NC01TA

Can oxygen sensors be malfunctioning or bad if the SES light is NOT on?? I always thought the SES light would automatically come on if a sensor was not 100%. Anyone know for certain?

redtan

iTrader: (5)

If they are dead or pretty close to dead yes you will get an SES light.

But you could have the sensor not work 100% but still be within the threshold of not tripping an SES light.

What's the reasoning for your question? Are you experiencing signs of a bad o2 sensor but not getting any lights?

1ltcap

I'VE seen very rare occasions where they may not trip a light. if they're in and out of spec, they may not trip it, or if there's enough crosscounts, but the sensor still doesn't make full range, it may not trip the light.

I SHOULD explain a little farther.

an o2 sensor generally ranges from .1 to .9v. it does this several times a second. the computer looks more for the speed, or frequency of this, than the amplitude. it judges the speed by the crosscounts. crosscounts are how many times the o2 crosses center, or about .45v. i forget the exact frequency, but lets say it's 10 times a second.
so in theory, you could only be traveling for instance .3 to .7v, and doing it 10 times a second. as far as the computer's concerned, it's working fine. yet since it's not traveling full range, it can and will cause some problems.

blade2222

I have a 2002 and whenever the o2 sensor starts failing I'll get multiple misfire and a lean code never a actual o2 sensor code.

1 FMF

iTrader: (1)

the recommended minimum frequency of an O2 sensor is 8 times or more in a 10 second period. Less than 6 times per second is definitely slow and should be replaced. I read this in an article in motor magazine.

if you read some of the obd-ii desciptions for the DTC's related to O2 sensors, the PCM will monitor readings over 1, 2 or 3 minute time spans and if the readings don't jive then it'll turn on the light. You obviously don't want the criteria to be too strict monitoring O2 sensors where you set false alarms, but it's very possible for sensors to be less than 100% and not set a code like 1ltcap explained.

http://www.motor.com/article.asp?article_ID=360
http://www.motor.com/article.asp?article_ID=131

pertainin to "does a bad o2 sensors always mean an ses light"
i would say mostly yes, it depends on your definition of bad.
and the ses light will be pertaining to some engine function impacted by the O2 sensor.
but a bad o2 sensor does NOT always mean an SES light with a DTC pointing directly to the O2 sensor.

1ltcap

depending on how it fails, an o2 will often trigger a code other than itself. the main o2 codes i see that are actually it, are for the heater circuit.

those of us that are good diagnosing ford eec4 systems are used to things like this, as fords generally would have one failure that could/would trip a half dozen codes.
my best friend used to get pissed off at me. he taught me a lot of what i know, but he always sucked with fords engine control systems. he'd laugh when i'd pull one in with a cel, thinking it was gonna drive me nuts. then an hour later when i had it figured out, he wasn't laughing so hard.

LS6427

iTrader: (2)

NOPE.....you will NOT get a code or a SES light when the 02 sensors on failing or even when they are real bad. Some PCM's will come on sooner than others, some PCM's won't do anything till the engine barely running....

All PCM's are NOT the same. Mine won't give me a SES or code for failing 02 sensors till the engine will hardly get above 35mph.......

Just scan it, see how they are switching. Change if necessary.

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1ltcap

p0153. cel on. just did that one today.

NC01TA

On a recent trip I found black residue (not oily, but like soot in a chimney) in the tail pipes, like the fuel mixture was off. Just guessing and trying to figure it out. Could it be O2 sensor related? The gas mileage was still good. Car ran fine overall. Thanks for the replies. 29,000 miles on the car. No sensors ever changed as I am the only owner.

LS6427

iTrader: (2)

The engine can run perfect and still be PIG rich....and black soot in the tailpipes is what you'll get. You could also, maybe, get a black cloud of smoke when you go WOT from a cruising speed like 50-60mph. Not WOT from a stop though.....

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1ltcap

at 29k you shouldn't have to change any sensors as yet. i'd check a few things though. if there's soot in the tailpipes, you're running rich. if you're running rich, something's not right.
to be honest, that could be caused by the o2's, IAT, CTS, MAF, or even something as simple as a dirty air filter.

VandykeT/A

When I was breaking in my 9 inch I noticed my car had alot more cam surge than normal. About 2 days later it through a code. I've heard people refer to 02 sensors as being lazy.

1ltcap

that's what i described a few posts up.

1ltcap

one other thing to remember....often times, the o2 sensor isn't the criminal, but rather is the victim. always check basics before trying to condemn any sensor.

SparkyJJO

iTrader: (11)

My Trailblazer's sensor died very suddenly. It started running really bad, lost power, and almost stalled, then the SES light kicked on and I got power back. It had switched to open loop because the sensor had quit. Replaced the sensor and all has been good since.

That's usually not what happens with those things however. Usually they get lazy and start affecting performance and MPG.