The "Check MX" feature on Autel tools is specifically designed to scan for unprogrammed Autel MX-Sensors, which is why it returns "no sensor detected" when scanning your factory or standard 433MHz OE sensors even though they activate normally. Given that your 2015 GTS is over a decade old, a dead internal battery on that rear right sensor is by far the most likely cause for the '--' dash warning. You should run a standard vehicle diagnosis trigger test to check battery status/hex ID directly, or simply clone the dead sensor onto a fresh MX-Sensor to solve the issue.
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