We have an inground pool. The heater is set at 80. The water is heating nonstop and can go up to 90 in the pool. Yet the heater is still showing 65. Any suggestions to fix? (The service heater light is NOT on.

We have an inground pool. The heater is set at 80. The water is heating nonstop and can go up to 90 in the pool. Yet the heater is still showing 65. Any suggestions to fix? (The service heater light is NOT on.