If you use a race header you have to use an O2 "defouler" on the secondary O2 (the one farthest from the engine). The stock ECU compares this sensors readings to the primary to see if your catalytic converter is working. The secondary O2 does nothing to affect engine performance.
Do not use a "defouler" on the primary!
mechanical o2 sim
If you're still using the stock cat then you probably have other problems...
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