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P0420 Fix !!

73K views 26 replies 16 participants last post by  Rmartin38 
#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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#4 ·
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?
 
#5 ·
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.
 
#6 ·
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
 
#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
 
#10 ·
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.
 
#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
 
#13 ·
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.
 
#19 ·
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.
 
#23 ·
Use a flared wrench, I used a standard long wrench and broke loose with ease.. Just give it a good whack with a hammer and it will break free.

"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."

- well I just gave you enough info so that you dont need to BUy anything but 1 resistor and some solder

"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)"

- Well now you can get rid of that damn light :D , and I believe that the 2nd cat is also a 3 way cat and lowers N0x and co2 among others. In theory, any CLEAN running engine should pass emissions with no cat (except in cali) All the cars ive seen with hollowed out cats, believe it or not were STATE INSPECTORS but just for shits and giggles did the emissions on their cars and they passed !!! not by too much though.

"So, this should, in theory, work for someone running a test pipe, correct? Basically fooling the ECU into thinking there's a cat there."

- Well, this is a fix for people who've removed their primary cat at the manifold. By saying test pipe, you mean taking out the downstream cat and placing an empty pipe in there, I have no idea. All I know is that this will fix the p0420 code when you remove the pre-cat (primary cat). If you think about it, there is no oxygen sensor after the downstream cat, so I dont know if youd get a MIL after installing a "test pipe". How would the ecu know the readings after the downstream cat, to know if its working properly?? Anyone every got a CEL after installing a "test pipe"... Sorry, I dont know about the 2nd cat
 
#24 ·
It's not that all that complicated. Your first o2 sensor reads the oxygen content of the exhaust right out of your ports. This is the o2 sensor that controls the A/F ratio, making sure that the mixture is 14.7 to 1 (I think) . This is the ideal mixture for max catalytic converter efficiency. The 2nd o2 sensor takes the oxygen reading after the exhaust has past through the exhaust manifold catalytic converter. This sensor is only there to tell you if your cat is working properly. If the 2nd o2 is reading a different oxygen measurement that then 1st o2, congradulations, your cat is functioning. but if the 2nd o2 is providing the same reading as the 1st o2, then you'll be thrown the "service engine soon" light.

My check engine light has never come on because I've relocated the 2nd o2 downstream of the 2nd cat.
 
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