Here's how to diagnose and hack around the P0420 Inefficient catalytic converter code on a Honda.
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Step 1: Oxygen Sensor Basics
The catalytic converter's performance is measured by the secondary Oxygen sensor.
If the output is less than 95% it throws the P0420 code. Graphing the output on an OBDII scanner shows an erratic curve with varying amplitude and frequency.
Step 2: RC Circuit Basics
To smoothen out the curve, an RC low pass filter circuit is used.
An RC circuit is a filter that will slowly charge and discharge giving us a curve that the computer will accept without throwing an engine code.
The time constant governs the smoothness and recovery of the output relative to the input.
Superimposing the O2 output and RC filter gives a nice smooth curve.
Step 3: Wiring Diagram
Here's the wiring diagram for the RC circuit in the Honda Accord. The values are starting points and not necessarily optimized.
Step 4: Oxygen Sensor Output - Before
Notice how the curve is erratic and the amplitude range is between 0.1V and 0.9V.
Step 5: Oxygen Sensor Wiring
The O2 plug is located under the seat. The white and blue wires were spliced into to wire in the resistor and capacitor.
Everything is soldered together and insulated with tape.
Step 6: Oxygen Sensor Output - After
Notice how much smoother the curve is and the amplitude isn't as large.
Step 7: Passed an Emissions Test
After driving 40 km, all of the monitors in the computer except for one read "ready", and the vehicle passed an emissions test! No more P0420 code.
Note: if you get another code for an O2 sensor frozen, its because the resistance and capacitance values were not optimized to match the curve.
The computer uses the readings from the narrow band oxygen sensor before the catalytic converter and the secondary sensor to determine if the cat is working properly.