E Type Fluval Heater

Just a heads up for those who have this type of heater, I unplugged the heater this morning whilst I carried out a PWC, which took about 30 mins. After doing the water change I plugged everything back in, after about another 20 mins I noticed that the heater display was blue indicating low temp and on closer inspection it showed 23° when it should be 24.5°. I checked on my digital LCD thermometer and that was showing 24.5°. I watched the heater for about 10 mins more but it didn't change, the heater is only a month old which was a bit worrying. I unplugged it for 2 mins then plugged it back in and watched it go through its self test and end at 24.5°. It might be an idea for other owners to keep an eye out for this happening to them as I had no idea if it was working or not as the heater doesn't seem to have an indicator light to show if it's on or off.