My heater displays r13 - Sta Rite Sr333na Max E Therm Pool & Spa Heater Dual Electronic Ignition Lo Nox 333,000 Btu Natural Gas

This is not an error code. 128 r13 is the normal programming or software version for your model. you need to press the spa on button again, make sure the temp set point is higher then the actual water temperature

Do you have automation?
If so, Is it in Auto?
You still need to, Manually press the Pool or Spa ON button.
The 128, then the R13 are = Normal.
They are just confirmation of firmware numbers and the Revision of the software Logic.
Then if the Heater is NOT on, display will go blank.

this is not an error code, its the startup sequence to your heater. after those numbers have been displayed you have to press the pool on button and set your temperature

There is no r13 code on your heater. The r13 flashes everytime you turn on your heater, wait, then see if it fires, if not, you will get an error code starting with EO then a number, or the "service system" or "service heater" light will come on.

A lot of things could be wrong. Sounds like your not to mechanical, you could start with the air flow switch. I think it would be safer for you to call a heater pro.

r 13 is not an error code that is a program that runs at startup when the power is applied then the display goes blank, now you push pool on button and it should work

128 r13 is not an error code. its the startup sequence of that heater

this is a start up display and is normal when furst turned on but should go away