A water heater with the light on but no heat could be caused by a number of different reasons. Below are some common problems and corresponding solutions.

1. Heating component failure

Heating component failure is one of the most common reasons why the water heater light is on but does not heat. There are many types of heating components, including electric heating wires, gas burners, heat exchangers, etc., and they usually require maintenance and replacement during use. You can take the water heater apart and check for faulty heating components. If something goes wrong, you'll need to purchase and replace damaged parts.

2. Thermistor failure

The thermistor is also called a temperature sensor. It can sense the water temperature and feed back to the controller. If the thermistor fails, it will affect the work of the heating component and cause the light to light up but not heat. You can use a multimeter to test the resistance value of the thermistor. If the value does not display correctly, you will need to purchase and replace the thermistor.

3. Power supply problem

Another reason why the light is on but not heating is the power supply problem. Check that the water heater is connected to the correct power source and that the power source is functioning properly. If there is no problem with the power supply, it may be that the circuit or controller inside the water heater is faulty and needs to be repaired by replacing the circuit board or controller.

4. Waterway obstruction

If the water heater light comes on but does not heat, it may also be caused by waterway obstruction. Scale and impurities in the water circuit will affect the flow of water, causing the water heater to be unable to heat water. Cleaning the water lines and replacing the filters can solve this problem.

In summary, there are many different reasons why a water heater may not light up but not heat, and careful inspection and diagnosis will be required to determine the problem. If you are unable to resolve the problem yourself, seek the services of a professional technician.