Originally Posted by TheOtherDave™

• Ditch the test pipe.
• If your state will let you, run a high-flow cat instead, matched to your header diameter.

• Once an operational catalytic converter has been fitted and both O2 sensors are plugged in and operational, pull the + (positive terminal) battery connector and let the car sit for 10 minutes.

IMPORTANT: If you have an OEM radio installed, be sure you have the Anti-Theft code in hand BEFORE DISCONNECTING THE BATTERY.

• Once 10 minutes have elapsed, reconnect the battery and go for a drive.
If the Check Engine Light comes back on during your drive, retrieve the code using the directions in our FAQ thread. Then come back to us for assistance.

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And ignore your friend's advice.
A wideband oxygen sensor would not fix the problem on it's own.
The OBD-II networks expect to see a drastic reduction in post-catalytic converter o^2 content. When both narrow-band sensors report the same level of o^2, the ECU figures that either a) the cat is not performing properly or b) it isn't there at all.