Here we discuss the main components of the heater circuit and the specific symptoms that may be associated with each.  Testing techniques and possible fixes are listed.

Note! Digital systems with printed circuit boards typically do not use mechanical thermostats and high limits, They use electronic temperature sensing devices.  They may use a contactor and pressure switch, and GFCI tripping issues may still involve the heater. Some tests will be the same.

Thermostat:  (Symptoms: no heat, too hot, not hot enough)
Mechanical thermostats consist of a switch, control knob, and temperature sensor.  The temperature sensor looks like a copper wire when in fact it is a tube connected to a bulb which is filled with freon which expands and contracts with temperature.  It is this expansion and contraction that activates the switch.  If your spa is too hot and does not respond to a turning down of the thermostat, this bulb could be corroded.  Check it.  If so, the gas may have escaped and thus while the switch will activate you'll have no sensing capabilities so it won't turn off at any designated temperature.  Bulbs cannot be replaced.  Time for a new Thermostat.  If your pump does not respond to thermostat demand you may want to check through the thermostat with your meter on ohms and power off to be sure the switch is opening and closing. You can usually hear this if your ears are good.  The switch will make a faint click as it is engaged and disengaged.  If it is engaging your problem may be up or down the line.  A power on check will assure you the T-Stat is getting power.  Meter to AC volts check each side to ground. 

If your spa is not hot enough, you may find that the thermostat is out of adjustment.  If you remove it from the heater (power off of course)  you'll find an allen screw adjustment on the bottom.  a quarter turn clockwise will usually  increase the temperature to comfortable levels... A word of caution here.  

It is UNSAFE to use your spa at temperatures above 104 degrees! It raises your core body temperature and can cause flu like symptoms.  It can also kill you!  Even at 104 degrees you should limit your time in the spa to no more than 15 minutes.  Some people shouldn't use a spa that's hotter than body temperature.  Consult your doctor!!!  Drugs and alcohol do not mix with spas either!  If you turn up your thermostat it's your responsibility to be sure your spa is not too hot for safe use.  Get a good thermometer that will give you an accurate reading.

High Limit: (Symptoms:  no power, no heat, high limit trips too soon)
High limits are also temperature sensing switches. They also use sensing bulbs so the same problems that exist above can influence your high limit.  If your high limit is tripping within a few minutes of activating the heater CHECK FLOW! Little or no water flow will cause this. 

If it trips at the end of the heating cycle it's probably because at the end, when the thermostat turns the pump off,  the element is still hot, and the water is hot.  The temperature at the element  can go up for a few seconds and will sometimes trip the high limit.  It's best to replace the high limit if this happens all the time. The high limit has an adjustment screw also.  If you choose to try this it is a counter clockwise turn and it should barely be tweaked.  You need this safety switch to interrupt if there is a problem. 

This adjustment is at your own risk!  Remember flow is critical!  If you adjust your high limit and your pump impeller is full of leaves you could have a complete meltdown of all pipes in contact with the heater.  Not a pretty sight!  If you have no power this can also indicate a "high limit tripped" condition.

Pressure Switch: (Symptoms: contactor not closing, no heat.)
Obviously, flow is going to come up again here!  You must have flow for the pressure switch to activate!  To check for activation:  with power off, disconnect the leads going to the pressure switch.  Make sure they aren't touching anything.  Power up and turn on low pump. With your meter on ohms, check across the poles of the switch. You should have continuity. If not and you know flow is not the issue, most pressure switches have an adjusting collar that will vary the pressure it takes to activate it.  Counter clockwise turns decrease pressure required.  Turn the power off  before attempting any adjustment!  Make all adjustments slowly and try again. WARNING, if you go too far the pressure switch may activate with the pump off. 

This is the last thing you want to happen!!!  Always check to be sure that the heater goes off when the pump does!   If you are certain flow is present, but your pressure switch isn't activating you may want to remove it and be sure no debris is clogging it up.  Pressure switches are in contact with the water, when you remove it water will come out (or should).  Power off!  Close valves. 

Sometimes the location of the pressure switch is such that removal is impossible without removing other components (I hate that!) Some use a plastic base and must be unscrewed very carefully or the darn thing will break off. Now you'll have to dig out the old threads and replace your switch.

Flow Switch: (Symptoms:  GFI trips, no heat)
I haven't said too much about these as they aren't as common as pressure switches.  They are usually in the plumbing close to the heart of the action.  They are gray or white with a small cord coming out of them that goes to the controller. They utilize a little paddle type device that is pushed by water flow until it activates a little switch.  The switch should be isolated from the water. 

If, when you open your flow switch water comes out, you must replace it.  Otherwise, with the power off remove the leads to the terminals, make sure they aren't touching anything and power up.  With flow established check the switch to be sure it is closing using your meter set on ohms.  You should have continuity through the switch.  If not, time to replace. The wires, though small, carry 120 volts.  Don't leave them hanging.

Contactors: (Symptoms:  no heat, buzzing, GFI trips)
Contactors close when the control loop is complete. They consist of a coil which when energized should pull down the contacts so the voltage to the element can pass through.  Troubleshooting a contactor consists of checking across the coil (not the contacts) to see if the circuit to it has been completed.  Coils can be 120 volt or 240 volt with 120 being the most common.   This is a power on test so be careful.  With your meter set to AC volts activate all controls necessary to activate your heater. Turn up the thermostat to engage your low pump. 

Check across the coil of the contactor for voltage (neutral and hot side for 120 volts).  If you find the proper amount of voltage present and your contactor is not engaging it is bad.  If you do NOT have voltage at the coil there is an open switch somewhere in the control loop, you'll need to go back through your switches.  If your contactor is buzzing replace it. There may be more than one contactor.  Make sure you are checking the heater contactor by tracing back the wires from your element.

The Element: (Symptoms:  GFI trips, no heat)
Before we start it is important to note that if the spa is heating AT ALL it is not the element. See thermostat.  Assuming this is not the case continue with the power off, disconnect all leads connected directly to the element.  With your meter on ohms test across the element terminals for continuity.  If there is no continuity your element is bad.  If  you have continuity it should be around 9 to 12 ohms.  If your GFI is tripping, check from ground to each element terminal.  The slightest flicker of continuity indicates a fault to ground and your element is bad even it it has continuity between the terminals. It may have a pinhole in it or be otherwise corroded and leaking current to ground. This is a potentially lethal situation and your GFI is doing it's job. Do not bypass GFI (ever!), even if your element looks okay!