Navigate to your home's main electrical panel (breaker box). Locate the double-pole circuit breaker dedicated to the water heater; it is typically labeled and requires two slots. Firmly move the breaker switch to the 'OFF' position. As a safety precaution, you can place a piece of tape over the breaker to signal that work is in progress and prevent accidental reactivation.

Most electric water heaters have two metal access panels on the side of the tank, an upper and a lower one. Using a nut driver or a multi-bit screwdriver, remove the screws securing these panels. Set the panels and screws aside. Behind the panels, you will find a piece of insulation. Carefully pull the insulation out to expose the upper and lower thermostats and their associated wiring.

This is the most critical safety step. Set your multimeter to measure AC voltage (VAC or V~) in a range greater than 240 volts. Carefully touch the two probes of the multimeter to the top two terminal screws on the upper thermostat where the main power wires from the house connect. The multimeter must read '0' or very close to zero volts. If you get a reading of 120V or 240V, the circuit is still live. STOP IMMEDIATELY and return to the breaker panel to correctly identify and turn off the power.

Take a photo of the wiring. For a standard upper thermostat, testing occurs in two parts. First, test the high-limit switch. Set the multimeter to continuity (Ω symbol or sound icon). Disconnect the wires from terminals 1 and 2 (left side). Touch a probe to each terminal. A reading of 0 ohms (or a beep) indicates continuity. No continuity (OL) means the thermostat is bad. If it passes, reconnect the wires. Next, test the temperature control switch. Disconnect the wires from terminals 3 and 4 (right side). Test for continuity between them. A reading of 0 ohms means it's working. No continuity means the thermostat is bad and needs to be replaced. Remember to reconnect all wires after testing if the thermostat is good.

If the upper thermostat tested fine, move to the lower one. The lower thermostat is simpler, usually with only two terminals. Ensure the temperature dial is turned up. Take a photo of the wiring. Disconnect the two wires from its screw terminals. Set your multimeter to the continuity setting (Ω or sound). Touch one probe to each of the two screw terminals. You should get a reading of 0 ohms or hear a continuous beep, indicating a closed circuit. If the multimeter reads OL (Open Loop) or does not beep, the thermostat has failed and must be replaced.

Once you've identified a faulty thermostat, disconnect all its wires. It is typically held against the tank by spring-like metal clips. Gently pry the thermostat away from the tank to release it from the clips. Take the old thermostat with you to a plumbing supply or home improvement store. Purchase a new thermostat that is an exact match in size, shape, and electrical ratings. Do not use a substitute.

Take the new thermostat and align it with the mounting clips on the tank. Press it firmly into place, ensuring the flat back of the thermostat makes solid, flush contact with the tank's surface. This is critical for accurate temperature sensing. Referring to the photo you took earlier, connect each wire to its correct screw terminal on the new thermostat. Use a screwdriver to tighten the screws for a secure connection, but do not overtighten, as this could strip the threads or damage the component.

With the new thermostat installed and wired, set its temperature dial to the recommended 120°F (49°C). Carefully tuck the insulation back into the openings, making sure it doesn't pinch any wires or obstruct the thermostat. Place the metal access panels back over the openings and secure them with their screws using your screwdriver or nut driver. Finally, return to the electrical panel and switch the water heater's circuit breaker back to the 'ON' position.

After restoring power, allow approximately 30 to 60 minutes for the water heater to heat the water in the tank. You may hear a humming or hissing sound, which is normal during heating. After waiting, go to a faucet close to the water heater and turn on the hot water tap. Let it run until the water feels hot. If you have consistent hot water, the repair was successful. Monitor for any issues over the next few hours.