# How does one check the O2 sensors?



## pyrophilus (Feb 8, 2005)

Hello,

I wanted to know if there was a way to check the O2 sensors.

I wanted to diagnose the problems by myself because everytime the SES light comes on, the local dealers want me to spend hundreds of dollars on needless things before they decide to tell me what's really wrong with the car.

For example, The SES light came on last summer, and after talking to few folks, I thought it was faulty MAF. Back then, the car (2001 Pathfinder LE with VQ35DE) was under warranty, so I asked them to check the MAF. Dealer service insisted that I do the "intake manifold cleaning" FIRST before they do any sort of code reading! I asked them to check the MAF and they would not do it unless I agreed to the $120 cleaning.

I drove around for month or two. Considering the price of a new MAF (and not even knowing if it was indeed faulty), I agrred to the $120 cleaning, and what do you know? Dealer calls back and says, "guess what? You have a failed MAF"...

Anyway, MAF was replaced under extended warranty, but now SES light is on again. No hesitation, neither during acceleration nor idling, which makes me think that it's not the MAF? My fuel mixture was runing rich for a long time (due to the failed MAF before). Could this have damaged the O2 sensors?

How does one check the O2 sensors? I am afraid that dealer will want me to spend couple of hundred $$$ before paying another $$$ for the actual fix.

Cheapest O2 sensors I saw on ebay for the 01 pathfinder went $120 for front sets (before CAT), and $240 for rear set (after CAT)!!!! Is there a cheaper alternative?

Also, does cracked exhaust manifold also produce SES light?

In very hot temperatures in the summer, after idling for while, the car would "whine" when revved. I was wondering if this was due to cracked exhaust manifold (common on Pathfinders).

I was wondering if cracked exhaust manifold would cause O2 sensor to not work correctly? I would assume that it wouldn't cause problems, because the exhaust manifold would have a positive pressure, so no outside oxygen would be able to come INTO the exhaust manifold through cracks (which is the only thing I can think of that would foil the O2 sensor)...

I also wanted to add that there is a "burnt plastic odor" in the car that started around the time that SES came on. Also, if I drive around for few days in local streets, the SES light would go off, but as soon as I go on a highway, it comes right back on. Anyone know if this is a sign of a bad O2 sensor, CAT or both??? Or something else???


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## xbrandonx (Mar 27, 2004)

autozone or advanced auto will read your codes for free.


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## pyrophilus (Feb 8, 2005)

xbrandonx said:


> autozone or advanced auto will read your codes for free.


Went to Autozone already. They guy just told me that it reads "Emissions system fault". No kidding...


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## KMG9185 (Dec 15, 2005)

if your engine is running rich it will cause damage to your o2 sensor along with your cats...if expose to this rich condition for a consider amont of time........ ..... but if your car is anywhere near 100k i would change the o2 sensors at a maintance thing any how...... just to guaruntte that you are getting the best fuel economy specially with these gas prices........ ...


kevin


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## technut (Mar 30, 2007)

Here is what I know about the o2 sensors in my Sentra. This is assuming you have 2 known good o2 sensors. They are 3 wire type. You have 12 volts that is feed to the o2 sensor from the ignition switch, a ground and an output to the ecm. The 12 volts from the ignition switch should be hot when the switch is in on or start. Once running and warmed up you should have approximately 1 volt to the ecm. So I believe that if you manually apply 12v to where the switch is connected to the o2 sensor and the problem remains you have eliminated the switch being a problem. Now apply approximately 1v to where the ecm input from the o2 sensor is. If problem goes away you know the o2 sensor or wiring to or from is bad. If the problem does not go away your ecm is bad.


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## datsun74 (May 7, 2007)

technut 



very clever


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## leigh08 (Aug 9, 2005)

If your car has lost several miles per gallon of fuel economy and the usual tune up steps do not improve it. If the computer has issued a code pertaining to the O2 sensor, the sensor and it's wiring should be tested. Usually when the sensor is bad, the engine will show some loss of power, and will not seem to respond quickly. Bad nissan oxygen sensors can cause high idling, rough idling, stalling, etc


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