Flame Sensor

My Jandy Lite2 heater attempts to stay lit but after initial ignition it shuts off. I am suspecting the flame sensor. (This model was manufactured in 9/2003 and the sensor is located next to the ignitor. Newer models have the sensor in a different location. Better design ?? don't know) My sensor is a metal rod that is attached to a ceramic casing which is then connected to a single wire that connects to the control panel. I have removed the sensor and cleaned it with sand paper. This has worked in the past, however not this time. I also checked continuity between the rod tip and the end of the wire. Positioning it over the flame, I am able to measure a temperature increase from the end of the wire using a i/r thermometer....

So my question is.. how does this Flame Sensor communicate it's status to the heater. Is it via continuity or heat ? Does it break continuity when heated ?

Any other suggestions. Any further testing I can do to troubleshoot my problem. Could it be the pressure switch and how can I test this ?

Thanks
Ollie

Jandy Lite2 LJ400N