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I'm having a problem the HP racing header and high flow cat. I have three holes for O2 sensors. Two are in the header, one is in the high flow cat. I have two O2 sensors on the car. One goes to the header, the other to the cat. The header is suppose to come with a plug to fit the third hole. I bought mine second hand and it didn't come with the plug. I've been to every auto part store in my town looking for a plug. A 3/8 and 1/2 plug don't fit. Some people suggested screws, but the hole is specially designed for an 02 sensor, so it tapers as it gets deeper. For those people that have installed the HP racing header, what did you use to fill the third 02 sensor hole? And what is the third hole there for anyway?

 
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The 3rd hole is for other applications where the o2 sensor would reach the lower left side easier than the upper right. The header came from the store w/ a plug for the unneeded hole (not stainless i may add..mine is rusty) but anyway...umm i can think of a few options
1. Find a bolt to fit
2. Re-tap the hole to a bolt you already have
3. Fill the hole with an old empty o2 sensor.

Basicly anything that will fill that hole and keep pressure in the header and you will be allset.
Goodluck