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  1. In an effort to preserve information about these cars and tuning, I figure I'd write this up. This information was figured out by the members of GrandPrixForums, but the thread that it's in is very long and full of off-topic posting. Overview: Owners may wish to delete the rear oxygen sensor in order to install a wideband in the factory bung location. In 2001+ cars, this causes a new cat diagnostic to run repeatedly, skewing fuel trims and negatively impacting fuel economy. Cause: Beginning in MY 2001 (and possibly some prior MY California emissions cars), GM added a cat diagnostic test in order to determine the efficiency and operation of both the catalyst and rear O2 sensor. This test works by richening the fuel trims to an extreme degree in order to saturate the catalyst, then waiting for the O2 to respond appropriately. It will run this test over and over during cruise until it is satisfied. Without a rear O2, the test never passes and fuel economy suffers. LTFT readings will also skew rich (high double digit LTFTs), causing tuning difficulties. O2 Simulators do not correct the issue because the PCM is expecting a specific response from the amount of fuel being dumped. Fix: Utilizing Tiny Tuner, this diagnostic can be disabled. As of April 2017, HPTuners still does not offer this ability. By and large, this patch is available for most 2011-2014 L36, L67, L32, L82, and LA1 OSIDs, but some may be missing. To disable the test, open your .bin in Tiny Tuner and navigate to the following: Diagnostics > Main > CAT Diagnostic Enable Coolant Temp Stock is 20. Change to 285 (positive, not negative like the screenshot). Navigate to Diagnostics > Main > CAT Diagnostic Tests per Trip Stock is 18. Change to 0. This should disable the O2 diagnostic test going forward. Errata: The original version of this document (and the screenshots) suggest changing the enable temp to -285. This value is too low and would allow the diagnostic to run anyway. The diagnostic would run every time the car is driven until interrupted. After interrupted, the test would be suspended. Setting it to a high value that the car would never see prevents it from running at all.
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