downstream o2 sensor

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I was doing the exhaust on my new aspire today, pulled the downstream o2 from the pipe and welded a cap on the bung..my plans were to move the sensor to another location like on babyspire, but for whatever reason I didn't get around to plugging the sensor back in.....

is it supposed to throw a code?! I don't have any lights on, it's making me think maybe the check engine light is out?!? I'm not getting any codes.. it's completely unplugged at the socket and separated from the car.

[standard235]

On anything OBD2, yes.

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Yea it was throwing a Code, I don't know why the light didn't come on, but it came up when I plugged in the OBD2 scanner. Now the code is clear and I'm getting a code for p0500, vehicle speed sensor.

It idles low..really low, probably 600rpm, and it vibrates a bit. It's pretty sluggish when accelerating, but when it goes close to like it should the shifts are smooth.

Just sitting and idling I can twist the throttle open, it hesitates slightly then takes off and revs smooth.

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The check engine light is also comming on now. I have no idea why it wasn't before.

[nitrofarm]

Hook the downstream o2 sensor back up

[standard235]

Eww downstream. Unless you need it for emissions sniffers, de-cat and use the spark plug anti fouler trick to keep the cel off. My Mustang is doing dandy with it like that.

[blkfordsedan]

It will take a while for the lack of feedback from the rear O2 to actually turn on the CEL.

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Yea I got the downstream o2 hooked back up, it's not throwing a code for that anymore at all... my problem now is a random misfire. I'm gonna post a new thread and maybe get some attention to the matter. The car is warming up and stuff is starting to go wrong now, stuff that probably blew the head gasket to begin with.

[ericsmith32]

downstream o2 sensor

Sounds like the 97 I played with a while! I couldn't get rid of the 2nd o2 code regardless what I tried. Also had the hesitation and the reason I stopped driving it. Clearing the codes did it for a while.

[nitrofarm]

Yea I got the downstream o2 hooked back up, it's not throwing a code for that anymore at all... my problem now is a random misfire. I'm gonna post a new thread and maybe get some attention to the matter. The car is warming up and stuff is starting to go wrong now, stuff that probably blew the head gasket to begin with.

Matt did you use new HLA's or re use the old oness?

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Used the ones that were in it, 4 of them are collapsed but they're quiet so I don't care. I've been diagnosing this problem all day and I've got it narrowed down..posting another thread on it now.

[XFSE71]

This may be more information than you really want, but I'll just say it anyway because I'm a Ford guy and I hope this helps. Just quit reading when your eyes glaze over ...
The Federally mandated Onboard Diagnostics 2nd Generation testing is the same for all vehicles since 1996. Some 1995 models (Mustang) had it as before the law was enacted.
The OBD2 PCM has a specific sequence of self testing. When you turn on the key (POST) the PCM "looks" at every input for a valid input signal. This is the Comprehensive Component Monitor.
Most signals are in the range of >.5V and <5V. If its 0V it’s shorted, if it's 5V it's open. If the voltage is not right then the PCM will set a code and turn on the light. Some sensors are not able to be tested until conditions are right.
O2 sensors are signal generators. They test exhaust gases for oxygen content compared to ambient air. They sniff your exhaust gasses in the upper manifold using zirconium dioxide to measure in voltage how much oxygen is present there, compared to how much oxygen is in the wire that goes to the connector [breathes through the wire]. The range is .1V lean to 1V rich. Out of range? Set the code turn on the light. They don't make any V when they are cold so electric heaters were added to quickly get them over ~1100ºF so they can start working (Open Loop / Closed Loop). The PCM can't test the O2s until the engine is running warm enough to go to closed loop and the O2s are hot enough to work.
During open loop operation the PCM ignores the O2 and adjusts fuel quantity, timing, EGR .... and so on based on the signal inputs comparing to a set of values in the "Look Up Tables" (ROM).
Once closed loop is achieved then the PCM reads the upstream O2 directly and writes the variation in values to a memory log (RAM). Next time those same conditions occur it will use the ROM value +- RAM to modify the fuel and timing for best performance and emissions. This is the "learning" that you erase by disconnecting your battery.
During your OBD2 "Drive Cycle" (1st trip of the day) in closed loop the PCM will test the O2 by ramping fuel rich ~.8v then lean ~.1V just to feel it move. If no pass, set a "pending code" and retest when conditions are right. The PCM can’t always test the rear O2. So if it’s too cold or you are accelerating hard (WOT) then conditions are not right, no test. If conditions are right and it fails the test then set a Pending Code. Fail 3 times, set a code/light. If the front O2 passes then the PCM will look at the rear O2 to see if it is in range and has a low enough signal frequency to indicate that the CAT is lit and working. The rear O2 is like a security guard … just kinda hangs around looking for something to happen. The downstream O2 sniffs gasses after the CAT has done it’s thing. There shouldn’t be much left after passing that 2400 º F bed of rare metals. The rear O2 and the CAT will not change performance, unless the CAT is plugged, then code/light. Gutting your CAT will cause the rear O2 to switch just like the front.
The light comes on anytime emissions go 25% above the Federally regulated Grams per Mile (GPM).
There is a lot more to this, but this may help you understand why no light with the rear O2 unplugged.