Originally Posted by AuthenticAMD
If you're running a stage 3 setup with greater than stock injectors / turbo, should the A/F Learning D value still be within 5%? Are the values determined by the rear O2 sensor? Just wanting to make sure a wideband isn't needed to determine the actual A/F of the tune during WOT conditions.
During WOT (or other higher throttle/load conditions), you are in open loop which means that the ECU does not monitor your actual AFR anyway. It has no clue if you are hitting your target or not. Only in closed loop does it look at AFR and adjust short and long-term fuel trims appropriately.
The A/F Learning 1 values (long-term trims) are determined by the front o2 sensor and they should be reasonable regardless of the mods assuming a good tune and the car in good mechanical condition. The different stored (and applied) values are simply across different airflow ranges, with the "D" range determined in closed loop (in the highest range) but also applied in closed loop and open loop (i.e. even WOT). So, if you have a problem with the car/tune, "D" range has additional significance as it will also impact high load operation, whether there is a problem there or not.