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.