2000 Hyundai Sonata P1751 and P1723 Fault Codes: Transmission Stuck in High Gear?

My 2000 Hyundai Sonata (140,000 miles) has developed a transmission issue where it is stuck in high gear and will not shift into lower gears manually. The gear selector moves to the perspective range, but the transmission only responds with DRIVE, REVERSE, NEUTRAL, or PARK — no lower gears are accessible. The vehicle displays two diagnostic trouble codes: P1751 and P1723. While P1723 is listed in the service manual as related to a speed sensor, P1751 does not appear in the official documentation. I contacted a dealership, and they advised that a diagnostic scan using specialized equipment is required to determine whether the issue lies with wiring, sensors, or the transmission control module. However, such diagnostics can cost between $80 and $120 — a significant expense for a non-essential repair. Has anyone else experienced this exact symptom? What does P175 or P1723 actually mean? Are there any known recalls or technical service bulletins (TSBs) related to transmission issues in the 2000 Hyundai Sonata? My car was previously functioning normally before this failure — is this a sudden mechanical breakdown or a gradual degradation? Any insights into what might be causing the stuck-in-high-gear behavior would be greatly appreciated.