I have a mhu45ng mr heater. When i turn it on nothing happens. It has 120v power and 24v power. The light on the circut board does not come on at all. The gas is on the thermastat is on and the power is turned on.

check the pressure switch and the snap disks

How old is the heater? Is the control fuse good? The issue you are describing is exactly what should happen if you power up the heater and DO NOT select POOL or SPA. If the membrane pad on top of the heater is not working or the user-interface board is bad then the heater will not be able to turn the heater on.

The green light being off simply means there's no power, if the fuse is blowing this isn't surprising. If your circuit board looks OK - no burn marks etc I think the most likely culprits are the components that get the mains voltage which are the two fans and the transformer. You tube has videos about checking these components with a multimeter.

On your heater, does it tell you a fault code for the number of blinks?

If you have cycled the power to the unit already, the fault code should have cleared. If the fault code cleared and then came back, sounds like either a issue the blower motor being shot, or a relay in between the ignition controller and or the ignition control is bad, not making the heater do anything.

I’m happy to assist further over the phone at https://www.6ya.com/expert/robert_54ae41aaaffbab00

There is most likely, a diode in line before the transformer. Also, check the switch and thermostat. To see if the thermostat works, connect an ohm meter to it, warm it up with a hair dryer, and than cool it down with a bag of ice or some canned air.

check to see if the gas line or gas valve is blocked

try national energy they are on line

check for 24vac between wires on gas valve about 5-10 seconds after ignitor starts to glow. If ignitor does not glow, check for 120 vac to ignitor after inducer goes on.
First...sequence of operation..power to W, inducer motor gets 120v and starts, 10-15 seconds later, ignitor gets 120v and glows, 5-10 seconds, gas valve gets 24v and opens, after 3-4 seconds the flame sensor approves flame and unit stays on. If this is not happening, it has to be in this sequence. Which ever is not happening and you do have voltage to that component, that component is bad. If you are getting a error code on board after it tries to light, pay attention to the code. if a 1 lock out code, that just means it tried 3 times and did not recognize a flame.

Check for a fuse either in-line or on the circuit board. There should be 24 volts going to the thermostat. If not, you have either a bad transformer or a bad thermostat wire. Jump out the terminals that the thermostat wires are connected to. This should force the unit to fire. If not, bad circuit board.

If nothing will come on check the breaker to the indoor section, check that the service switch by the in door section is on and the blower door cover is tightly closed. If all these junctions are transmitting power then it may be a burned out transformer. If you can check for 120v on the primary coil of the transformer and 24v on the secondary that would tell you where the problem is. If theres no voltage on either side then backtrack from the transformer on the 120v side till you find 120v. If theres 120v but no 24v your transformer secondary is burned out. If theres both then the problem is downstream from the transformer: circuit board 3amp fuse, loose wiring.