How To Test High Temperature Cutoff Switch Located
at the Upper Heating Element
Before testing the electic water
heater elements we should check out the high temperature cutoff switch since a
problem there will simply turn off power to both heating elements.
On an electric water heater the upper heating element may have two, three, or
four electrical terminals, and above the upper thermostat is a high temperature
cutoff switch with four terminals. You should be able to identify these parts by
looking at the water heater's wiring diagram found on the tank or inside the
The electric water heater high-temperature cutoff swtich will turn off the
entire water heater if temperatures are too high. If this cutoff is not working
the water heater will simply not heat at all. You can use this switch to test to
confirm that the incoming electrical power circuit is indeed bringing power to
the water heater.
Turn off electrical power to the heater
at the electrical
panel: WARNING: possible fatal electrical shock
Set the multitester
or VOM to its 250 VAC scale, and attach
the tester probes or alligator clips to the two uppermost terminals on the
control (L1 and L3 on the top left and top right in our photo below). These are
normally the incoming electrical wires from the power circuit to the heater. "L"
refers to "Line voltage" on most controls.
Test for line voltage:
Now you can turn on electrical power
to the heater. Your VOM should show voltage close to the voltage on the water
heater's identification tag: typically this will be 208V or 240V for a 240V
water heater, or it might be just 115V or 120V for a 120V water heater. If you
do not see voltage close to the rated voltage you need to call an electrician to
diagnose and repair the problem.
Turn off Electrical Power to the
Water Heater again.
Don't skip this step or you may be killed or you
may ruin your test equipment.
Test the High Temperature Cutoff Switch
itself: set the VOM
to its Ohm or resistance scale using the most sensitive setting (R x 1). Touch
one probe to the upper left terminal on the control (L1 in our photo at left).
Touch the other probe to the lower left hand terminal on the control (T2 in our
photo at left).
The ohm meter needle should move (or your digital multimeter DMM should show
some number other than infinity). If no continuity and infinite resistance is
what you read, try pressing the reset button on this control (the red "RESET"
button in our photo at left).
If you cannot reset the control then this device is not working and it needs
to be replaced.