Spa Info: 1998 Cal Spa C2000 premier2
240v 40amp Digital ControlPack
two 5.0hp pumps

Late last August our Cal Spa started to display a HL error code on the digital read out, and reset itself.

In an attempt to troubleshoot, I cleaned out the filter checked for any obstructions and continued to reset the breaker and restart the spa. All of the jets seemed to be working fine with no apparent water flow issues but after about 20 to 30 seconds after restarting, the same error code appeared on the display and the spa would reset itself. I checked around the heater manifold and it did not seem to be excessively hot, so I decided to purchase a new high limit switch to see if that was the problem.
After installing the new switch the same problem happened all over again.

We decided to drain the spa and weather-proof for the winter months and address the problems this spring.

I was wondering if the heater element could be bad and is tripping this error code, or is it the circuit board, or does the controller as a whole need to be replaced.

A high limit device is not a switch, but a sensor at the end of a grey wire. Hopefully this is what you replaced.

If it is, then what I would look for is a heater casing that is like very warm to the touch when it trips off line and gives you the HL code. If not then replace your Temperature sensor also.

This is why it's always a good idea to replace the temp sensor and high limit as a set.