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My 2002 Sundance Optima 850 died in the winter of 2015 after 13 years of operation. The water was cold and the display showed OH (over heat). When I started troubleshooting, everything looked correct and the water started heating. Then after a short time the temperature started rising quickly. I noticed the circulation pump wasn’t running but the heater was on. This made me think the circulation pump ($200) might be bad. As an electrical engineer I tried to get some schematics of the controller so I could determine the exact cause. If the pump wasn’t running it could be a bad pump or it could be the controller wasn’t telling it to run. Unfortunately schematics are not available. Due to the high cost of parts, shot-gunning the problem can be very expensive. I decided to write this post to explain what I did so it might be of use to other people. Looking online I saw several suggestions. Capacitor failure (<$20 for the parts) – It is possible for capacitors to fail but this is not very common. The DC voltages on the PCB are < 17 volts and the capacitors are rated at 25V. If a capacitor fails it can load down a power supply or cause ripple (AC voltage) which could affect the circuit operation in strange ways. Typically you should design with more margin but technically these parts will work. I looked at the caps and didn’t see any signs of swelling. I also checked the power supply for signs of AC ripple but there was none. Changing caps on the PCB is not as simple as it looks. The board needs to be heated so both leads can be removed at the same time and this is difficult with a soldering iron. In the process of changing the parts you can easily damage other parts. I wouldn’t recommend changing caps unless you actually had a problem that was visible. Bad temperature sensor ($30) – Several blog postings said the most common cause of OH or ---- was the temperature sensor. This part is a thermistor, a resistive element with a negative temperature coefficient. As the heat rises the resistance drops. Online I found a table listing the temperature characteristics of the thermistors used in this spa. These are 30K ohm at 25 degC. When I unplugged the sensor connector and measured the temperature sensor it read 27K and the water was around 27 degC so it looked fine. Bad Hi-Limit sensor ($30) – I checked the hi-limit sensor in the same way as the temperature sensor and it also was around 27 degC. The sensors seemed to be fine. Bad controller board ($350 – 650) – Since controller boards are expensive I really didn’t want to buy one until I knew it was bad. The controller has a microprocessor as the brains of the system. To fully understand how the system works you would need to have a schematic for the board and some information on the firmware (software for the microprocessor). I traced the circuit and discovered how the relays were wired. The board uses two driver chips to control the 12 relays. During various phases of troubleshooting I had seen cases where the heater was on and the circulation pump was off. This of course causes an overheat situation. It could be caused by a bad pump, bad relay, bad driver chip or the microprocessor not telling the relay to activate. By running the system and measuring the 5V inputs to the driver chips I discovered the board seemed to be working correctly. At this point the system started displaying --- which means it has shut down due to some problem. I was about to spring for a used controller board ($350) when I read a post that said the factory never changes the board without using a tester to simulate the sensors first. What a good idea. For the Optima 850 LDC controller I built the following test circuit http://www.pdf-archive.com/2016/06/25/optima-850-lcd-sensor-tester/ . Parts are available at places like www.jameco.com or www.digikey.com . I used a 14 pin IDC connector with a flat ribbon cable so I didn’t have to deal with crimp pins. The resistor are just 1/4W 5% parts, nothing special is required. I put the circuit in a small plastic box so I could label the switches and protect everything from touching any other electrical connections. http://www.pdf-archive.com/2016/06/26/optima-850-lcd-test-box (The controller logic runs off low voltage DC but there is 240VAC on the board for input to the power transformer and at several of the relays, so be very careful. It’s not obvious which circuit connections are high voltage.) Design of the test box allowed me to test the flow sensor and both temperature sensors. The controller requires No Flow on the flow sensor when the system powers up or it will shut down. With the flow sensor switch open, turn on the power. Close the flow sensor to simulate the circulation pump operating. Since the lowest temperature you can set on the heater is 80 degF, the design allows you to simulate heating and non-heating conditions. With this I checked the operation of the heater to verify it turned on and off properly. The hi-limit switch allows checking of overheating condition. After testing each sensor it appeared everything was working properly. When I reconnected the actual senor connector the system went back to its previous state and would not turn on. It simply displayed ----. Even though I couldn’t see anything wrong with the sensors when I measured the resistance, I decided to buy new sensors since they were cheaper than replacing the controller board. When the two sensors arrived, I replaced the temperature sensor first since it’s the easiest. The spa is now working again. Apparently there was some intermittent on the sensor that I couldn’t see. Not knowing the exact logic running the controller didn’t allow me to troubleshoot any further before taking a guess at the problem. I now have a spare hi-limit sensor in case that fails in the future. Thermistors aren’t under any stress so I don’t expect a problem; in fact I don’t know exactly what was wrong with the one I changed. The bottom line, the most common cause of this problem turned out to be exactly what several people had stated and fortunately it was one of the least expensive solutions. It's now been three weeks and the spa is working fine. PROBLEM SOLVED!