Hi, I have a minimax 400 heater it will only bring the spa up to 85 degrees, the power light and temp light are on.

Your heater may have an overheat safety shutoff device that is calibrated for 104 degrees.

You have a bad thermistor. The heater thinks that water temp is over 114 degrees.

themal bypass.. looks like an automotive thermostat ,locater in the header.
temp should roughly 20 degrees difference from in and out

i have the same problem and i jumped the two red wires to bypass the thermal switch

Your EO4 message tells you you have a fan problem. So ether your fan is diying wich you would hear or you have a bad presure sensor in your exaust. A new fan is a little pricey but the sensor is reasonable for heater parts. I can't be sure but I believe it ether screws onto or into your exaust and it just plugs on to your control board. If your sensor is going bad their is a reason and this should be addressed. It should be in your by-pass or your gas pressure. But as far as a new heater, not untill you have water coming from the cabnit. It is sometimes easer to sell new than keep up with new, Good luck let me know how you do.

change the assembly valve

I am not sure if you have received assistance in this matter, or not, but just in case.

If the display is reading HI, then the Thermistor "read" the water temp over 115 degrees. The easiest fix would be to replace the Thermistor, doing this will reset the Hi Limit Switch.

This sounds like you have a problem with your pilot. You can check this with a small mirror to see if their is a spark when you turn it on. If you have any soot build up on the pilot that will stop the spark and keep it from lighting also. If you have a short on your pilot wire this could be the problem also. I hope this helps, good luck.

Turn the temp set knobs on the front of the heater all the way up do both pool & spa temp.
If that doesn't help flip the selector switch on the front of the heater the pool temp. This switch doesn't control what heats up Pool or spa it just selects the heater on and which thermistat to use