I just bought a 2003 Wrangler 4.0L X model with 107k. It has a custom header installed on the motor, down near the the six in one collection tube on pipe one and six there is still installed sensors. I have a service engine light on the dash, I took a friends Snap On Diagnostic tool to it and it shows error code P0420 and P0423 from the ECM. They both are "Catalyst System Efficiency Below Threshold Bank 1 (Catalyst 1/1 efficiency below required level)" I cleared the error code and it stayed off for over a week. It is back and is the same code. Is this issue from the custom header setup or could the sensors be bad or something else? If you need more info please ask or if you need a pic of the header. Is there something I can do so the light will remain off for this issue?
Thanks for any help, Jeff