Grande (Hot Springs) Spa Power Up (Red Light) Issue

[hge]

I have a similar to reported  "flashing red light at startup" issue with my Hot springs Grande.  

Power up goes thru a start up routine,... temp flashes 70  (now 55 since outside is colder) then off and then red light flashes and no control of temp or pumps.

Circulation pump comes on (but after some delay), and generates proper bubbles thru the bottom of tub..  

Had replaced the heater unit few years ago (>10 yrs use), and the circulation pump shortly after and Spa ran great for several seasons.    Just bought and installed new thermistors (replaced originals). and the same problem still occurs.

Other threads have reported similar issues but did not see a final resolution in those reports.  hge

NOTE Appreciate any advice here before my next step to repeat replacing the circulation pump. since it cleared the problem a  couple years ago. 

Also concerned that Cold weather approaching.

[RDspaguy]

Test thermistors with an ohm meter in the 20k ohm range. A difference of a few hundred ohms is a fault, replace both thermistors.

Post pics of the circuit board, wiring diagram, and equipment area.

[hge]

Thermisters test results at RT  (these were originals  with Heater change ~3 years ago)

4 term plug   A1335      12.06   12.31   12.60,   12.50  Kohm

2 term plug   A1336      11.60   11.90   11.50,   11.70  Kohm 

seemed not steady and different enough so I changed them both with new ones

[hge]

Changed Circulation pump,  all filters out, tied hose in first filter hole to push out any air bubbles   still the same problem,

 

Getting below 10 degrees in two days ,        any last advice / ideas before calling Spa company ???