how do you know your oxygen sensor is BAD??? - PeachParts Mercedes-Benz Forum

mushedroom · #1 10-08-2002, 02:55 PM

is there a way to find out if the oxygen sensor is bad, without taking it in to a mechanic?

i've got a 1992 300CE with 166,000+ miles...
i'm not sure if the sensor has ever been changed (i'm third owner), but i am experiencing strange throttle response changes....in the morning after a good warm up, the car is peppy...but afternoon evening or after the car's been driven, the car is sluggish off the line...even hesitates sometimes...

i've notice a drop of 1-2 mpg also and all this happened so suddenly and quickly...within a 2 week period of time...

i read somewhere on ebay (a vendor selling non-spec o2 sensors by bosch (where you have to splice them in)) that gave all the similar symptoms i'm experiencing, and also pegged my mileage in the range when these things go dead... i.e., 10-12 years or 150-170k mileage....

is this true?
anyone?
please help.....

Vronsky · #2 10-08-2002, 03:18 PM

O2 element shoul read 450mV at 1500 rpm
heater should read 0,5-1,3 A
If not, replace

Cheers!

mushedroom · #3 10-08-2002, 03:26 PM

hey vronsky
in non-mechanic terms how do i gauge these readings?
i'm no mechanic nor do i have special diagnostic tools, but i am mechanically inclined and i am capable of doing minor/intermediate repairs without too mush of a problem...

so with that said how do i find out these readings again?

thanks

Vronsky · #4 10-08-2002, 03:46 PM

Find a multimeter that's sensitive enough to read millivolts, and ampere's. Pull out the cable that leads from the sensor at the small box (NOT at the downpipe), and read the voltage between the contact and mass/earth, while engine revs at 1500 rpm. Anything less than 450mV means replacing.
Next, pull out the cable of the heater element. Switch on ignition (do not start). Connect the brown cable to earth/mass, and the measure current between the other two contacts. That should read between 0,5 and 1,3A if okay.

Good luck.

Cheers!

AtlBenz · #5 10-12-2004, 09:22 PM

Quote:

Originally Posted by Vronsky

Find a multimeter that's sensitive enough to read millivolts, and ampere's. Pull out the cable that leads from the sensor at the small box (NOT at the downpipe), and read the voltage between the contact and mass/earth, while engine revs at 1500 rpm. Anything less than 450mV means replacing.
Next, pull out the cable of the heater element. Switch on ignition (do not start). Connect the brown cable to earth/mass, and the measure current between the other two contacts. That should read between 0,5 and 1,3A if okay.

Good luck.

Cheers!

Anybody know where the leads connect in a 1994 E420? What (and where) is the small box that you speak of. And do you mean to totally disconnect the O2 sensor and measure the resistance between it and ground, or just pull up the leads enough to stick the meter in parallel?

Vronsky · #6 10-13-2004, 09:24 AM

Quote:

Originally Posted by AtlBenz

Anybody know where the leads connect in a 1994 E420? What (and where) is the small box that you speak of. And do you mean to totally disconnect the O2 sensor and measure the resistance between it and ground, or just pull up the leads enough to stick the meter in parallel?

Leave the O2 sensor itself alone. Follow the lead from the sensor to the black box and disconnect the lead there to do the measurements described. HOWEVER: that procedure concerned my former, 1991 200E (4cyl.), and I'm would very surprised if your V8 is the same. Better look somewhere else.

stevebfl · #7 10-13-2004, 03:35 PM

Before you all go buying O2 sensors, I would suggest that the proper voltage output of a fully heated O2 sensor depends on the mixture it is sensing. Its goal is to sense and the systems goal is to achieve an average reading of Lambda or .45 volts. A good sensor will not likely ever stay at .45 volts. It is easily overwhelmed by mixture reading close to 1v at CO percentages of 1-6%. At 6% the engine starts to run bad. At 1% the engine is way out of emissions range.

The easy, cheating, way to evaluate the system is to disconnect the sensor portion if possible (leaving the heater hooked up - even if one must jumper it - many applications can't keep the O2 lit off without the heater). Place your volt meter across the sensor pair of wires so that you read the sensor output.

Find the wire going to the computer that carries the sensor signal and ground it. With the engine reved one should get 1v output from the sensor which your voltmeter is hooked to. Now take the sensor wire and place one hand on a wire connected to the sensor to computer input and place the other hand on a 12v source. This tricks the controller into leaning the system and the voltage reading at the sensor should go to close to zero.

Modern cars concern themselves with how fast the change of voltage can take place. This testing will only qualitatively address that criteria. It is a way to see if basic function is capable.

Most O2 sensor codes originate from static sensor output voltage over a period of time. if the mixture is farther rich or lean than the controllers capability to correct then it will stay at one of the end stops 1v for too rich and 0v for too lean. This is NOT a sensor problem.

NDavies4 · #8 11-11-2004, 07:48 PM

Don’t want to hijack this thread, but do have a similar/associated problem and questions. Have just done a rather detailed new thread “Can O2 sensor be causing my poor idle problem??” that I’d much appreciate if the experts participating here could help me with.

Regards,
Neil (London)

mushedroom · #9 10-08-2002, 03:48 PM

hey vronsky
thanks

next dumb question...
is the o2 sensor located under the hood or under the car?

someone told me under the hood another said under the car
ugh
i've got a 1992 300CE

thanks

rich

Vronsky · #10 10-09-2002, 02:20 AM

Its either under the car or under the hood

If its under the hood, its mounted in the manifold with the cable leading to the area where the battery is placed.
If its under the car, the cable can be reached by pulling up the carpet under the left front seat.

Cheers!

drbrandini · #11 10-09-2002, 08:08 AM

mushedroom,

We're in this togethor, I am having similar problems and you asked this question before I could get to it.. nice work, btw did you ever find some thule racks for your car? Let me know !

pentoman · #12 10-09-2002, 10:22 AM

There can be multiple lambda (Oxygen) sensors along the exhaust system. I recently attended a short Bosch training course on lambda sensors and posted this text when I got back:

For the record:
The lambda sensor's main purpose is to reduce emissions. It measures the amount of oxygen in a car's exhaust gases and adjusts the fuel/air mixture to compensate. If its voltage is low, the ECU sees this and richens the mix (longer injection duration) and vica versa if voltage is high, to get the perfect mix for clean combustion.

When working ok, and on constant throttle/idle, the sensor should alternate between high and low voltage around once a second (it is a simple device with no state for 'mix is just right').
Under acceleration, the ECU ignores it and adds more fuel, so in this case the sensor would be 'high' for too much fuel.
I won't go into more detail

in the post here:

California officers plug into your car to see how illegal you are?

good luck!

mushedroom · #13 10-09-2002, 12:14 PM

ok...
here's another question..
my car has always ran very very rich..i mean i can't stand behind the car for too long if it's warming up due to how heavy the exhaust fumes are...could this be a strong indication of an o2 sensor going awry?

i don't have any check engine lights on but these are my current symptoms...

1. random hestating/sluggish off the line performance
2. loss of 2-3 miles per gallon
3. heavy exhaust fumes (and i mean make you high and die heavy)
4. burning 1 1/2 quarts of oil 600 miles before next schedule oil change.

also an autozone sales person told me that if i change any exhaust items on the car, that would mean i need to change the O2 sensor too....is this true?
all i have done as far as modifying the car is change out my stock muffler to an arospeed muffler....

please help
thanks

mushedroom · #14 10-09-2002, 12:15 PM

oh also,

still haven't done anything on the rack for my mercedes....

the only one that i have found has been from mercedes and they want a million dollars for it... (ok just kidding but sheesh....700.00 is close enough...just for a bike/surf rack?????)

mushedroom · #15 10-09-2002, 12:16 PM

hey brandon, i'm still interested in a thule rack for my integra though....
please let me know
thanks

now back to this O2 discussion
thanks everyone