A pool heater takes water from the pool and takes it to a heating tank that sends warmer water back into the pool. This is an exchange of cold and warm water that keeps your pool at a comfortable temperature, even if it is cold outside.

The Pilot Light:
First check the pilot light - does it ignite? If not, it could indicate that the gas pressure is low, the heater isn’t venting properly, or the air getting to the heater isn’t great. If the pilot light is working, make sure the heater is turned on and the gas valve is in the “on” position. 

Leaks:
Sometimes, the chemicals used to clean your pool water can cause damage to the heat exchanger. It could just be a loose connection that you need to tighten or colder winter temperatures, but you may also need to replace the heat exchanger. 

Exhaust:
If your pool heater is pumping out dark exhaust, it could be building up way too much heat when it’s running. If this is happening, check the gas pressure and make sure your heater is venting properly. 

Rust:
Remember how I said your pool chemicals can cause damage to the heat exchanger? Well, those chemicals can also cause your heater to rust and corrode. You’ll want to replace those corroded parts to prevent any further damage. 

Replace Parts:
If you have tried these simple steps in troubleshooting your power heater and are still having no luck with getting it to function properly, then this might be an indication that there is an internal issue with your unit. 

Source: 
https://poolresearch.com/