Controller maintenance is the same for many types of AC-based solar thermal system. Some of the tests require a multimeter.

Almost all controllers have two lights on the front: one that indicates the power is on and the other indicating that the pump is on. If the system isn't working, check the lights and follow these steps:

Both lights off

Check that there is power at the controller, using your multimeter. If there is, your controller is not working. Shut off the power, check the wiring and try again. If it's still dead, you will have to replace it.

Both lights on

Check that power actually reaches the system pumps, using your multimeter. If power reaches them but they're not running at all (whatever the circumstances), there's a problem with the pumps.

If there's power at the pumps and they run, but not at the right times, your sensors might be messed up or the controller faulty.

Check the controller first by switching off the power and taking off the casing. Look for black marks on the controller board, which will indicate a blowout from a power surge – this often happens after a lightning storm, though it can be caused by any kind of surge. The marks may be small or they may be really obvious.

If you can't see any burn marks on the controller board, you can test the functionality for each sensor set in turn. Most controllers have a jumper wire you can use to short out a sensor circuit, which convinces the controller that the collector array is hot – the pumps should switch on when you do this. If the pumps come on, there's a problem with the sensor you shorted.

To check a sensor, start with the wires. They should be soldered to connect the sensor. If the connections look good and are not corroded, you will need to check the sensor itself. This is done by removing the sensor and connecting the wires across your multimeter, with the tool switched to measure ohms resistance.

If the meter measures zero or infinite resistance, the wires are fine and the sensor needs to be replaced.

If you get a reading of a number of ohms, you can compare that to the sensor chart (which should come from the manufacturer along with the sensor) and see what temperature the system thinks is at the sensor location. An obviously erroneous value (like 500 degrees or something) shows the sensor is messed up and needs replacing. As a guideline, a 10k sensor reads 10,000 ohms at 77°F and 2,043 ohms at 150°F.

If the sensor checks out on all fronts and the controller still won't kick off the pumps at the right time, there's an internal problem with the controller and you should replace it.