P0420 Oxygen Sensor Simulator Hack

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Introduction: P0420 Oxygen Sensor Simulator Hack

Here's how to diagnose and hack around the P0420 Inefficient catalytic converter code on a Honda.

DIY video:

https://www.youtube.com/watch?v=n-upYHEQu0w

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.

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    29 Comments

    1
    JohnB524
    JohnB524

    9 months ago

    I fixed the P0138 high voltage by using 1M and a 4.7uF along with another 1M resistor across the 4.7uF cap (signal to ground right next to cap). I now have a nice smooth line at about .8V and a happy ECM

    0
    mike71898
    mike71898

    Reply 4 months ago

    I just wired up a 02 downstream sensor with a 1m ohm resistor and 1 uf capacitor. But now im getting that code everyone complains about p0138. Here is the graph from my scan tool.

    Screenshot_20210106-104507_Torque.jpg
    0
    oracle911
    oracle911

    Reply 8 months ago

    I'm having the same exact issues as you and about to connect two 1M resistors. Should I connect them in parallel or in series? Thanks.

    1
    JohnB524
    JohnB524

    Reply 8 months ago

    the extra one goes from the power sensor wire to ground paralleling the capacitor. Maybe try a 750K as 1M is a tad high for my cars computer though it isn’t complaining yet. A California emissions car would I suspect.

    0
    oracle911
    oracle911

    Reply 7 months ago

    Thank you so much for that info. I will try it and report back.

    0
    oracle911
    oracle911

    Reply 6 months ago

    Ok I just put in the second 1M resistor parallel with the capacitor. After a few miles, the p0138 and p0139 comes right back. Any suggestions?

    0
    JohnB524
    JohnB524

    Reply 6 months ago

    Lowering the resistance to like 500-600K on that extra 1M from signal to ground.

    0
    oracle911
    oracle911

    Reply 6 months ago

    Do you mean to solder a 500K resistor to the second 1M resistor in parallel? Like this?

    o2sensorSmoothing.jpeg
    0
    Boat2020
    Boat2020

    Reply 6 months ago

    I have 07honda civic hybrid California
    Car did hack. P0420 code clear but after 40 miles I have p0139 now
    Have anyone able to clear this code if so how and what resistor and capacitor is right value

    0
    p9689985
    p9689985

    Question 4 months ago on Step 7

    Hello. thx for this solution. I installed with a 1 microfarad capacitor and cleared the p0420 and 0430 faults but as you mentioned, o2 sensor stuck faults arose. I tried 4.7 and 10 microfarad capacitors with no luck. i'm trying a 3.3 microfarad cap now. any suggestions on how to optimize the circuit and eliminate the faults? thx.

    0
    Boat2020
    Boat2020

    Question 7 months ago on Step 7

    I have 07honda civic hybrid California
    Car did hack. P0420 code clear but after 40 miles I have p0139 now
    Have anyone able to clear this code if so how and what resistor and capacitor is right value

    1
    poiihy
    poiihy

    9 months ago

    I tried this with a 1uF capacitor and 1.03Mohm of resistors on a gen 3 prius but I got p0137 (low voltage) instead :( I removed one of the resistors to make 560Kohm i think it was, but I still got p0137 :(

    I will try to find a ~100K ohm resistor and keep trying lower values if that doesnt work. With no resistor (only the 1uF cap) i got p0138 (high voltage) and p0607 (i guess it messed with ecm). So it looks like the right value is still to be found between 0 ohm and 500K ohm

    1
    poiihy
    poiihy

    Reply 9 months ago

    I will try 104K ohm now

    IMG_20200809_225807.jpg
    1
    poiihy
    poiihy

    Reply 8 months ago

    104009.1 ohms actually, idk why I included that 9.1 ohm resistor. Anyway 104K didn't work, so on the way home I stopped and rewired it to 11K ohms. And I think I might've found the sweet spot; there's no pending code and both oxygen sensor monitors are OK now, so I think it passed its tests. However the catalyst monitor is still incomplete.
    Before with 104K the catalyst monitor got OK but the oxygen sensor and oxygen sensor heater monitors were incomplete when I checked the status soon before the light came on (and i saw the p0139 pending).

    1
    poiihy
    poiihy

    Reply 8 months ago

    100 miles later and P0420 is back! >:(

    I think I need more capacitance. But I wonder if changing that would require a different resistance again as well.

    Well hopefully the smog test will allow an incomplete catalyst monitor so I can just clear the code and do the test after all the other monitors get ready but before P0420 sets again. Then I'll just live with the light.

    If not though I guess I will have to keep testing values, but I still don't know if it's possible for this hack to make the catalyst monitor OK without any DTCs, or if it can only inhibit the catalyst monitor from ever completing!

    But even if this could work out I think that because it may take you a lot of trial and error to find the right values, this isn't such a good idea and you should try a spacer instead. They sell spacers designed to eliminate P0420; there are also angled ones if straight ones won't fit.

    1
    poiihy
    poiihy

    Reply 8 months ago

    Well it looks like my plan isn't gonna work as I hoped! I cleared the codes last night and after just FOUR miles the P0420 came back on!! How, when last time it took two days and *124* miles to get P0420 back?? 😩

    Oh and yeah, we called the smog test place and they said an incomplete catalyst monitor is passable if it's the only one.

    1
    poiihy
    poiihy

    Reply 8 months ago

    Ok we just passed the test using the spark plug non-fouler trick. (If you don't know that is where you get a pair of spark plug non-foulers from any auto parts store, and using a half inch drill bit (i used a conical one) drill one of them out to fit the sensor, and screw the other one on that.)

    It worked PERFECTLY. All monitors completed (except evap but that's passable; evap can take days to get ready) with no DTCs. I highly recommend doing that trick instead of trying this, I think it will just work on any vehicle, while this hack is tedious to get the values right, especially if you don't have any diagnostic tools to monitor the sensor voltages. You can also get spacers designed for this purpose, on ebay, (you can get 90 degree ones so you dont squash the sensor), but they'll take over a week to ship. This hack you can probably do and pass the test all in one day!

    IMG_20200814_184817.jpg
    0
    poiihy
    poiihy

    Question 8 months ago

    Did the catalyst monitor ever complete? Or will this hack make it never complete? I can't find the relevant smog test requirements for my state :/

    Edit: ok my state (nevada) allows one incomplete monitor including the cat mon.

    0
    Danieljjvv
    Danieljjvv

    Question 2 years ago on Step 5

    Hi there
    I have a 1uf 50v capacitor, will it work?

    0
    poiihy
    poiihy

    Answer 8 months ago

    yes voltage doesn't matter