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Which O2 sensor needs replacing?!

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Old 07-29-2008, 04:30 PM   #1
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Which O2 sensor needs replacing?!

I'm new here, but I have a question that may cost me a lot! I have a '98 Tacoma 2WD manual, and got CEL some time ago. I moved to Atlanta and now have to have emissions inspections. I have not noticed any problems with the vehicle at all (conditions-wise), but got codes P0171 and P0420. Tried to have the guys at AutoZone to help, but I got responses varying from "replace the before cat o2 sensor", to "it's the MAF" to "it's the catalytic converter". I seriously got all different answers from 10 different people. Anybody know definatively what is the answer?
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Old 05-24-2009, 12:26 PM   #2
Its Wicked Flow BITCHES!!
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Working at Autozone, I can tell you that they could print out the trouble code for you, BUT it lists several causes, usually 4 the first usually being the one to fix. So if they told you it could be this, no this, no that, it may be a combination of one or all of the above. If you really wanted to know what it could be, get a diagnostic on it. Sure its about 80 bucks, but at least you'll you what is wrong. Thats what i tell my customers, I never tell them oh its this. Just a little FYI.
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Old 05-24-2009, 12:32 PM   #3
With sufficient thrust, pigs fly just fine.
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The P0171 code most likely means the MAF is dirty, and can be fixed by cleaning it.
The P0420 Code lists These as possible triggers:
Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings
And these as possible fixes:
Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor (HO2), replace if necessary
Replace the catalytic converter
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Old 05-24-2009, 12:32 PM   #4
With sufficient thrust, pigs fly just fine.
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Oh...Welcome to TW!
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Old 05-24-2009, 02:47 PM   #5
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i originally had the p0420 code and then later i had a code for running lean. i ended up replacing the upstream 1/1 o2 sensor and that fixed it.
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