Diagnosing Error code OH

[vaughanatworld]

I have a Marin 850 about 5 years old.
The unit is showing an OH code as the temperature approaches 100F.
The software then goes into economy mode. Everything works as expected in economy mode (pump 1 on, heater comes on...)
The mode switch is disabled so I can not enter standard mode.
I turn off the main breaker, wait an hour to a day (water cools to low 90sF), turn on the main breaker, and the unit is in standard mode and starts to heat up until 99F and the cycle repeats.

I found debris in the circulator pump impellor and cleaned it. I removed the filter. The effects are temporary and always return to the cycle described above. I put my hand over the inlet (filter nozzle) for the circulator pump and there is good suction and flow.

My attention is now turning to the Hi-Limit Sensor (I believe part number 6600-168). It looks to be silicone-d into the heater box. Before I dig it out, I would like to take measurements on the sensor. Is it an open/close switch or is it a variable resistance temperature sensor? If a resistance type, could someone direct me to a datasheet with a resistance curve. (Alternatively, depending upon the circuitry, at what voltage or frequency (across the sensor) does the error OH get tripped?)

Thanks in advance.

[vaughanatworld]

I have a related question.

At the top of the heater assembly, (very close to the hi temp sensor) coaxial to the water exit port and pointing towards the main control box there is a 1/8" to 3/16" rod about 8" long.

This rod passes through a soft silicone bushing into the water jacket. The horizontal position is regulated by a couple of nuts pinching an aluminum dust shield.

Originally, I thought the rod was there to hold the dust shield. However, it looks too complicated for that purpose. Does anyone know why it is there? Is the horizontal position/placement important?

[vaughanatworld]

Has anyone seen debris get through the circulation pump impeller and get hung up inside the heater core? This might be an alternative explanation for the OH code , if debris collects around the sensor.
Might a reverse flush clear any debris?

[Sam]

Has anyone seen debris get through the circulation pump impeller and get hung up inside the heater core? This might be an alternative explanation for the OH code , if debris collects around the sensor.
Might a reverse flush clear any debris?

I'm not a service tech, but OHH is usually caused by a flow restriction.  This can be dirty filters, or debris in the impeller area, even closing too many jets can cause it.  If you have debris in your impeller, that is a problem and likely the cause of your error code.  Have you run the spa without the filter in?  This would allow debris to get sucked in.  Most hot tubs also have auxiliary intakes in the footwell that have a plastic grate over them to prevent debris from entering.  One of these could be cracked.

As to the previous question about a rod:  That sounds strange.  Can you post a picture of it?