boochyan Posted January 11, 2012 Share Posted January 11, 2012 New member here - been browsing and searching the site and have a conundrum. I have a 1999 Subaru Legacy that has been producing some error codes along with a very rough idle. Here's the breakdown: 1. Initial code was P0170 (fuel trim malfunction, bank 1), which I cleared out. I took it for a diagnostic to a mechanic who said the O2 sensor was dead. 2. Replaced the O2 sensor on my own (Bosch OEM). The rough idling continued regardless and a new error code of P0130 (O2 sensor circuit malfunction, upstream) popped up. 3. The code went away on its own, while rough idle continued. 4. Took the car to my father who has a diagnostic tool and we discovered that the car was stuck in open loop and wasn't reading the O2 sensor….showed 0.00 V for the sensor. Thinking we got a bum sensor, we bought another one - same result. Dang. We checked the electrical connections going to the O2 sensor on B18, B20 and E1 - the sensor seems to be receiving the proper voltages…the ground does seem to be sending a tiny (negligible?) voltage of .06 V. We re-seated a bunch of the the connectors as well just to see if there was a faulty connection anywhere...no dice. Here are a few other readings from the scan: Coolant Temp: 86 C Intake Manifold Pressure: 36 kPa Airflow Rate: 2.87 grams/second O2 sensor 1 (upstream): 0.00 V O2 sensor 2 (downstream): .5 V Anybody have any thoughts on this, or experienced anything like it? Thank you much for your input! -Nic Link to comment Share on other sites More sharing options...
bstone Posted January 11, 2012 Share Posted January 11, 2012 Nic, you have to use an original Subaru sensor. Aftermarket ones are notoriously bad on Subarus. Link to comment Share on other sites More sharing options...
ocei77 Posted January 11, 2012 Share Posted January 11, 2012 Nic, you have to use an original Subaru sensor. Aftermarket ones are notoriously bad on Subarus. Disagree. I have replaced three O2 sensors on different cars and I used Bosch (the originators of oxygen sensors btw) and have never had a problem. Unfortunately I can't help the op with his issue, but the Bosch sensor is not the problem. O. Link to comment Share on other sites More sharing options...
1 Lucky Texan Posted January 11, 2012 Share Posted January 11, 2012 I wouldn't think open loop would LEAD to rough-running so(maybe poor gas mileage), perhaps there is a different....or second problem. But that is weird about the sensor. Are you the original owner? Cause, a different problem someone else had was traced to a bundle of poorly repaired wires at the ECU. Link to comment Share on other sites More sharing options...
johnceggleston Posted January 11, 2012 Share Posted January 11, 2012 (edited) this doesn't sound like an o2 issue, this sounds like a wiring issue. i would test the resistance on the wires between the o2 sensor and the ECU. i'm not an electrical guy or know much about o2 sensor reading, but i thought the voltage was suppose to vary from 0 to 1 volt when working properly. or am i thinking of something else.? Edited January 11, 2012 by johnceggleston Link to comment Share on other sites More sharing options...
jarl Posted January 11, 2012 Share Posted January 11, 2012 If you have a small voltage on the ground lead that means the ground is not connected anywhere, i.e. the wire is open somewhere. The O2 sensor doesn't take a lot of amps, so the voltage drop between the battery and that pin should be 0V. Link to comment Share on other sites More sharing options...
boochyan Posted January 13, 2012 Author Share Posted January 13, 2012 Thank you for your input everybody! Out of desperation (and after checking wiring, etc) we decided to go ahead and get the "official" Subaru oxygen sensor from the dealer. We ran a scan and the engine is now operating in closed loop and the o2 sensor checks out and all that. The ridiculous part? The official sensor is also a Bosch with the SAME PART# as the 2 we bought ourselves from NAPA. But the one from the dealer checks out, while the other supposedly identical ones failed. WTF? That is very frustrating to say the least. But glad things seem to be working out now. Thanks again! Nic Link to comment Share on other sites More sharing options...
naru Posted January 13, 2012 Share Posted January 13, 2012 The non-working ones likely got contaminated somewhere down the distribution line or maybe even by your engine. I worked on a 280SLK that was regularly losing O2 sensors. Culprit was a leaking cam sensor that was a long ways away. Oil travelled down the wiring harness until it fouled the sensor.Filled the ECU too! I would put a digital meter on the output of the bad one(s) and hit them w/a propane torch to see if they generate any voltage just for fun. Link to comment Share on other sites More sharing options...
porcupine73 Posted January 14, 2012 Share Posted January 14, 2012 You're not the first person to have that problem with the non-genuine O2 sensor on the front. Especially the years '99 / '00 era for whatever reason seem to be really sensitive and picky. Not sure what to say about the genuine being Bosch with the same part #...other than to say, it is possible the genuine one they sell to Subaru is different from what appears to be the same that they sell to the aftermarket. Link to comment Share on other sites More sharing options...
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