Heater problem

Hi all. New to this site and have an issue with my heater. I had my pool built almost 3 years ago. Free-form pool about 25k gal. With spa and grotto features. Saltwater with pebble tech. My heater is a pentair mastertemp 400. The service heater light is on and will not heat. The blower does not turn on. I took the side off and saw i have the hls (high limit sensor) light on. After looking at manual and troubleshoot section it says to replace the hls. I looked that up and I'm confused as I've also seen a high limit safety switch. Are they the same thing or different? I called local pool store and i was told they are different and heater has both. Same store different location said it only had high limit sensor. So i don't know what i need to replace or the part number. Please help. I do have a pic of the board and error light. Also cud debris in the lines make this error come on due to lack of waterflow?