This whole issue has come up many times.
I believe that many times it is nothing more than a false code. In some cases, Ford eventually published a TSB that showed the cure was to reprogram the PCM with wider perameters to set the cat effeciency codes. Not all models have the new program available. My SVT is one that does not.
On my own car, code would set ocassionally. I would read the code and reset the system and the code did not repeat for several months. On one of those ocassions, I took the time to actually check the downstream oxygen sensors real time reading compared to the upstream sensors and found everything working properly.
Also, when taking the car in for smog tests, the tailpipe readings were near zero, which would have not been possible without all the cats functioning.
I have not had the code now for over a year. I'm not sure what that can be attributed to. I did replace both upstream oxygen sensors a few months back after receiving codes for the #1 bank upstream sensor not switching properly. The code repeated a couple of times after reading and resetting. Since there had been no episodes of cat effeciency codes for several months before this, I suspect that in my case that upstream oxygen sensors really had no impact on setting the cat effeciency codes.
On a slightly related topic. One of the things required for the latest upgrade in engine oil certification when the industry moved to the latest level of GF-4 and SM was a reduction in the level of ZDDP, an anti-wear additive. ZDDP is a form of Zinc. It coats the metal surfaces to p[rovide protection when there could be a brief episode of metal to metal contact such as cold start up. There are other additives that also provide similar protection, but they are more expensive. One of the more common alternatives is Moly. The reason that ZDDP levels needed to be reduced is that it tended to poison cats.
So I know of nothing that points to high ZDDP levels causing problems with our cats, the newer oils should be more friendly to cats in general.
Most of all, I suspect that the precats are just too small to always be up to he task. I also suspect that the downstream oxygen sensors (cat monitors) should have been placed by the factory AFTER the main cat. Should I ever install headers, I will investigate extending the downstream oxygen sensor wires and installing bungs to the pipe right after the main cat.
If you are not aware of it, the main reason for the precats is to clean up the exhaust sooner on cold starts as they will come up to proper operating temperature before the main cat can. The system does rely on all the cats to clean up the exhaust suffeciently to meet smog requirements. That is why car that have the precats removed have much higher readings (but if all is healthy, still low enough to pass from the readings alone, hoping that the smog tester does not check to see if the precats are missing).
Also, you should understand how lthe cat monitor works and how it sets the code for low cat effeciency. In the 96 and newer systems (OBD II) there are several monitors in the PCM that only run ocassionally. Two of those are the fuel evap monitor and the cat monitor. The cat monitor is the last to run. And even then it does not run until several driving conditions have been fulfilled. Such conditions are something like 1) after engine has come up to temp 2) lhas been driven above a certain speed for a certain length of time 3) etc. Then the cat monitor may come on, and it compares the readings of the upstream oxygen sensors to the readings of the downstream oxygen sensors.
What it is looking for is the frequency of oxygen sensor switching between high and low readings. When an oxygen sensor is cold, it switches very slow. Once it warms up, the upstream oxygen sensor will switch rapidly. An oxygen sensor generates voltage in the absense of oxygen. The voltage range is 0 volts to .9 volts. 0 volts is a lean reading (lots of oxygen) and .9 volts is rich (little or no oxygen). The reason that it switches rapidly is that it is reading the firing pulses from the engine, with puffs combustion product and the voids between the puffs. When a cat is working effeciently, the switching is moderated, that is what comes out of the cat will be more even in consistancy, with greatly reduced pulses.
So when the downstream oxygen sensors are switching at close to the same rate as the upstream sensors, the cat monitor in the PCM recognizes that the cat is ineffecient and sets a code.
Now this is what I would do (and have done) when you get that code. If you have the means, get some real time readings on the downstream sensors and see how they are doing. If the readings are dampened, son't worry about it and erase the codes and drive the car, probably for several months before the codes return. If you don't have the means to read the upstream and downstream sensors in real time and compare them, catch up on any maintenance that may impact this. Confirm the condition of the plugs and plug wires. Clean the injectors and throttle body. The cleaners may have the additional effect of helping clean up the cats. Change the engine oil. Confirm the condition of the air filter. Erase the codes and drive to see if it repeats soon. Because of what it takes to run all of the monitors and different driving habits, that may take a week or two or three. When the codes have been erased, you will be able to reae a code P1000 until the monitors have run. Once the P1000 is gone, the monitors have all run.
If the cat effeciency codes do repeat often, then you need to either replace the weak cat or look into finding a way to fool the system into thinking the cat is OK. That is what MIL eliminators do. They slow down the readings from the downstream oxygen sensors so that the PCM thinks it is healthy.
