I own a 2004 Blue Water Quality Spa with the Hydro-Quip Heater components. The main circuit panel has a red error light for the "Auxiliary Fuse (Optional)" and it does not heat above 49 degrees. I have turned off all power to the tub, pulled fuses from the panel to check for continuity and all checks out. On the panel there are two 25A/300V SC-25 Time Delay fuses and a standard 2A/250V 3AG-SLO-BLO fuse that all test good when checked for continuity. This optional auxiliary fuse must somehow control the heater and the Pump 2 but where is it located at and how to I check or replace it? Everything was working until it was recently moved. Does anyone have any ideas as your help would be greatly appreciated!!!!

So I find this odd that I am posting a reply to my own post, but I have learned and hopefully by sharing, I will be able to help many others. My first mistake is that I thought that all fuses are created equal and when the "pop" they are done. So when I tested each fuse for continuity, I thought it seemed logical that if there is continuity then the fuse would still be good. Unfortunately this is not a good test for the Time Delay fuses that are used with hot tubs and swimming pool pumps. Someone makes a specific time delay fuse and relay testing piece of equiptment but it seemed far too expensive for this one time fuse test.

Here was my work around:
I turned the power off to the tub by turning off the circuit breaker
I pulled both of the time delay fuses and swapped them in the tub's panel as they were both the same identical fuse and numbers
I turned the power back on to find the aux fuse light went out and the aux pump worked but the main pump fuse light was now on
I visited a local electronic's supply warehouse and bought new fuses (they come in a 2 pack)
I installed both new fuses and turned the power back on
Now there are no fuse warning lights showing, both pumps and the heater now work!

Again, hopefully this can help anyone else who may ever need to troubleshoot heating and/or Time Delay fuse issues!

Happy hot tubbing!!!

Okay so perhaps I spoke too soon and still am in need of some help. I am located in Iowa and it is cold outside this time of year.
After I replaced the time delay fuses, it seemed that everything was working correctly, but I am finding that the temperature only increases when running with both pumps on high. I read a ton of other similar posts and have tried much of the same troubleshooting. I am trying to post some photos and hope this works...
Here is what I am finding:
The heater element reads 235V when running the pumps on high or low and since it the water temperature increases when running on high, I would like to think the heater element is okay. By continually turning the pumps back to high I have gotten the temperature up to 84 degrees so far. If I turn the thermostat display down below that point, then the tub will return to an idle state and all is quiet. I have removed the filter and the same happens, so it seems the flow is adequate. The pressure switch (the round grey thing with the flat adjustment-I have not turned this) measures continuity when the pump is running both on high and low speeds-which I understand that it is opening and closing to tell the heater there is flow.
I do not know if this tub has a high limit switch and/or where it would be located or how to test it? Is there a picture of what it looks like as that may help? Could that even be my issue or have I overlooked something else?
Any help would be greatly appreciated!

Your high limit sensor is the small black two-wire plug on the bottom right of the board. If it were having problems you'd get a 'HL' error or 3 flashing dots. What's the continuity reading on your heater?

Oh, so that is what those three red dots mean!!! I seen those yesterday but had no idea what they meant. Since I didnt know, I just turned off power to the tub, waited about 30 secs then turned it back on and they went away. I had not seen them on prior and they have not came back yet. Should they?

I just turned power off now so I could measure continuity between the two poles for the heater and the digital multi-meter read 1.563. It did not go to an infinity reading. Is there a high and low side to the heating element as it warms the tub when the pumps are running? I only found those two poles to test so perhaps the heater has a high and low mode by varying the voltage to it?

No, the heater works at the same resistance all the time, though it should be more like 10-15 ohms. Since you have the dots, I'll assume that measurement you got had a fuzzy connection. You got some resistance and the heater worked until you had the error, presumably.

See how far it heats up. High limit sensors sometimes go permanently bad, or sometimes they start tripping at say 90 degrees instead their original set temp. You might also have a different issue, though. The three dots are a generic error code that can mean any number of things. The fact that they disappeared after you reset the tub makes it likely to be a HL, but you'll have to watch and see.

Okay...so a slight delay on my part here as the work thing had to happen... If I keep turning the pumps on high, they will run for a predetermined amount of time, which I think is like 20 minutes or so. During this on/off/on cycling I have been able to bring the tub up to 99 degrees but have not been able to break 100 even though I have the temp setting turned all the way to max 104. Last night I had it up to 98, then went to bed and awoke to a 94 degree tub and the main pump running on low and not adding or maintaining heat again. The three dots have not returned to the display board yet either.

And is the "Heater On" light lit? Check the continuity one more time across the heater terminals with the power off. If the heater light is on and you're losing heat, it's a bad element. You may just be getting an increase in temp from the heat of the motors.

I apologize for my lack of electrician experience as I think I had the wrong setting last time checking for continuity. This time I used the range designated with the upside down horseshoe looking emblem on the 20K range with the power to the tub off. The multimeter was reading 12.63.

When the pump is running at low speed the heater light is illuminated but it does not maintain temp. When I turn the pumps to high, the water temp will increase and the heater tube definitely gets hot to the touch too. How much heat do those motors really make?

Do I just replace the heater element? Where can I get one and what do they cost?

Just to ask as I do not know and this tub did not come with an owner's manual... is there an economy/normal mode settings for summer/winter use? If so how do I change it?
The lady at my local pool supply warehouse asked if there is a flow control that could be bad?
If the heater heats at full speed but not at low, could the flow control reading create the issues that I am experiencing?

Thank you greatly for all of your help too!!!