Resolve P0420 by poisoning downstream oxygen sensor with silicone

P0420 has been coming and going but then stayed as smog test time is getting close.

I got the idea somewhere to try applying silicone on the downstream sensor so I tried it because it was the easiest solution I could do with what I had.

So I completely covered the sensor with ultra black RTV silicone I had left over.



But then at about 13 miles I got P0139 instead (O2 Sensor Slow Response)!

I took the sensor out; all the silicone had burned to ash.



I thought the sensor was probably ruined but I cleaned it out and put it back in to see what would happen.



I test drove 15 miles and surprisingly did not get any DTC! Now there are not even any pending codes, there's nothing. All monitors except Oxygen Sensor Heater Monitor are OK, so this would pass smog test right now.

So I think the silicone poisoned the sensor just enough that it's responding slow enough to not indicate an inefficient cat, but fast enough to not indicate a bad sensor.

I will update if a DTC comes back or when it passes smog test.

 

P0420 means Catalyst System Efficiency Below Threshold and is a common problem with older cars. Catalytic converters are expensive and from what I've heard the threshold is very high, like 95%. So a lot of people want to hack the system to pass the smog test instead of replacing the catalytic converter, which only goes bad due to problems upstream such as with the engine, so a lot more work is usually needed or the new cat will be damaged as well.

The catalyst efficiency is measured by a second oxygen sensor after the catalytic converter. I believe a good catalyst would take out most or all of the oxygen from the exhaust stream, and a failing cat will let more oxygen through (i'm not entirely sure though but I know the computer measures the fluctuation in voltage).

So to get rid of P0420 without replacing the cat you have to inhibit the sensor's ability to measure oxygen or modify the data the computer receives to look normal. I think poisoning the sensor has a similar effect to the non-fouler trick.

Just cleaning the sensor is the opposite of what you want to do, and probably achieves nothing.

 

Awww the P0420 is back
It was still ok for 50 miles another day but the next day by four miles the light came on.

So if the silicone did anything well the effect wore off. But maybe it didn't even do anything at all and it just takes a while for P0420 to set?
FWIW the ultra black silicone I used said "sensor safe" on the package... but I thought being that it's still silicone it'd still have an effect when smothered directly on the sensor

It was passable though; I wonder if P0420 just takes a while to set then maybe you can pass smog test by clearing the codes, getting enough monitors ready and getting the test done before it sets P0420. There weren't even any pending codes though so idk, maybe the silicone did have a temporary effect.

I don't know what "sensor safe" really means; maybe there's something else you could apply to permanently make the sensor "lazy".

But at this point I think I will try something I know will work. I don't think there's enough space in gen 3 for the non-fouler trick so that leaves only the low pass filter.

 

Hmm interesting, they put the resistor and capacitor both in parallel between the white and blue wires without being connected to the oxygen sensor; unlike the low pass filter hack where you keep the oxygen sensor connected, but with the capacitor in parallel and the resistor in series on the blue wire between the cap and sensor.

I do not see how that would work... the sensor produces the voltage.