Caldera geneva not heating

[Bazzo]

So.. just acquired above spa for free.. and its in really good condition.

Temp says 10deg c.. when turned on, although I'm sure it's more like 20 as I'm in Spain.. temp light flashing.

Flow is very good, filter is clean, so I'm thinking temp sensor or heater...

Any tips on testing either ?.. I'm good with a multimeter.. ir should I start elsewhere ?

 

[RDspaguy]

Post pics of the circuit board, wiring diagram, and equipment area so I can see what you've got. Some systems use a pressure switch, and some have a separate 24/7 pump for the heater.
Remove filter(s). If problem goes away, get new filters.
Check pressure switch for continuity with pump on and open with pump off. Some systems can bypass the pressure switch with a jumper, others can't.
Test thermistors (sensors). They are in the thousands and tens of thousands of ohms, so set your tester appropriately and read it correctly. Post results.
Test heater for continuity, though it's likely not the cause.