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P0420 Still (after O2 sensors replaced)

New 11/26/2004

P0420 is a catalytic inefficiency, I replaced the O2 sensor on the bank 1 (both sensors), and still have the issue. Is it time for a new catalytic converter? How hard is it to get a new cat on the Y pipe and still have it bolt up correctly, should any muffler shop me able to do it? How much am I looking at here? Any high flow cats that anyone recommends?

Thanks in advance,

This code comes from a failed converter test. The computer runs cat efficiency tests every cold start drive cycle when the conditions allow it. What this means is that you have to be going a certain speed (39-62mph for 3min without accelerating or decelerating) before it'll start the test. When the computer starts the test, it's checking the function of the front HEGOs along with the back HEGOs. The computer runs one side of the engine really rich and one side really lean and then swaps sides. While doing this, it watches the HEGOs to see what they do. The front HEGOs have to respond in a certain timeframe and in a certain way. The rich side should go to almost 1v and the lean side should go to almost 0v. The downstream should also respond in a keratin way, but on a much lower scale. The front HEGOs should have a wild swing in both directions while the downstream should have a much smoother swing without being as wild as the front HEGOs. As the converters are working and cleaning up the passing thru them, the oxygen signal that gets to the sensor is smoother. When the downstreams closely mimic the front sensors, it means that the converters aren't working any more and the converter efficiency is below the trigger threshold and the code is set.

This isn't something that just happened, it's happened over time. Heck, you could've gotten a bad tank of fuel somewhere that had lead in it.

Lead and a bunch of other things will poison the converters. Inside the converters, there is a honey comb substrate that's made out of silicon (not silicone) that's coated with a platinum mixture. It's this platinum that does the catalyzing of the exhaust as it passes thru the substrate. The reason that we don't have leaded fuels anymore is lead adheres to platinum and coats it. When it does, the converter simply stops working. Silicone sealers that are not "Sensor Safe" can kill the sensors as well. There are a lot of things that can kill the converters over time. The more poisoning the less efficient the converters are. Eventually, they cross the threshold and trip a code.

Once this happens, your options are slim. None of the aftermarket or Hi-Flo converters on the market are efficient enough to work in these cars. The only ones that come close are the Car Sounds converters that are OBD-II compliant. They are pretty pricey, tough to get and even tougher to install in our configuration. There must be two of them and the sensors must be configured in a certain way. The packaging in our cars doesn't lend itself to easy installation (gee, imagine that). So it's back to stockers. New ones from Ford are better than $2K. We sell used units for $500.

There other options that can be discussed off list if anyone so desires...

Hope this answers a few questions for you and anyone else that comes across this problem.

Doug Lewis

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