GJK921

I’ve been having an issue with my 2000 Marquis Spa. I found the water level low due to a leak and screen displayed ‘Sn1’. I filled with water, reset power at the breaker box and ‘Sn1’ no longer showed. The ‘Sn1’ message was gone and the pumps ran on High and Low, but I was getting a ‘FLO’ message. The pump might have sounded a little bit louder too, not 100% sure, since I don't normally have the access panel off. I don’t think it ever really heated up the water and I noticed the ‘Sn1’ message the next day. I reset power and now get ‘OH’ message with cold water in the spa. Any thoughts on what might be going on? Suggestions of things to test? I'm concerned about the cold weather and the hot tub not being warm enough to not freeze.