Tankless water heater hot then cold

I have a Navien tankless water heater that worked fine for about 1.5 years. Recently, however, I’ve been having an intermittent issue with the hot water. It starts out hot, then turns cold, and sometimes becomes hot again, all within about 4–5 minutes. Other times, it works normally without any problems.

I also noticed an error code 762-00, which indicates that the vent pipe may not be fully inserted. However, the error is dated 2023, so it does not appear to be recent. Could this old error code still be related to the hot-water issue I’m experiencing now?

Thanks