I have a DreamMaker Fantasy Spa. It recently started flashing "OH" (Overheat) on the topside control pad. When tested, the water temperature was 91 degrees. I unplugged the tub and let it set for 5

Make and model would help. How about my best guess?
OHH means OverHeated Heater caused by an error
One of the Water Temp Sensors has detected 118°F (47.8°C) In the Heater and Spa Functions are Disabled. You Must Press Any Button to Reset when Water is below 108°F (42.2°C). Check for Water Level, Flow Restrictions, Closed Valves, Air Bubbles, and Pump Prime, or Continue for a Detailed Test Procedure.

WARNING:
A Heater Overheat Error Message may indicate Excessively High Heater Temperatures which can Scald or Burn!
If the Heater is Not Cool Enough to Safely Service, allow the Heater to Cool before Continuing Testing.
A Heater Overheat Condition that generates an "OHH" Error Message is Most Likely the result of a Low Flow Condition. Therefore, "OHH" is considered to be a Flow Related Error.
Check for All of the following Conditions:
1.Low Water Level.
2.Blocked Suction Fitting.
3.Blocked Skimmer.
4.Dirty Filter.
5.Too Many Closed Jets.
6.Closed Slice Valve.
7.Heat Related Pump Not Running.
8.Heat Related Pump Not Primed.
9.Contact your Dealer or Service Organization.

DO NOT remove the protective covers from any electrical enclosure, or attempt to service any related electrical device, unless you are a qualified electrician or service professional.
As a responsible adult, always check the topside control panel display for errors and verify safe water temperature before using a spa, and before allowing children to use a spa. Safety should ALWAYS be your first consideration

This may be a little late as you may have already found the answer. But most "Fantasy" spas have "Balboa" control system. They are either a heat control knob, or Up/Down arrows on the spa side control system.
If you have the know & it is all the way turned to max position, and still not hot enough, then you may well have a faulty "Temperature Sensor". Get a replacement from your local DreamMaker dealership. Easy to replace.

This might be a problem with the topside control. When the spa is clicking, try unplugging the topside control and see whether or not the problem stops. If the jet button is stuck on the topside, the pump will turn on-off-on-off-on-off until the topside is unplugged.

If unplugging the topside worked, replace it.

Topside control is in "Locked" mode.
If you accidentally locked your control

Hold temperature UP & DOWN together for 6 seconds.
or
Balboa 7 button controls, both buttons to the left of the display screen & down temperature.
or
One of the buttons might be stuck in the down position

If these do not help let us know your control make and model & we'll see what we can do to find out more for you.

OHS, OH, or SPA TEMP LMT Message There is an "Over Heated Spa" (OHS or OH) Water Condition.

One of the Water Temp Sensors has detected 110°F (43.3°C) Incoming Water Temp and Spa Functions are Disabled. System will Auto Reset when Water Temp is below 108°F (42.2°C). Check for Extended Pump Operation or High Ambient Temp, or contact your Dealer or Service Organization.

Spa Water Cool Down
• In order for the Spa Water Overheat Error Message to Automatically Reset and Clear, the Spa Water Must Cool to below 108°F (42.2°C).
• Check the Topside Panel Display to see if the "OHS", "OH", or "SPA TEMP LMT" Spa Water Overheat Error Message has Cleared. If the Error Message has Not Cleared, the Spa Water is still too Hot to Continue Testing.
• Either Wait for the Spa Water to Cool Below 108°F (42.2°C), or Exit Now and Retest at a Later time.

You can find answers to COMMON SPA MESSAGES at the following website:

COMMON SPA PANEL MESSAGES



Hope this helps.

This is a GFCI and will trip at the least bit of water at the plug or at the wiring inside the spa controls. Just go through the system and make sure all is dry. Use a hair dryer in the outlet and on the controls and wiring. If you still have problems you may need to call an electrician.

I have this exact same problem on my dream maker ez spa for the last few years and can`t trace it .

"OH" is an overheat error. In normal function, it is used as a safety shut off if the spa water temperature reaches over 110 degrees in the event of a thermostat failure. But more commonly, (if the water temp hasn't gotten too hot) it indicates that the temp sensor(s) have failed. If you have a digital display on the topside controller of your spa, then it is a circuit board controlled spa pack. Most spa builders use the same control pack manufacturer - Balboa Watergroup. The sensor kit usually consists of 2 probes and wires that plug directly onto the circuit board. Depending on the spa, they could both be located onto the heater manifold, or one on the manifold and one mounted to the tub making direct contact with the water via a thru-wall sensor mount. If I knew the spa manufacturer, I could further assist you. If you replace the sensors, and encounter the same problem... then the circuit board is bad, and usually a costly replacement. The sensors are usually listed as a "BA sensor assy" Hope this helps you,

sound like you probably have a bad temperature sensor, or high limit sensor. "OH" usually means overheat and will shut the heater off. Have the sensors tested and go from there.