If your check engine light is going on and off, it isn't the MIL Eliminators. If it were a rear o2 sensor problem, the code would stay on all the time, and would only go off if you reset the codes by either unplugging the battery, or clearing them with a code scanner.
Without seeing the codes, I'm afraid we can only offer the following advise. Keep in mind you are using a competitors MIL Eliminators.
Pull the codes, and email the code #'s to me, and we'll walk you through how to fix your problem.
Dan Smith
www.ponyexpressperformance.com
info@mileliminators.com