Pool heater cycles off and on

It should be just inside the door, maybe up behind the thermostat. It is connected to a 1/8" copper tube on one side and has two wires coming out the other side.

"LO" is simply informing you that the Heater is not sensing proper water flow through it! A Flow Problem.
A special sensor, called a pressure switch is this source. But... That does not mean it is bad - No, it is in most cases just doing its job!
Most common reason - Dirty Filter.
A good cleaning of the cartridges, or maybe a breakdown and recharge of a DE Filter, or a good cleansing backwash of your sand filter, will remedy this situation.
Or... Check for any OTHER reason or situations water flow may be reduced through the heater:

• enough water in pool or spa?
• If you have a weir gate in skimmer, is it stuck in the UP position?
• Are any valves closed?
• Do you have a bypass valve at the heater?
  • If open, this will reduce the amount of water going thru heater, close it to the point, LO goes away.
• Could be debris in front of Pressure Switch, in the copper tubing
  • Unscrew and remove pressure switch, turn pump on. But first, align end of tube so that when water squirts out it will shoot out from heater and not onto electronic heater components!
• If all above is good, you may have failed pressure switch?
• Etc

BO Message...
This means "By-Passed Operation"
This mode is for systems with 'Automation' controls. If you don't have an automation system, then your heater has a control dip switch or logic jumper that needs to be undone. But if you have automation, do Nothing, your automated system has control of your heater. Good Luck - hope this helped.

This could be a water flo problem. you need to chech to be sure your by-pass valve is closed down enough to force water threw heater, There is a internal by-pass that could be a problem also, or a bad pressure switch. If the by-pass isn't the problem then you should check these also.

Check for proper flow at your pump and at the return jets. You have to sustain proper water flow in order of rthe heater to function properly. If the flow is adequate, then use a jumper lead across the pressure switch terminals on the heater and see if it will fire and remain on. If so then the pressure switch is either faulty or you have a flow problem which is not generating enough water pressure to trip the switch. DO NOT adjust the pressure switch, as it is factory set. Check your pump for a clogged inlet or other possible blockage before the heater.

My pentair matertemp 400,000 BTU natural gas heater cycles on and off before reaching desired temperature. I completely cleaned the DE filter and flow should not be an issue. When running the spa the pressure is 25. When running the cleaning system (polaris pop ups) the pressure is 30. Burner fires for 80 seconds then shuts off, reignites after 53 ceconds.Any tips on what might be the issue with the heater

On the side of the heater where the pipes are connected; under the top cover. It is a round disk threaded into the header. It is likely that your filter needs a good cleaning if your heater has always worked reliably in the past. If you have a cartridge filter, it may be time for a new element if you can no longer get it clean enough to get the heater to work.

insert on hot water and see if open or close than on cold water and must reverse from hot. if nothing happen replace