Here are some other tests that you can do.
With the power to the heater off, remove and inspect the voltage selector plug. Reinstall it.
Disconnect the blower plug from the connector and check each wire on the supply plug for voltage.
There should be no voltage.
Then, check each wire on the supply plug to ground. There should be no connection to ground.
Then check each wire on the supply plug to the other wires for connection. The only wires that should be connected are the blue and yellow wires.
Then, restore power to the heater and check each wire on the supply plug for voltage. The only wire with voltage should be the red wire.
Then, turn the heater to on and check power between the red and black wires. You should get 240 volts.
With the power to the heater off, remove and inspect the voltage selector plug. Reinstall it.
Disconnect the blower plug from the connector and check each wire on the supply plug for voltage.
There should be no voltage.
Then, check each wire on the supply plug to ground. There should be no connection to ground.
Then check each wire on the supply plug to the other wires for connection. The only wires that should be connected are the blue and yellow wires.
Then, restore power to the heater and check each wire on the supply plug for voltage. The only wire with voltage should be the red wire.
Then, turn the heater to on and check power between the red and black wires. You should get 240 volts.