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Discussion Starter #1
Everyone who has installed a header on their ka24 altima most likely has had the check engine light drop "P0420 Catalyst Efficiency Below Threshold" code. After about 2 weeks of research and trial and error, I am 90% positive that Ive found a fix.

Before you understand the fix, you must be aware of why the CEL is on. When you install ANY header, your removed the stock exhaust manifold which contains a pre-cat. There is also another catalytic converter downstream which is used as a smog cat. The new header does not contain that pre-cat, and there just pipe where once this pre-cat existed.

The Post Oxygen sensor is placed to monitor the efficiency of this cat. Each exhaust manifold is considered a bank. Bank 1 and Bank 2. 4 cyclinders contain one bank. On each bank is usually 2 oxygen sensors, the first being the furthest to the block. P0420 refers to bank 1 sensor 2 (downstream sensor).

There are certain measurements that the downstream oxygen sensor must be in accordance with. The ecu monitors the first and second oxygen sensors. The Ecu also measures the 2nd oxygen sensor based on the upstream oxygen sensors readings. The upstream oxygen sensor is also used to control engine conditions based on o2 readings.

Here is the formula as stated in the 1999 Altima service manual

As long as the rear o2 sensor is within 75% voltage reading as the upstream sensor, then the pre-cat is working.


Now to trick the ECU, when you install your header, you increase the voltage sent to the ecu by the downstream sensor. I am using an obx header, and at 60mph, the 2nd oxygen sensor was approx. 94% reading as the front o2 sensor. By lowering this voltage sent to the ecu, it will trick the ecu into thinking the cat is working.

How o2 sensor harness is defined:

There are 4 wires on the downstream sensor. It is a TYPE A sensor. The White wires are to heater circuit, grey is a sensor ground, and the black wire is the signal to the ecu.

By Soldering a 1.2k Ohm 1/2 watt resistor in series with the black wire on the downstream o2 sensor, I was able to decrease the ratio so that my reading at 60mph is now at 45%. you must be careful not to use a resistor too large or the ecu will not see any reading from the sensor.

So far my CEL has not been on since, but if it does come on, I will up the resistance of the black wire.

Hope this helps.
 

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Damn seems like too hard a job than just getting a hole made on the pipe and relocating it. If i get someone to help maybe i'll try this. Good job man, thats alot of research.
 

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great job, where did you get the resistor from?

and by changing the rear 02 reading, did the fuel consumption and engine power change?

I know the first 02 sensor determines the air/fuel ratio. does the second one play a part in this or its just to determine if the pre- cat is working?
 

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i'll be getting my HS header soon so i may have to check it out while i'm installing it.

speaking of the o2 sensors and installing the header, i am supposed to reset the ecu after the car has warmed with the new header right? well my problem is my temp sensor broke so i just left it off, will my car still automatically adjust to the new settings and calibrate itself?
thanks and sorry if i'm getting OT.
 

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altimas.net OG
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i got the hotshot 4-1 header on a 96gxe...

light turns on sometimes.....sometimes it doesnt

im pretty sure the valuable info you have posted is
direct for the 2nd gens? am i right?

i dont know..

-shek
 

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Discussion Starter #7
se_12alty said:
i'll be getting my HS header soon so i may have to check it out while i'm installing it.

speaking of the o2 sensors and installing the header, i am supposed to reset the ecu after the car has warmed with the new header right? well my problem is my temp sensor broke so i just left it off, will my car still automatically adjust to the new settings and calibrate itself?
thanks and sorry if i'm getting OT.
Why would you need to reset the ecu?? The ecu monitors by driving cycles, Cold - Hot - Cold. It "resets" every driving cycle when ign is off and no faults were present
 

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I AM LEGEND
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Danny,I talked to my female co-worker who drives an Older gen Impreza with a WRX engine swap,and I mentioned this method to her and she stated it is an old Subaru trick and does usually kill that CEL regarding the 02 sensor.Let us know how it goes...

cause if it works,then I am just going to forget about getting a cat and just install the HS w/o it.

Thx.
 

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yeah, the rear o2 sensor basicallt just makes sure the cat is working. the front 1 is what the ecu reads for engine management.

danny, in series to what? the gray wire? so black wire to resistor to gray wire,right?
 

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Discussion Starter #12
no I Cut the black wire on the 2nd oxygen sensor and I placed the resister "in line" or in "series" with the black wire

Black wire ---------------------------- X cut X------------------------

--------( | | | | )------ 1.2k

Black Wire with resister is series:

o2 harness--------------------------( | | | | )------------oxygen sensor
 

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There is no reason why you should have a cell for changing the Header. The only reason I got a cell is cause I had to cut the EGR pipe. that was all. Got the replace and everything was fine.
 

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Discussion Starter #14
^ You dont know what your talking about, these alot of people on her that get a CEL after installing a header.. if you havent then your lucky but dont go assuming that what is true for your car is true for everyone elses.
 

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ALTIMA FREAK said:
There is no reason why you should have a cell for changing the Header. The only reason I got a cell is cause I had to cut the EGR pipe. that was all. Got the replace and everything was fine.
Ooooh, look who knows so much....:rolleyes:
 

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btw, nice writeup Danny. I just ignore the code since I know why the ecu is throwing it.

And I've passed emissions twice now (a span covering 4 years)
 

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There are a couple of vendors out there that provide a small device called O2 mimick or some kind of name used to plug into the harness connected to the ecu. This device provides the correct amount of resistance to "fool" the ecu into thinking there is a properly working O2 sensor there. That is what I plan to get when I aquire my header. I have the first gen but the obd2 system with the 2 for and aft sensors.
 
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