2013 Hyundai Veloster P0638 Throttle Body Fault: Two New Units Fail, Code Returns Immediately

Hi everyone! I'm troubleshooting a recurring P0638 fault on my 2013 Hyundai Veloster with no modifications—stock setup, 68k miles. Battery voltage at rest is 12.7V. The car enters limp mode with both P0638 and P0639 codes lit, and the Check Engine Light stays on. I cleared the codes, but they returned within less than 5 minutes—idle becomes very rough. Initially, I suspected carbon buildup in the throttle body (TB). I disconnected the hoses and inspected it visually—it didn't appear dirty. After fully removing the TB for cleaning, I noticed a distinct clicking sound when moving the butterfly valve, suggesting detents. Upon drilling out rivets and removing the plastic gear cover, I found that the #1 tooth on the plastic gear attached to the throttle shaft was severely worn—indicating a defective throttle body at 68k miles. I ordered a new throttle body for $310 from Delray Hyundai (vs. local prices of ~$680). After installation and clearing codes, the issue resolved for four days. However, P063-8 returned immediately after code reset—no drivability problems, just the CEL. I installed a second new throttle body, which also triggered P0638 but failed to allow any idle or accelerator input—the car won't respond at all. Both new throttle bodies move smoothly and freely when manually operated. I had to reinstall the first one to get the car running again. I inspected all engine bay connectors—clean and intact. I cleaned all three ground connections on the driver's strut tower (had to remove the PCM for access), which were previously loose. The wiring and throttle pedal connector appear undamaged. The vehicle has been well-maintained by its owner, who is very cautious about upkeep. I also confirmed that the engine bay was never power-washed—this would have risked damaging wiring, a known concern in Hyundai vehicles. The P0638 code appears immediately after clearing memory every time. I've tried multiple 'learn' cycles (ignition on for 10 seconds, off for 10 seconds) with no success. Given that both new throttle bodies are functioning mechanically and show no signs of defect, is there a deeper issue? Could it be a faulty sensor, wiring harness, or ECU calibration? Is P0638 possibly tied to the throttle position sensor (TPS) or a software bug in the 2013 Veloster? Any insights or known fixes for this specific code on this model? I'm at a loss and wondering if it's a rare but documented issue with early-gen Velosters. Thanks in advance, Matt.