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Old 05-26-2009, 04:24 PM   #1
Howlleo
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O2 sensor and TPS: What do they do, and their relative importance?

If this is in the wrong place, please let me know.

I just bought (my first ever) used car - a '96 Infiniti G20, automatic. 4-cylinder engine. The mechanic told me it needed a new O2 Sensor and a new throttle position sensor.

I'm trying to figure out what they do and which is more important, because I can't afford to replace both. Or, for that matter, if I should replace either.

The car runs well (a little slow on the uphill), but otherwise very nicely. It vibrates a drop, but I've been told that that's because it's 4 cylinder, not because of engine problems.

So, what do these sensors do, and which is more important to a well-running vehicle? (Please try to keep the jargon down.)
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Old 05-26-2009, 05:11 PM   #2
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Re: O2 sensor and TPS: What do they do, and their relative importance?

O2 stands for oxygen sensor. It tells the computer how much oxygen is left after combustion. TPS stands for throttle position sensor, it tells the computer how far you have pressed the accelerator. Both sensors are important for you car to run properly. If only replacing one now and one later I would go TPS, but until you do both, your car will not run right, or get its proper fuel mileage.
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Old 05-27-2009, 02:59 PM   #3
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Re: O2 sensor and TPS: What do they do, and their relative importance?

So without a TPS, I'm probably having acceleration issues, and without O2 sensors, I'm probably not getting my full miles per gallon? And presumably acceleration problems leads to lower mpg anyway. Is there anything else that could be very wrong due to not having those sensors?

Also, I priced the repair at several mechanics, and one of them claimed that if the TPS is out, I will need a new distributor. Is he selling me snake oil because I'm a clueless female?

Thanks.
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Old 05-27-2009, 09:21 PM   #4
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Re: O2 sensor and TPS: What do they do, and their relative importance?

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Originally Posted by Howlleo View Post
Also, I priced the repair at several mechanics, and one of them claimed that if the TPS is out, I will need a new distributor. Is he selling me snake oil because I'm a clueless female?
Unscrupulous mechanics are apt to use nonsense bafflegab to take advantage of any poorly informed customer. The fact you are female has little to do with it.

Distributors as an entire unit are very rarely faulty and have absolutely nothing to do with the TPS.

The distributor cap and rotor do need periodic replacement (every few years or so) as part of a complete tune-up. They are quite inexpensive parts. But the entire distributor.... that is very likely nonsense. Don't go to that mechanic again, and tell your friends to steer clear too.

As for the TPS or O2 sensor..... A truly bad TPS sensor often gives you drivability problems, such as stalling, hesitation etc. If you do not have these, and the car drives nicely and normally as you say, you likely do not need a TPS.

The oxygen sensor does need occasional replacement..... at about 50,000 miles or so. Often they do last longer but do eventually fail, causing poor gas mileage.
However, I have found often people neglect to change the O2 sensor, so it is quite likely that your 13-year-old car actually does need a new O2 sensor.

So.... replace the O2 sensor and enjoy the car.

BTW is your "Check Engine" light illuminated? (It should be somewhere on the instrument cluster, beside the speedometer.)
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Old 05-31-2009, 07:47 PM   #5
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Re: O2 sensor and TPS: What do they do, and their relative importance?

Yep, the light's on.

An ASE certified mechanic said it was these two sensors that are the problem.

I'm just confused because I'm not getting the symptoms - no stalling or coughing, for example.

But I think I'll get the O2 replaced. It's the cheaper of the two.
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Old 05-31-2009, 11:52 PM   #6
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Re: O2 sensor and TPS: What do they do, and their relative importance?

If it's your first car you may not know this, so just for future reference - you don't need to automatically go to your mechanic when the Check Engine light illuminates - there are several places which will read the code for free (Autozone, amongst others, I believe). They'll likely tell you what fault is indicated by the code and what action might be required, and by all means take note of that, but more importantly be sure to take a note of the actual code number, then you can come here, quote the code number and ask for advice, and then you'll be better informed when you do visit your mechanic.
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Old 06-01-2009, 07:40 PM   #7
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Re: O2 sensor and TPS: What do they do, and their relative importance?

Just found out about that and went to Auto Zone. One code was 1400. Underneath it says for Infiniti - EGRC solenoid valve. Probable cause (according to the print out):
- Open or short circuit condition
- Poor electrical connection
- failed EGR solenoid valve

That doesn't even sound like the TPS. :-/

The other is 136, which is the rear oxygen sensor.
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Old 06-01-2009, 08:02 PM   #8
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Re: O2 sensor and TPS: What do they do, and their relative importance?

Quote:
Originally Posted by Howlleo View Post
Just found out about that and went to Auto Zone. One code was 1400. Underneath it says for Infiniti - EGRC solenoid valve. Probable cause (according to the print out):
- Open or short circuit condition
- Poor electrical connection
- failed EGR solenoid valve

That doesn't even sound like the TPS. :-/

The other is 136, which is the rear oxygen sensor.
The EGR valve is an emissions control device, and is not related to the TPS at all. Look here for more info:

http://en.wikipedia.org/wiki/EGR

IMO an EGR valve which is stuck closed will not significantly affect performance. It will increase the oxides-of-nitrogen emissions only. If you are short of cash, leave this one alone for now.

The rear oxygen sensor should be attended to. Modern cars made using OBD II (after 1995) use two oxygen sensors, both mounted on the exhaust system.
The front one is upstream of the catalytic converter. The rear one is downstream of the catalytic converter.
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