1990300se I had cel come on 3 weeks ago.checked code ,faulty 02 sensor
so I replaced it with new OEM sensor cleared code after a week cel back on!
checked code same bad 02 sensor now what? what else can I check.only thing I can check is the wiring .any ideas?
also how many 02 sensors are on this car? I'm pretty sure there is only one

You can probe the wires while it is plugged it to figure out what is going on.

Both white wires are the heater circuit and the black/green wire is the sensor wire to the ECU. The heater circuit has no influence in the CEL, so probe the black/green wire for a reading.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

what kind of voltage should I have there? 12volts? key on or engine running?

YOU SHOULD BE GETTING A 12 VOLT SIGNAL FROM THE WIRES WHEN THE KEY IS IN THE HOT IGNITION ON POSITION,LOW VOLTAGE IS USUALLY A FAULTY O2 SENSOR,BUT IF YOU REPLACED IT YOU MAY WANT TO CHECK THE WIRES,MAKE SURE THE CONNECTOR ENDS ARE CLEAN NO GREEN CORROSION ON EITHER END.MY 02 HAS 2 CONNECTOR ENDS ON IT AND THERE LOCATED UNDER THE PASSENGERS RUG,SOMETIMES THEY GET DAMAGED FROM BEING UNDER THERE OR MOISTURE DAMAGE,IF YOU FIND ISSUES WITH CONNECTOR END YOU CAN ALWAYS HARD WIRE IT WITH BUTT CONNECTORS.

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enjoy life,take the coal that is handed to you and turn it into diamonds. { PHILLIPIANS 4:13}

The O2 can also trip if its out of range. In other words, the O2 is working to its limits. Hows the idle and exhaust scent?

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Car runs good .idle a little rough but not horrible.ex does smell a bit
drove the car to LA last weekend ran fine no problems
this car has been cared for. at 100k the dealer rebuilt the top end T-chain and lot of stuff .the old owner was a stickler.I have all the reciepts for it from Fletcher Jones Newport Beach Ca.
I replaced the plugs and wires
water pump
also the srs light came on today for 20sec than went off?

Run some Seafoam through the gastank. Cleared up my check engine light on my '05 Altima SER. I also got a Duty Cycle Meters from Sears and adjusted my air/fuel mixture on my 300E (do a search for how) which, if out of adjustment, may cause issues. I live in OC, Ca. Have 195K on my car. When I bought it, it was running and idling like crap and struggling to start. No issues now (well, except for the delayed reverse and drive engagement which Ps2cho has a detailed fix for on his website...thank you in advance!). Also, spray some contact cleaner into the steering wheel space behind the airbag and might fix your SRS light. Mine was coming on a lot and I did that...nothing in 6 months so far!

Steve

I love Seafoam.

http://www.youtube.com/watch?v=vRAV5ln5cUc

And as sptt says, check the duty cycle. If your air/fuel is off you will have a rough idle and probably bad economy. I used a simple digital volt meter to check and adjust mine.

M117 how to test lambda?

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

I will check duty cycle.? what about the Cat converter.160k on it .if it is plugged up could this trigger the o2 sensor?

lowesguy, remove the air cleaner housing and check if the air flow sensor is dirty. Cleaning the air flow sensor on our 1995 E320 resolved a persistent CEL.

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Fred Hoelzle

Thanks all this info really helps!
on my 93 escort that was the problem to