Here’s how to hack your oxygen sensor to get rid of a P0420 OBDII code and pass an emissions test.
The P0420 code is thrown when the computer detects irregularities in the signal coming off of the secondary oxygen sensor. If the catalytic converter is operating at less than 95% efficiency, the code comes up often indicating the catalytic converter needs to be replaced.
Catalytic converters can get quite expensive for an OEM replacement, and since these codes often show up on cars that are older, it isn’t cost effective to replace. Aftermarket replacements may work, but are known not to last as long as the OEM catalytic converter.
Using a spacer on the oxygen sensor is another method to deter the P0420 code from showing up, as it exposes the O2 sensor to less oxygen passing through the cat, thus fouling the readings.
However if your car’s underbody is rusty and the Oxygen sensors are likely frozen, another method can be deployed, that is to change the O2 sensor’s outputs before it goes into the ECU. This will fool the computer into thinking the readings coming in from the cat are legit and it can pass an emissions test.
This is done using an RC circuit, which acts as a low pass filter. It only costs $0.50 or so for a capacitor and resistor, which is to be soldered in a series-parallel circuit on the secondary oxygen sensor line.
On this Honda Accord, the oxygen sensor wire is under the seat. The white and blue wire represent negative and positive. The resistor is spliced in series and the capacitor is spliced in parallel. Further explanation about RC circuits is demonstrated in the video.
Once the vehicle was driven for 40 km, the check engine light stayed off and the emissions test was passed with only one “not ready” monitor.
Note: The capacitor and resistor values presented here are a starting point. It may require some tweaking to get the correct value that works with your vehicle.
Here’s a good reference video: