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Old 10-29-2011, 07:19 PM   #1
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Default What does the secondary hO2 sensor do?

I'm working on my lady friends' 96 HX civic with MT. It's giving code P0141, secondary O2 sensor. For four years this codes been on and she said her old mechanic told her it was no big deal. I just started working on this car because the CEL started flashing. I pulled the codes and it was insufficient lift in the EGR valve (P1491) and of course the 2nd O2 sensor. Well I cleaned the EGR chamber out (it was fully clogged). Understandably I still get the P0141.
So, if we have been driving this car for years with a bad secondary HO2 sensor, what does it do and do I need to replace it??????? The car got over 40mpg before I cleared the blocked EGR system.
I'm going to check the resistance in the sensor tomorrow to make sure the actual sensor is bad and not just an open circuit.
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Old 10-29-2011, 07:20 PM   #2
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Default Re: What does the secondary hO2 sensor do?

The secondary O2 sensor serves only an emissions function (monitors health of cat).

Did cleaning the EGR chamber clear the EGR code? If not, did you clean the valve?
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Old 10-29-2011, 07:29 PM   #3
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Default Re: What does the secondary hO2 sensor do?

Monitors CAT effeciency.

First o2 sensor reads exhaust gas measurements then after all that stuff flows passes through the CAT and is "cleaned", secondary o2 sensor reads the exhaust gas and compares what it is and what it should be.
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Old 10-29-2011, 07:30 PM   #4
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Default Re: What does the secondary hO2 sensor do?

Ron, thanks a bunch for your posts. They've helped me alot.

Yeah, the EGR code (p1491) is cleared. Everything is running good. I didn't have to replace the valve which saved me over $150. I did clean the valve out a bit but the clog was in the EGR chamber, what a mess! I'm hoping for MPG in the upper 40's now.
So if the secondary O2 sensor measures the health of the cat, could this mean my cc is bad?
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Old 10-29-2011, 07:35 PM   #5
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Default Re: What does the secondary hO2 sensor do?

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So if the secondary O2 sensor measures the health of the cat, could this mean my cc is bad?
As the secondary O2 sensor has not been working, the health of the cat has not been monitored. There's no way to know whether or not the cat is bad until the secondary sensor is working.

Is the secondary O2 sensor code still thrown?
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Old 10-29-2011, 07:44 PM   #6
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Default Re: What does the secondary hO2 sensor do?

Yep, getting just the P0141. If this thing just "reads" the exhaust, I don't understand why it's necessary to replace. I have great MPG, no black smoke and an engine that purrs (as much as a 1.6L can)!!! Am I missing something?
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Old 10-29-2011, 07:53 PM   #7
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Default Re: What does the secondary hO2 sensor do?

It's only important if you have emissions testing in your state or if you want to ensure that your car is not a gross polluter.
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Old 10-29-2011, 08:17 PM   #8
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Default Re: What does the secondary hO2 sensor do?

I'm going to assume it's not a gross polluter due to the high MPG and lack of noticeable exhaust. However, I found a replacement for around $65 so maybe I'll just swap it anyway, especially if the ohm resistance tests show a bad sensor.
Moving from CO to ID in about a month. I don't think Idaho has emission testing, Colorado definitely doesn't.
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Old 10-29-2011, 08:22 PM   #9
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Default Re: What does the secondary hO2 sensor do?

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It's only important if you have emissions testing in your state or if you want to ensure that your car is not a gross polluter.
Another possible issue being that the CEL is on and so if another code is thrown it might not appear any different from the dash.

Basically you could throw another code and not know about it. I have two codes in one of my cars right now and it's just a solid CEL. Same for all the cars I used to scan at my old job. The most codes I've pulled from one car was something like 12. A coworker pulled 17 once though iirc.
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Old 10-30-2011, 06:55 AM   #10
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Default Re: What does the secondary hO2 sensor do?

a flashing check engine light means a catalyst damaging condition . usually a sever misfire or a very rich a/f ratio
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Old 10-30-2011, 09:25 AM   #11
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Default Re: What does the secondary hO2 sensor do?

When the CEL started flashing, it threw the EGR insufficient lift code and also misfire in cylinder #3. I'm assuming the misfire was related to the EGR malfunction as it hasn't come up since clearing the carbon blocked EGR chamber.
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Old 10-30-2011, 09:26 AM   #12
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Default Re: What does the secondary hO2 sensor do?

I'm going to replace the secondary O2 sensor, just to make sure the CC is all good.
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