P0420 - A Primer

New 01/27/2005

Here's your "primer". There are three cats on our cars, but only two of
them are monitored by the ECU. As you know, there are exhaust manifolds
on each bank of the engine (front and rear). The exhaust from the front
goes through one cat, and the exhaust from the rear goes through another
cat before the two exhaust streams join together at the Y-pipe to go
through the third cat (which is not absolutely necessary for emissions,
but is really just a "safety net" to keep emissions clean when the car
gets really old and the two primary cats begin to lose their
effectiveness.) Before each of the primary cats, there is an oxygen
sensor which measures how much oxygen is in the exhaust to help the ECU
determine if there is too much or too little fuel being added to the
engine. After each of these two cats, there is another oxygen sensor to
determine whether or not the cats are doing their job in reducing
emissions. If the oxygen level after the cat is too close to the oxygen
level before the cat, then obviously the cat isn't burning up (or
"converting" to be technical) the extra crap in the exhaust.

Now, just because you are getting a cat-related code doesn't necessarily
mean the cat itself is to blame. An engine misfire allows fuel to pass
through the cylinder unburnt, so it just flows out the exhaust, where it
can leave trace deposits on oxygen sensors and cats. Over time, this
can cause an oxygen sensor to report false or skewed readings, so it may
look like the cat is to blame when it is actually a faulty oxygen
sensor. That is why we are curious as to whether or not you have known
of any misfires. Of course, a bad coil on the SHO will not always be
noticeable, but it might sporadically misfire and contribute to oxygen
sensor problems. Most of the SHO owners who have had coil failures have
put up with them for many many miles without a misfire code being
thrown. Coolant and other contaminants in the exhaust stream can do the
same thing.

**ASIDE** I have heard of persistent misfires contributing to cats
getting clogged up with soot, but I think that is an
illusion/coincidence. Soot is created when oil burns at low
temperatures. An engine which is burning oil at a high rate will often
have fouled plugs, contributing to a misfire. So really the clogged cat
AND the misfire are both EFFECTS, caused by the burning oil.

I hope I got all that right. :-)

Dan
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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 down streams 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 down streams 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 down streams 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. Hell, 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
Ford Performance Specialists Inc.

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