Q: Auto/Emergency Locking Retractors not locking on 3rd row 2011 Tahoe. How to fix?

These (seat belt assemblies) are not repairable, or serviceable units, and must be replaced by OEM dealer supplied parts if faulty. However, these systems are very robust and thus it is not likely at all that the seat belts are malfunctioning but rather the mode of operation is not obvious. In particular, your vehicle may be equipped with dual-sensing locking retractors (the GM dealer can confirm whether that is the case). If so equipped, and if you are simply "manually" testing the belt payout, because you cannot "mimic" the vehicle G-loading that actually occurs in a crash, the belt will not respond (lock) as you are expecting in a "static" test. The procedure to determine if the vehicle is equipped with emergency locking retractors, versus automatic locking retractors (auto retractors work different than emergency retractors) is to 1) Pull all the webbing slowly and gently out of the retractor; 2) Allow some of the webbing to go back into the retractor; 3) Try to pull the webbing out again VERY slowly. If the webbing goes freely in and out of the retractor after you have pulled out all of the webbing, you have an emergency (not "auto") locking retractor and it is working as intended. When you do this test, do not pull quickly or jerk the webbing because such "might" trigger the emergency locking features of the retractor. However, as noted, if the belt payout (locking) also depends on g-loading (i.e., if dual-sensing locking retractors are installed in your vehicle) the belt may very well not lock no matter how hard you pull on it because the coincident g-force is not present. If you have further questions or concerns, do not hesitate to re-contact YourMechanic and we will contact GM on your behalf with your specific model and model year.