Evening all,

 

New to the forums, and new to Subarus in general. So far, pretty impressed with my snowbeater! Bought it pretty cheap, it's a 97 Legacy Outback. It is in need of a LOT of maintenance, and to get started I figured I'd clear the check engine light. Pulled codes, and one was for the knock sensor (plenty of forum info on that one) and my second code is for the downstream O2 sensor. How does this sensor affect engine performance when it has failed?

 

Thanks!!

I've noticed it will increase fuel mixture, and run rich.

 

You're code was P0136 yeah?

it doesn't affect engine performance at all. you can remove the rear sensor altogether and the car runs fine.

 

if you have the catalyst efficiency code (common issue), make sure you have no leaks. if not, then do the $5 spark plug anti fouler mod, very common and removes the nonsense of this glitch.

 

in the future - be sure to post the exact code you're getting, that helps us help you. P0420?

Do apologize for not having the codes, I pulled them the day when I got the car, but sitting here I couldn't remember the P codes off the top of my head. I do recall the P0325 for the nock sensor, and the one other code refered directly to the second o2 sensor. There were no additional codes.

 

Thanks for 'yall's input! I'm pretty sure I'll be around here for a while, I've got timing belt, cam oil seals, and all kinds of other things to do on this car!

The downstream oxygen sensor is there only for emissions purposes. The computer evaluates and compares signals from the upstream and downstream sensors to assess whether the cat. converter is doing it's job. Without a scope/DSM/scan tool to see the signal in real time, it is hard to say whether to sensor is working well or not.

 

Be that as it may, as far as diagnosis-by-parts changing goes, if the trouble code is for the rear sensor, it's good odds that replacing it will likely resolve the issue. However, it is possible for a bad cat. converter to set a rear oxygen sensor code, and likewise, a bad rear oxygen sensor could set a cat. efficiency code.

 

If you have an infrared thermometer handy, you could measure the temp of the cat with the car running at at least 1500 rpm...a functioning cat. should be at least 100 degrees warmer at the outlet.

well, when, if, the code comes back, get the number. until then enjoy your new ride and welcome to the board.

 

if you are crawling around under the car, check the o2 sensor wire connection, on the driver side near the rear of the trans.

 

they can difficult to undo , but an unclick, re-click may help, depending on the code.

 

there is one rear o2 code that doesn't come up very often, it's rare, but when it does, there is usually snow, sleet, and salt on the roads. the connection gets dirty and corrodes.