What can cause a transformer to fail?  Not much. Rarely happens.

What does a transformer do?  Simply said, it takes incoming power (primary) and reduces to a lower power (secondary). You have only one primary but you can have several secondary output voltages. Most boards only use one secondary but some use 2.

Its purpose is to take the incoming power and lower the power to a voltage the board and other components operate on. So the primary can be 115v or 230v depending on the spa. Note that some spas that operate on 230v have a 115v primary transformer. Don't assume your spa is 230v primary.

The secondary is the lower voltage. Most output to boards are around 12-15vac. This operates the board and often the outside mood light(s).

How to I test a transformer?  Testing a transformer should not be performed by someone who is not experienced and comfortable with electricity. If you are not qualified, have a licensed electrician do the test.

What you need to test is the incoming power to the transformer and the output. In general, incoming power travels through the board to the transformer and back. So look for your two primary wires from the transformer plugged into the board. Note: This test can only be done on spas with a separate transformer from the board. Boards with onboard transformers can not be tested this way.

Again, concern is this test has to be performed while power is being applied to the spa. See picture. You identify the 2 primary lines and slide your meter's probes down the wires until they touch the metal on the plug. You should get either 115v or 230v depending on which voltage the system is designed for.

If proper primary voltage is present, test one or two pairs of secondary voltage. They should be decreased into the 12-20vac range. If they are, the transformer is good. If not and you do have proper incoming power, the transformer is bad.

Let me say that since 1995, I probably have replace 2 bad transformers. They are highly reliable.

On picture below, I'm checking the primary voltage for this spa; in this case, white and black wires. I'm sliding the probes down the wires until the lightly touch the metal clips inside the actual plug. This gives me a 230v reading which is correct. The incoming power to the transformer is good.