxygen content which makes catalyst efficiency codes go away.

 

The front O2 sensor is used for adjusting how much fuel is injected into the engine. A high oxygen content will tell the computer the engine is running lean, and it will then inject more fuel to correct the condition. But the same deal applies when it is placed in an anti fouler. It pulls the sensor out of the main stream, which can make it read a higher oxygen content. The computer will then enrich the A/F mixture driving down performance and fuel economy, while at the same time driving up the work load on the catalytic converter which will eventually lead to failure. Then that catalyst efficiency code will actually be set for a good reason.

Which is why you don't put anti-foulers on the front O2 sensor.

Make sense?

 

perfect explanation. thanks a lot.

any suggestions for the easiest way to find the leak??

 

eh?

any suggestions for the easiest way to find the leak??

 

If you suspect you have a leak. Get a friend/helper/wifey/etc. to cram a rag into the tail pipe and hold it there for a few seconds while you hunt around for the source. I like to do this when the pipes are relatively cool so I can put my hand around them to feel for any leaks. Ever put your hand over top of a hot catalytic converter? Kinda like sticking it in an oven with the broiler on.