Spa comes on and pump comes on but doesn't heat.

If the heater light or icon doesn't come on most common reason is the filter. To check is easy. Turn off main power and remove and leave filter(s) out for this test. Turn on power. If the spa heats (1-2 degrees per hour for 110vac spas; 4-5 degrees per hour 220vac) then you know either the filters need to be replaced or cleaned better.

If the heater light doesn't come on even with the filter(s) out, then it's likely 1 of 2 things. First and most common is the pressure switch. For interruption of heater because of the pressure switch, it's always best to replace. If not the pressure switch, it's the pcboard itself but this is rare.

If the heater light DOES come on it's either the heater itself or the pcboard. The only way to check is with a voltmeter. You set the meter for above 250vac. While the spa is running and the heater light is ON you place the voltmeter probes on the 2 posts of the heater. Be careful not to short anything. IF the meter shows you are getting proper voltage, then it's the heater that is at fault. If the voltage is not present, the pcboard has to be replaced. One last test for checking the heater. Turn off main power. Set the meter for OHMS. Test the 2 posts of the heater. They should read 11-15 ohms resistance on most heaters.