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O2 Sensor Location Help


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#1
snowyfield

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My 2000 626 V6 got CEL with code P0141 and P0161 ( from Autozone ). I researched the OBDII code and got these defition:

P0141: O2 sensor Heater Circuit Malfunction ( Bank 1 Sensor 2 )
P0161: O2 sensor Heater Circuit Malfunction ( Bank 2 Sensor 2 ).

Any one can tell me in more detail where these two sensors are located? :rolleyes:

Thanks in advance.

#2
flora macdonald

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Hi:

Welcome to the board!
Fix your P0141 code first. This could cause your P0161 code.

On Mazda 2.5 V6 engines, the rear cylinder head (nearest the firewall) is bank 1, and the front cylinder head (nearest the radiator) is bank 2. A common mistake is to replace the front sensor when a bank 1 sensor code is stored and visa versa.

Some O2 sensors here:

http://www.1motormar...h...sensor&my=1

Flora

#3
snowyfield

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Thanks for quick response. Here are more questions ( you can see I am a really newbie ): when I was shopping autozone.com for oxygen sensors, I was asked to choose "before or after catalytic convetor", "front or rear convertor". I am confused.

#4
NickR

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I don't exactly know the answer, but this may (or may not) help: "sensor 2" refers to the second sensor away from the engine. Sensor 1 means the sensor closest to the engine, sensor 2 is next as you follow the exhaust flow, sensor 3 comes after that, etc.

#5
flora macdonald

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Hi:

Sensor 1 is before catalyst, sensor 2 is after. See this link for more info:

http://www.automedic...2.5L exc. Calif.

Flora

#6
brian passarelli

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so like both of the rear sensors came up as a code ... and not that ocygen sensors ARENOT regualr maintenance replacement because they are but ... i would think that there is something wrong with ecu's determination, or there is possibly something wrong with the Cat (catalytic converter) or other issue causing the Cat to run wrong . so its not cut and dry.

easy home mechanic tests:
1) do a vacuum test and perform the procedure to determine clogged exhaust which would indicate bad catalytic converter
2) pull the 02 sensors and do a bench test with a propane torch and multimeter
3) visually examine condition of ignition parts like spark plugs

often, the 'after cat' 02 sensors come up as an error when something is causing the cat to run improperly (which could even be caused by faulty before-the-cat 02sensor(s))... or when they have a really lagged or incorrect response time caused by their wear or by other things -- bad connections, for example.

good luck!!

#7
NickR

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Forgive me if I wander just slightly off-topic. And forgive me, Snowyfield, if I just slightly hi-jack your thread (though you may be asking yourself the same questions).

With the help of Haynes, and Brian's and Flora's posts, I think I am now getting the feel for the 02 sensors on a 1998-2002 626. (I need to revise all my knowledge, now I've switched from a 1993 to a 2000.)

So, there are 4 O2 sensors: Bank 1 sensor 2 is at the bottom of the rear downpipe, and bank 2 sensor 1 is at the bottom of the front downpipe (easily visible when you open the hood). These two sensors are in the same positions as in the earlier OBD1 cars, and it makes sense that you would need two on a V6: one for each bank of cylinders.

And the bank 1 sensor 2, and bank 2 sensor 2, are both downstream of the cat.

But, why are there two sensors downstream of the cat? Wouldn't one be enough? (Or are there two cats, one for each bank?) Sorry, but my garage floor is cold and wet and oily at the moment, after getting my car rustproofed.

And Flora: why do you think he should: "Fix your P0141 code first. This could cause your P0161 code."? Wouldn't these two sensors be symmettrical, in their positions, and functions?

#8
flora macdonald

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Hi Nick:

I think you have it figured out....but why they put 2 O2 sensors after the cat is beyond me!
I said to fix the P0141 code 1st because on my car that would be the sensor in the exhaust pipe before the cat and the P0161 is the last sensor (in the exhaust pipe after the cat.) I posted not realizing til later that you guys have 2 before and 2 after, not 3 before and 1 after. Should have edited my post.

Flora

Bank 1 Sensor 1 firewall side before cat, Bank 2 Sensor 1 rad side before cat. Bank 1 Sensor 2 after cat, Bank 2 Sensor 2 after cat (for your car).

#9
twiggy144

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But, why are there two sensors downstream of the cat? Wouldn't one be enough? (Or are there two cats, one for each bank?)


Some cars have dual ( 2) exhaust lines all the way to the tail pipes. That might explain why the generic OBDII code exists for 2 downstream O2 sensors: one for each exhaust line. Dont know about the 626 or the Mazda 6.

#10
jpit

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Those 2 codes are just for the heater built in to the O2 sensor itself, not for the signal line. If you have a meter, you can disconnect the connector and measure the resistance between the two like-colored wires leading to the O2 sensor. Should be about 10ohms of resistance. If you have the correct resistance I would say that your O2 sensor heaters are most likely good and that you should be looking at a common point that supplies the voltage to the heater.

#11
flora macdonald

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Hi:

If the 'heater" fails, then the O2 fails.....you'll always get the code until you replace it!

Flora

#12
jpit

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Just trying to show a way of checking it.

I agree that the heater is inside the O2 sensor and if the heater fails then the O2 sensor is bad and replace it like you said. All I was trying to give was a way to check the heater to see if it is actually bad or whether it may be a circuit issue. If you measure about 10 ohms then it's not a bad O2 sensor heater, check the wiring. If you measure a high resistance then or an open, then it's a bad O2 sensor.

I just measured an old O2 sensor I have and it was 13ohms. That would be a good heater.

#13
snowyfield

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Today, I finally got Bank 1 Sensor 2 replaced. The reason I only replaced one of them is because, first flora suggested replace the first might get the second fixed too; senond AutoZone near my place has only one O2 sensor left in stock. So far after reset, the CEL has not come back yet. I will keep my finger crossed.

I mesured the old O2 sensor. There are four lines, two white, one black, and one grey. The resistence between the two white lines are 7.7 ohms. All others are open. The instruction of the new O2 sensor says the black line is signal line, the grey line is ground line, and the two white lines are heater lines. So I suspect my old sensor is fine. If that is the case, I do not know where the Code P0141 comes from.

Also, the location of O2 sensors are different for California Emission Control and Federal Emission Control. I got a diagram from Mazda dealer shop. Compare to the diagram, my car is for California Emission Control even I live in and bought my car in Maryland. I will find a scaner to scan the diagram and post it later.

#14
flora macdonald

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Hi:

Your "heater" should have measured between 10 and 40 ohms...so that low reading could have caused your code.

Flora




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