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Downstream O2 sensor?


TomB985
08-13-2007, 09:42 AM
My '95 Blazer has been throwing a code for awhile for the downstream O2 sensor, and I'm curious as to what kind of effect this is going to have.

The way I understand things, the downstream sensor is just to monitor the catalytic to make sure it's working correctly. Will this have any effect on fuel trims, mixture, or driveability?

Is it worth fixing now, or can it wait?

MT-2500
08-13-2007, 10:39 AM
My '95 Blazer has been throwing a code for awhile for the downstream O2 sensor, and I'm curious as to what kind of effect this is going to have.

The way I understand things, the downstream sensor is just to monitor the catalytic to make sure it's working correctly. Will this have any effect on fuel trims, mixture, or driveability?

Is it worth fixing now, or can it wait?

What engine and engine code? PCM or VCM computer.
What was the 02 sensor code no?

TomB985
08-13-2007, 11:27 AM
It's the 4.3L, Vin w, VCM, codes P0137, P0141, 91,000 miles

MT-2500
08-13-2007, 12:03 PM
It's the 4.3L, Vin w, VCM, codes P0137, P0141, 91,000 miles


The codes indicate a heater problem and low voltage on Bank 1 sensor 2.
If this is a 4 02 sensor system. It would be the sensor in front of the cat.

The downstream behind the cat 02 sensor would be Bank 1 sensor 3.

I would check the 02 sensor heater circuit and wiring and if the wiring is ok I would replace the sensor.

The VCM checks all 4 02 sensors readings and if one is dropping out it can throw off fuel mix.
MT

TomB985
08-13-2007, 07:40 PM
My blazer only has two oxygen sensors, one before the cat and one behind it...

http://i195.photobucket.com/albums/z162/tomboy376/42406164.gif

So, even though this is the downstream, it will affect mixture?

blazes9395
08-13-2007, 10:03 PM
Yeah the '95 was the only year that had two sensors, the pre and post convertor sensors. We had a good discussion on this a few months ago, essentially the post sensor doesn't do "too" much, its main purpose is to monitor convertor effeciency but if its throwing codes, it may be the convertor is actually failing. If the convertor is failing, you could run into problems such as a restricted convertor and thats where many driveability problems occur.

TomB985
08-14-2007, 07:25 AM
Good point. Is it common for a cat to fail with just 91,000 miles?

blazes9395
08-14-2007, 10:12 AM
Has your CPI leaked? Missfire, or more specificly unburnt gas is usually the leading cause of a convertor going bad. Basiclly your overloading it and it begins to overheat and self destruct.

TomB985
08-14-2007, 10:28 AM
Yep, I've actually had to replace that damn thing twice. Once when I bought it, and I have no idea how long it was running like that, and once when it wouldn't start for my wife, but it didn't run like that for more than a few miles.

Come to think about it, it's been rattling quite a bit since I bought it, and it's not coming from the heat shield.....

BlazerLT
08-14-2007, 03:04 PM
Replace it, you cannot keep on driving without it. O2 sensors are so cheap today, you might as well replace them both at the same time.

My engine ran a lot better once I replaced them both and was don't with it.

The rear sensor is not just a cat monitor, it also ajust long term fuel trim levels for efficiency.

I had a increase of 2mpg and more pep after I replaced them both.

MT-2500
08-15-2007, 12:19 PM
I rechecked the code no for the one with 2-02 sensors.
It is a code for rear sensor.
Check your fuel trims to see if it is affecting fuel mix.
They should be around 128.
Also Evaporative emissions will cause the o2 sensor to set the 141 code.
And watch fuel pressure and tune up end.
Good Luck
MT

TomB985
08-15-2007, 04:51 PM
Okay, not to kick this one around too much, but...

Back to what blazes9395 said about the cat, is there a good way to test it? It's been rattling like crazy for the past 7 months, and when I checked, it's coming from inside the cat. Any thoughts?

MT-2500
08-15-2007, 05:26 PM
A engine vacuum gauge will check for blockage.
Check vacuum at idle and at 2k rpm.
If vacuum drops under idle vacuum you have exhaust blockage.
If it is rattling it is lose or broken up inside.
To see if it is working good or doing it job you will need to watch your front and rear 02 sensors on a scanner.
As long as 02 sensors are good???
You should see more and higher rich to lean on front 02 sensor.
MT

old_master
08-15-2007, 07:43 PM
Like MT said... with everything working as designed, the rear O2 sensor should remain steady around .4 volts. If voltage fluctuates similar to the front sensor reading, the converter is not doing its job.

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