Fairtax4me Posted November 6, 2014 Share Posted November 6, 2014 Before you put money on that oxygen sensor, the DOWNSTREAM sensor does NOT AFFECT the air/fuel ratio. The downstream sensor only checks the efficiency of the catalytic converter. If you think you have an A/F problem the UPstream sensor is responsible for that. Hesitation on acceleration could be the knock sensor. Those are cheap on eBay and they do fail often. 1 Quote Link to comment Share on other sites More sharing options...
Bushwick Posted November 6, 2014 Share Posted November 6, 2014 I'd drive the car a bit on the freeway, say 50 miles before thinking anything else is wrong. Run the car a little hard, put in "3" instead of "D" to force the rpms up a bit. Any time you have a poorly running engine, it can negatively effect sensors like your O2 sensor from improper running i.e. too rich. Had a CPS wire in a 92' SHO ground out from rubbing on pulley, under timing cover. Eventually had to take it to Ford to diagnose this was pre internet. Anyways, when I went to go get it, the Ford mechanic said to "drive it on the highway" for the same reasons I listed. TBH, I was skeptical, but being Ford I listened to him. Sure enough about 45 miles later the engine was running as it was supposed to, so that always stuck with me. Granted a coil isn't a CPS, but the end result is the same as both are needed for spark to occur. The rear O2, despite being there solely to monitor if the cat is doing it's job, CAN still impact the way an engine is running if it's shorting out. It may or may not throw a code. Don't dismiss it. Get an ohm meter from Radio Shack for $20 and set to "Ohms". Disconnect the sensor in question and touch the test probes to the prongs for the sensor. Get the Subaru values and compare yours against them. Works for any sensor from knock to TPS, to O2. If the sensor is beyond the acceptable specs, replace it. 1 Quote Link to comment Share on other sites More sharing options...
bendecker Posted November 6, 2014 Author Share Posted November 6, 2014 Before you put money on that oxygen sensor, the DOWNSTREAM sensor does NOT AFFECT the air/fuel ratio. The downstream sensor only checks the efficiency of the catalytic converter. If you think you have an A/F problem the UPstream sensor is responsible for that. Hesitation on acceleration could be the knock sensor. Those are cheap on eBay and they do fail often. The knock sensor was replaced with one from Ebay about 200 miles ago (should have mentioned that before, but forgot about it). The old one was the original OEM and had a cracked case. Quote Link to comment Share on other sites More sharing options...
bendecker Posted November 6, 2014 Author Share Posted November 6, 2014 (edited) I am definitely going to be testing both O2 sensors when time permits. I have two ohm meters, so I'm good to go. Specs and pin-out are in my repair manual. And, I will definitely be driving the car hard during testing :-) Edited November 6, 2014 by bendecker Quote Link to comment Share on other sites More sharing options...
bendecker Posted November 7, 2014 Author Share Posted November 7, 2014 (edited) Drove it hard and long today. Other than the very small burp on startup and a bit less power than I remember, it drove fine and had no issues for 50 or so miles. In the afternoon, I took it to run an errand and the CEL came on. P0141 -- heating element in the rear O2 sensor. Could be because I monkeyed with it the other day (unplugged, then plugged back in). Could be going bad. Looks like I need to jack up the car again. Edited November 7, 2014 by bendecker Quote Link to comment Share on other sites More sharing options...
Bushwick Posted November 8, 2014 Share Posted November 8, 2014 That's what originally went out on my Saab. Actually, I bought it like that and ignored for a good year or so as it didn't interfere with day to day driving initially and our e-check is ever other year. That failure of the rear O2, is what eventually turned into a short within the sensor- whether they were directly related to one another, I dunno. I just know the external casing on top of the sensor where the pig tail entered, was metal and it had a bunch of cracks and the chrome covering was flaking off. So moisture could get into the exposed epoxy and settle, and cause an open circuit. The heater element's purpose is to aid with better cold start emissions. With mine, the 1st 5 minutes or so caused issues randomly once moving (misfiring, stumbling) then became more predictable as it occurred more frequently. The muffler blowout (muffler was a year old BTW) made me take it more seriously as there had to be a decent amount of raw gas that made it past the turbo somehow to deform a muffler the way it did. I originally replaced the up O2 as both heater elements were pulling codes. It was during the backfire that only the up O2 had been replaced, so it left me with the rear O2. Replaced the rear/down O2, reset the battery, and the issues never came back. IIRC, the actual O2 sensor was still close to spec ohms, but the heater element wasn't. For what it's worth, the Saab is a 99' with OBDII and it never threw a code for the O2 shorting. It only threw a random misfire (P0300) if the engine actually misfired, Once or twice it threw a specific misfire (P0304), and both O2 sensors were throwing heater element codes. It's quite possible while wiggling the wiring that it might have caused an issue, or there is corrosion where the wire enters the crimping, and wiggling it is opening/closing the circuit. If water pools in the sensor and allows a short to ground, or to another wire right next to it, your stumbling will return. Since the heater element code is already getting thrown, replacing the sensor makes sense. Quote Link to comment Share on other sites More sharing options...
bendecker Posted November 9, 2014 Author Share Posted November 9, 2014 Agreed and already ordered on from Rock Auto. Bosch. $59.00 delivered. I'm pretty sure the one in there is original, so it had a good run. Quote Link to comment Share on other sites More sharing options...
bendecker Posted November 20, 2014 Author Share Posted November 20, 2014 (edited) Got the O2 sensor, jacked up the car and found that the old one was not even finger tight. It was actually loose by about 1/2 turn. So, it was the easiest under car part replacement I've ever had. The old one was a bosch as well. Exact same part number. So... I'm guessing it had been replaced at least once. That's the good news. The bad news is the car threw the same code after driving it about 10 miles. AAAARRRRGH!!! Now, I'm thinking that the wires were fragile under there the first time I unhooked the rear O2 and they need some TLC. So... looking for some time to jack it up again and take a really good look. I have not tested the old sensor with an ohm meter, but I would bet it's fine. Oh well. Stuff happens. The car is still running much better than when this all started. Just a hint of hesitation at startup and none of the really bad intermittent symptoms like hard deceleration and outright dying -- yet. I haven't driven it enough to be 100% confident, though. Edited November 20, 2014 by bendecker Quote Link to comment Share on other sites More sharing options...
Fairtax4me Posted November 20, 2014 Share Posted November 20, 2014 If you worked on the rear sensor there is a long extension harness that runs from the top right corner of the bellhousing, across the top of the transmission, then down and along the side of the trans back to where the sensor plugs in. Check the plugs at both ends of that harness. Make sure none of the pins are bent, or that any of the wires have backed out of the plug. Ive seen this many times lately on customers vehicles after they've replaced a sensor or other electrical part. You'll also want to check all of the fuses. The heating elements normally get power from the fuse panel. 1 Quote Link to comment Share on other sites More sharing options...
bendecker Posted November 22, 2014 Author Share Posted November 22, 2014 (edited) You were spot on with the harness being the problem. The sensor was put in fine, it was the other end of the harness that was not plugged in properly. Pilot error on my part. DOH!! (I hate it when that happens...) But I've saved a lot of money by doing all this myself and have learned a ton about the car, fuel injected engines, etc. (I've never really worked on it, unlike my '81 Brat, which I have always done everything on). On a side note, there appears to be a spliced-in wire the full length of the harness and there was some electrical tape wrapped around all of the wires on both ends, just before they went into the end connectors (I can't imagine it looking like that from the factory). I cleaned everything up, removed the skanky old electrical tape, and replaced it with some good quality heat-shrink tubing. I drove it quite a while (mostly surface streets) and no CEL, no major issues. Gaining confidence, but need more time on the freeway at speed before I'll let my wife or kid drive it. Edited November 22, 2014 by bendecker Quote Link to comment Share on other sites More sharing options...
bendecker Posted January 20, 2015 Author Share Posted January 20, 2015 (edited) ARRRGH!!!!!! I don't drive the car often (we have newer cars that the fam likes better), but when I've driven it over the last couple of months, it's worked fine (despite the slight hesitation at startup). Until today... It crapped out going up a long, hard hill. I kept pressing the gas and could feel the power slowly slipping away. Having been through this before, I knew what to do: I pulled over before it completely died, turned off the ingnition, then started t back up and went up the hill. I think it's a fuel starvation issue and will start another thread on it. Edited January 20, 2015 by bendecker Quote Link to comment Share on other sites More sharing options...
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