So there's this rig for fooling your ecu when you have either installed high-flow pre-cats or you have a (Warm Up Catalyst Below Threshold) OBDII code because of a weak pre-cat. Oxygen sensors happen to share the standard 18mm thread size with spark plugs. Auto parts stores happen to sell a device for reducing spark plug fouling in heavily worn engines. So the idea is to make 1 + 1 add up to three and save mucho dinero.

You can try this trick with only one non-fouler, but seasoned riggers recommend stacking two of them. In order to do that, you must drill out the center of one of the non-foulers with a 1/2" drill bit so that your oxygen sensor head will pass through it as you screw it into the stacked non-fouler.

So you've done your drilling and you've stacked the two non-foulers together, with the non-drilled non-fouler on top.

Now you remove the oxygen sensor that is downstream of the offending pre-cat. In it's place you install the non-fouler stack and then re-install the sensor into the stack. This takes your oxygen sensor out of the direct line of fire of your exhaust stream. Supposedly, this will be the kiss of death for your P0421 & P0431 codes.

I don't know this for a fact. I would never do anything that might be considered illegal, immoral or excessively fattening. But I hear that it works.

I'm going to attempt to attach some pictures:

 

Well, I finally spoke to someone who actually tried this trick. They said it didn't work for them. It sounds reasonable enough, but apparently the Oxygen sensor doesn't need to be placed directly in the exhaust stream in order to detect variations in the Oxygen content.

Add this one to the Urban Myths list.

 

UPDATE: It turns out the guy's truck popped a code for something else altogether. So far, the hack seems to be working.

 

This is not true it wont work because the upstream o2 sensor is always trying to get the perfect air / fuel ratio 14.7 to 1. = lambda 1. no matter where or how many stackers you fit it will always try and get the closed loop to lambda 1. the reason your getting p codes come up is due to cat effeciency all the rear sensor does is makes sure the cat is doing its job correctly . The reason you might be using more gas is because the cat is dying .