I have a FloorStat 500650-120B; the Test button just started coming on during a heat cycle recently. No GFI message appears. The GFCI tests normally.
The light will not go out by setting to Standby. The only way to reset is to recycle power at the breaker. The light will remain off until some future heat cycle (unpredictable which one, but usually several go by); once on, it never resets until I recycle the breaker.
The unit appears to work normally whether the Test light is on or off. No additional messages are on the display.
Hey Maynard. The GFCI light acting like that is a good indication that the actual GFCI inside the stat is starting to fail. At some point it will probably actually start tripping, cutting power to the floor, and then eventually it will just get stuck closed. It's probably a good idea to start planning to replace it.
You'll have to replace the whole thermostat. The easiest way to do that would be to pop on in to Home Depot and get one there. If they don't have any on the shelves, they can order it, or you can just buy it from their website.