http://www.k-series.com/forum/showthread.php?t=8833
its only for code P0420, which i think means your Cat is bad. With the nonfouler trick, it limits exhaust to get to the o2 sensor.
And if cat is rattling, your gonna get the p0420 code. And when you get this code, your MPG is gonna go to s#it.
P0420 is a code for catalytic converter performing under threshold code
The catalytic converter has an oxygen sensor in front and behind it, upstream and downstream. When the car is warm and in operating temperatures it runs in closed loop, and hence looks at data from the o2 sensors on how to behave. The upstream oxygen sensor reading should fluctuate from around 0.2-0.8v. If it stays up or down for long periods of time it could indicate a faulty sensor. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This would mean that the cat converter isn't doing what its meant to do and not behaving up to ECU specs.
A code P0420 may mean that one or more of the following has happened:
-Leaded fuel was used where unleaded was called for
-An oxygen sensor is not reading (functioning) properly
-The engine coolant temperature sensor is not working properly
-Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
-Retarded spark timing
-The oxygen sensors in front and behind the converter are reporting too similar of readings
What to check
-Check for exhaust leaks at the manifold, pipes, catalytic converter.
-Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
-Inspect the downstream heated oxygen sensor (HO2), replace if necessary
-Replace the catalytic converter
Usually a P0420 will come up with after-market cat converters who do not perform up to oem standards.