The raypac 2100 pool heater turn on but when you go to fire it al

Check inducer fan motor. Unit has to have airflow. The air is acting as as safety. If not heater will shut down every time

make three attempts to get heater running. most electronic ignition heaters will lock out after 3-4 times. after it locks out error codes should present themselves.....such as ignition failure, prs, etc.
ensure proper gas and water pressure. check voltage supplied to your raypack's igniter aswell. if heater fires up then instantly shuts off, then it may be the flame sensor

ignition lockout. heater has tried to fire up 3 times without success. after three attempts unit will temporarily lock itself out

Here is the directions from raypak on this code.

1. Turn off power and disconnect ignition wire and gas valve wires from cpu board.
2. Turn on the power and watch for spark at module.
3. If no spark is observed, turn off power and replace the board.
4. If spark is observed, turn off power and reconnect gas valve and ignition wire.
5. Turn on power and unit should fire.
if unit does not fire and code reappears, turn off power and disconnect gas valve wires from the board and remove MV,PV and ground wires from the gas valve. Clean and reconnect wires.
7.Plug the gas valve back into the board and torn power on. Unit should fire.
8. If the unit fails to fire, Replace the gas valve.

Its not a good code to get. Hope this helps

You didn't indicate what model heater you have, but here are some of the fault codes you might see. Could it have been GVO? You can download a manual for your exact model at www.raypak.com
BD1 Board failure
EEP Microprocessor error
FAN Blower pressure failure
FFL Flame sensing when pilot and gas
valves are closed
GVC Gas valve closed
GVO Gas valve open
HL1 High limit switch #1 open
HL2 High limit switch #2 open
IGN Ignition failure
ILO Ignition lockout - Propane units only
PRS Water pressure switch open
ROL Heat roll-out safety switch open
SNS Sensor failure, Water temp. below 36°F
or above 110°F
VNT Vent switch open

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.

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.