If original problem was caused by sediment burning out lower element, did you clean sediment out of water heater when changing lower element?
Two element water heaters are 240V.
Thermostats cut power on one hot wire, but 240V circuit has 2 hot wires. So lower element has power at all times coming from the red wire.
1) Use ordinary tester or multimeter set to 240V <> test each screw of lower element to bare metal part of water heater. Each screw should show 120V. If there is no power, then upper thermostat is defective, screw on upper thermostat is loose, or red wire between upper and lower parts of water heater is bad.
2) Remove wires from lower element. Use continuity tester or multimeter set to ohms <> test across both screws of element to see if power travels through element <> if element is bad, then continuity tester will not light up and multimeter will show no ohms. Put wires back on element after test.
3) Start with cold water in tank. Turn on water heater & upper element will turn on. Put ear on side of tank to hear if upper element is heating water. Test #3 screw to #2 screw on low left side of thermostat, should read 240V > Test #3 screw to #4 screw on low right side of thermostat, should read 0V.
Put ear on side of tank. When upper element turns off: Test #3 screw to #2 screw on low left side of thermostat, should read 0V > Test #3 screw to #4 screw on low right side of thermostat,
should read 240V. If this test shows 0V and 0V, then upper thermostat is bad. If reading is correct, immediately move to next test.
4) Turn temp on lower thermostat to 150. Using ordinary tester or multimeter set to 240V <> Test across both screws of lower element. If there is no reading, and you know element is good from step 2, then lower thermostat is bad, or wire is bad between lower element and lower thermostat, or wire is bad between lower thermostat and top part of heater.
5) Test across both lower thermostat screws, should read 240V.If this test is good, it means wires are good. Put jumper wire across both screws of lower thermostat, and test lower element again. If element finally turns on with jumper wire, then lower thermostat is bad.
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If wire is bad between upper and lower parts of water heater, and water heater has tight-fitting 2" foam insulation, it usually indicates water heater replacement.