We have an older Coleman Horizon 208 spa which we hadn’t used in a while and recently got back into working condition. After inadvertently locking the panel, I realized I had forgotten how to unlock the darn thing and searching on google showed that apparently other people have run into this too. Anyway, I thought I’d share the solution here just in case it might be useful for someone.
This works for both the panel lock and temperature lock features.
To unlock either, press the following 3 buttons in rapid sequence. Program, then Mode, then Set Down. The manual says you have 2 seconds but the timing actually appears to be quicker which is the issue I was having.