Spa not heating past 89 degrees...

[Spatropy]

I've got a 19 year-old Marquis 214 (a small two person 115v tub, aka "Rendezvous") hot tub, and it's not getting hot. Just warm.
It's been working fine for over a month.
I just cleaned the the plumbing with Ahhsome, drained, refilled, and now it won't get up to full temperature.
Maybe running continuously (tub normally takes 18-20 hours to reach max temp because of 115v) finally caused a problem.
Tub is correctly set to 104 degrees, but is stalled out at 89, sometimes 88, sometimes 90, but around there. Digital readout says that, confirmed with in-tub thermometer.
From looking around on some forums with similar problems, sounds like it could be:
Temperature probe
Hi temp cut out switch (heater safety switch)
Temperature sensor
Thermal regulator

I'm not even sure I have all these things or if they're the same thing. Well, I've got a sensor in the tub and a sensor right after the heater (see photo below). I guess my question is: can somebody point me toward how to troubleshoot/diagnose the sensors?

Thanks!

 

[RDspaguy]

Do you have a multimeter?
Open the box and post a pic of the guts and wiring diagram.
Are there any error codes on the display?
Most likely is the pressure switch, top left on the heater. Unplug and test for continuity when pump is on.
Test voltage between heater terminals.
If it has a manual hi-limit it will be accessible from the outside of the box, look for a little button through a hole or a rubber gromet on the side or back of the box.
Post results.

 

[Spatropy]

Thanks for the response. This is good info to keep in mind. I just ended up solving the problem: I had corrosion on the plug of the in-line temperature sensor after the heater. I was pulling off wires and one was coated in green copper corrosion, so I hit it with some contact cleaner (DeOxit in my case), and sprayed the other connectors for good measure, and now it's getting up to full temp.

 

[Methuselah]

For an old tub it seems the acid test is a drain and refill. Mistakes can happen, and heat cycle from fill temperature to set point runs it up to a day. Glad you found the problem!

I noticed a label in you picture that your tub can run on 220v. Since you mentioned DeOxit, I'm suspecting you may be savvy enough to know the advantages of a 220v system...