Upon start heater comes on for about 5 seconds and shuts down. Then starts again and stops again, and so on. "PRS" appears

Most likely, the thermostat is bad and the heater is getting too hot. It shuts off to keep from catching fire, then cools down and works again.

ive had two of these to fix...its the battery contacts get dirty on the lawnmower battery compartment ive checked wires switchs all fine as other stated,ive cleaned the contacts on the lawnmower side with a fibreglass brush ,and they dont shut down down...anymore rich ,the contacts are shown in picture below ,its worked for me

AGS - Auto Gas Shutoff = Because of a problem sensed, the gas valve is closing, stopping gas supply and hence heater shutoff.
Improper water flow through heater! Check filter, pump, water supply lines, anything that will reduce water flow into heater.
A faulty Pressure Switch may cause this problem as well.
or...
In coming water to heater is over 104°
If all is good with the above, replace the AGS Switch.

change the oil to the correct specification first. As the engine runs the heat changes the viscosity and overheats or trips the oil alert switch. If its over 10 years old change the oil alet magnetic switch

Your true actual time second hand is on one of your subdials. That big "second" hand is really your chronograph seconds timer. It should start and stop when you depress one of the plungers/ buttons on the side of your watch.
Again, that big hand does not need to be turning all the time (or ever if you never use your advanced stopwatch/ chronograph). The true seconds hand is very small on one of the smaller dials.
Good luck, I hope that's all it is

one of two things it could be the flame sensor or the control board for the entire unit

Heater must have water before it will fire up. There is a delay relay that may be dozing off. Not a bad thing but should be chk'd by a pool sevice.

Dustin, The ignitor only glows for 5 seconds, just long enough to ignite the fuel. The heater is shutting down because the photocell does not see the correct flame color.
Since you say "comes on for 5 seconds", I can only assume that there is a flame. You do not specify a lack of flame. I am also assuming that you have had the heater running this heater season and the problem just started.

YOU must find out if the problem is fuel delivery related or electronics related. Start by checking the pump pressure.

Follow these troubleshooting tips

If it's not beeping during the first start up then it's not POSTing.  Beep means everything connected to it seems to be functional ie PnP  
Try pulling out your memory and reseating it.   Try pulling out your battery for 5 minutes then plug in ac(without batt) and see if it will boot.  Try pulling your hard drive and reseat.  (access by covers on bottom...dont be scared. Touch something that's grounded just before you remove) If all this and still no full POST(with beep/s) then sounds like a new mobo or processor.
James Mac

reset switch my heater did the same thing after 5 sec of running