One part of the rear O2 sensor's job is read the exhaust gas and determine the efficiency of the cat. Based on input from the sensor, the ECU logic will change the "tune" to adjust AFR for cat efficiency. However, this reading can also compensate for other mechanical issues as the rear sensor can dictate fueling corrections over 20%. If you use an O2 simulator or spacer, the ECU will always see optimal conditions or won't see the full exhaust stream. Therefore, if anything goes wrong or needs to be adjusted for any other reason, the ECU would have no idea because the sensor input would be false.My personal car will be going on the dyno this week for map improvements! Our current maps are working great, but we usually just keep a "beta" tag on them until we have full Stage 1 (tuning + filter), Stage 2 (tuning, intake, FMIC), and Stage 3 (tuning, intake, FMIC, TBE) maps.Ian