it may run a "CAT diagnostic" where the PCM will command fuel trims super pig rich and then expect to see a corresponding change in rear O2 readings to see if the cat is there. If it does not see this it will continue to run the test continuously until it does...and if you don't have a cat...you guessed it...your car is basically stuck locked running pig rich. A custom tune can eliminate this from happening. Does your state do a plug in emissions check to read the status of various tests prior to renewing your registration? that may be another issue even with the custom tune as the tune only masks it and keeps the MIL from turning on due to the failed cat check.
