legacycentral bbs

on my 90 legacy i got a check engine light that said i had a bad o2 sensor. i got one from the local parts store and the light would be out when first started in the morning but after 5-10 min. it would come back on. i was told i probably got a bad sensor. i went to the subaru dealer and got one and the light is still on. what should i do.

sounds to me like you have something else wrong with the car that is making the O2 sensor have an out of spec reading. How does the car run? bad gas milage? Does it have any exhaust leaks?

I'd check the resistance on the sensor. I'd also check the wiring harness to see if there are any breaks or high resistance spots.

you cant really check resistance on an 02 sensor, the best way to test one is to do a function test which is pritty easy all you need is a voltmeter and some propane, or any other hydrocarbon sorce you can inject into the intake in small amounts. A scanner can be used to but i doubt many of us have that capability. But sence you've replaced it twice i still doubt thats the problem spot. And yea deffinatly check the harness.

the car runs good and there are no exhaust leaks. today when i started it the light was out until the car warmed up then it came back on.

where does the wiring go to. is there a plug at the other end

If you have tried that many O2 sensors, then the check engine light may be another code. Did you ever try to reset/clear the ECU memory? Do you still have the diagnostics codes check plugs unplugged now (green and black connectors under the steering column tray cover), or are they plugged in? etc.

I would say after trying 2 new O2 sensors, that you are probably good there, maybe need to reset the ECU, and if you still get codes after that, it is most likely something else.

yes i did reset it, the only code was the o2 code

did you try another known good ecu? sure someone has one you can borrow to check that out. Also what is the visual code, how many blinks are you getting.......on the light with diagnostic connectors plugged together. I want to know the number of blinks to make sure you even have the right code.

i don't remember how many flashes it was but i am sure it was the o2 sensor code. the light was not on when i got the car a year ago. one day it just started to come on. since i replaced the sensor it will sometimes be out when i start the car but after about 5 minutes it comes back on and stays on.

so a bad ecu can do that. is it a common thing. how hard are they to swap.

First things first. You will get a CEL for the O2 sensor only when the car is warm. This is because the ECU runs an internal circuit until the exhaust heats up. Once at running temperature it switches over to the actual O2 sensor.

ECU is easy to swap if you need to. In my opinion its unlikely that its the ECU. But if you wanna try its under the dash right next to the steering column (just to the left of the column).

But since you say its only happening when the car is warm it is more likely external to the ECU. I know you said the exhaust is tight, but my 93 is extremely sensitive to leaks. I needed a sniffer to find my leak. It wouldn't have made a differenece on most othercars.

sometimes the light is on when i first start it and other times it is off until it runs for 5 min.

Could it be a wiring issue not fixed by the sensor replacement?

hmmm.
are you sure its the O2 sensor code?
My 93 SS was doing what yours is, and I checked the code.
Sometimes I could drive the car 5 miles without the Check Engine light. then it would come on

Sometimes I could drive it 70 miles and it wouldnt even come on at all.

other times I would drive it and right away it was on, then it would go out.
My code was for Purge Control Solenoid valve. when I replaced it, no more check engine lights.