Our pool heater display shows SPK when we turn it on.

Sounds like your ignitor is shot. You may have to replace it.

spk I think means spark for ignition and plt means pilot so i assume the heater tried to light but the pilot light is not staying on. could be dirtly contact on the ignition spark plug or dirty pilot flame need to be cleaned and the unit serviced.

i hope this was useful.

You can try pulling your pilot assembly out and clean it. Sometimes it will get sooted up and cause the sensor to short so the heater get's a falce reading and shuts the heater down. Then sometimes the heater will start and sometimes it won't. If this dosen't help check the ignition wire for any cracks or anyplace it may be shorting. Good luck.

CFH is call for heat not sure what spk is, but will find out and let you know.