Rheem Error Code 33 indicates a specific malfunction within the Heat Exchanger Thermistor circuit. This diagnostic code signifies that the water heater’s control board is receiving an out-of-range resistance signal—either an open circuit or a short circuit—from the sensor responsible for monitoring the temperature of the heat exchanger fins and piping.
📖 Warning: Check Manual First
For your safety and to avoid voiding the warranty, please check the official docs.
If you see this code, you are likely experiencing a total loss of hot water, or perhaps the unit begins to fire up only to shut down abruptly after a few seconds. You might also hear the internal fan running excessively as the system attempts to purge heat. Don’t worry; while “Heat Exchanger” sounds intimidating, this is typically a sensor-related issue that is entirely fixable with a bit of patience and the right diagnostic approach.
| Specification | Detail |
|---|---|
| Repair Difficulty | Intermediate (Requires Multimeter use) |
| Estimated Time | 45 – 75 Minutes |
| Tools Needed | Phillips Head Screwdriver, Digital Multimeter, Needle-Nose Pliers |
| Estimated Cost | $30 – $70 (for replacement thermistor) |
Symptoms of Rheem Error 33
- Control Panel Display: The most obvious sign is a flashing “33” on the digital remote control or the main PCB display inside the unit.
- Immediate Shutdown: The unit may ignite for a brief moment, but as soon as the control board performs its safety check on the heat exchanger temperature, it kills the flame to prevent overheating.
- Cold or Lukewarm Water: Because the system cannot verify the safety of the heat exchanger, it defaults to a lockout state, preventing the burner from heating the water.
- Audible Clicking/Fan Noise: You may hear the unit clicking as it tries to ignite repeatedly, followed by the blower fan running at high speed to cool down the internal components, even if the water isn’t hot.
Detailed Diagnosis: Root Causes
Detailed Diagnosis: Root Causes
Understanding why Error 33 occurs is vital for a permanent fix. This isn’t just a random glitch; it’s a safety response to a perceived thermal emergency or a hardware failure.
1. Thermistor Component Failure: The thermistor is a resistor that changes its electrical resistance based on temperature. Over years of operation, the constant expansion and contraction of the heat exchanger pipes can cause the internal ceramic element of the thermistor to crack or degrade. This results in “drift,” where the sensor sends the wrong voltage back to the board.
2. Wiring Corrosion or Loose Connections: Tankless water heaters vibrate during high-output operation. This vibration, combined with the humidity often found in mechanical rooms, can cause the molex connectors to loosen or the copper pins to oxidize. If the connection is “noisy,” the control board triggers Error 33 to prevent a false reading.
3. Mineral Scale and Overheating: If your heat exchanger is “limed up” (heavy mineral deposits), the heat from the burner cannot transfer efficiently to the water. This causes the metal of the heat exchanger to reach much higher temperatures than intended. The thermistor detects this spike and shuts the system down to prevent the copper from melting or cracking.
4. Control PCB Logic Error: While rare, the Integrated Circuit (IC) on the main control board that interprets the thermistor’s resistance can fail. This usually happens after a power surge or lightning strike, causing the board to permanently “think” the heat exchanger is overheating.
Troubleshooting & Replacement Instructions
Troubleshooting & Replacement Instructions
Step 1: Safety Protocol & Power Down. Before touching any internal components, you must disconnect the power. Unplug the unit from the wall outlet or switch off the dedicated circuit breaker. Additionally, turn the yellow gas shut-off valve to the “off” position (perpendicular to the pipe). This ensures that no electrical discharge occurs while you are testing sensitive circuits.
Step 2: Remove the Front Access Panel. Using your Phillips head screwdriver, remove the four screws located at the corners of the front cover. Carefully pull the cover toward you and set it aside. Be mindful of any wires connecting the display panel to the interior; you may need to unclip a small harness to fully remove the cover.
Step 3: Locate the Heat Exchanger Thermistor. Look toward the top of the unit where the large copper heat exchanger is located. The thermistor is a small sensor—usually with two white or black wires—clipped directly onto one of the copper tubes. Do not confuse this with the “Outlet Thermistor” located near the hot water exit pipe at the bottom. The Error 33 sensor is specifically on the exchanger body.
Step 4: Perform a Continuity and Resistance Test. Unplug the thermistor’s wiring harness from the control board. Set your digital multimeter to the Ohms (Ω) setting (specifically the 20k range). Place your probes into the connector pins of the thermistor. At room temperature (approx. 68°F / 20°C), you should see a reading between 10k and 15k Ohms. If the meter reads “0” (short) or “OL” (open), the sensor is dead and must be replaced.
Step 5: Replacing the Faulty Sensor. If the test fails, use needle-nose pliers to gently remove the retaining clip holding the thermistor to the pipe. Pull the old sensor out. Before installing the new one, ensure the contact point on the copper pipe is clean and free of debris. Slide the new sensor in, reattach the clip, and plug the harness back into the designated spot on the control board.
Step 6: System Reset and Verification. Replace the front cover and screws. Turn the gas supply back on and restore power. Open a hot water faucet to trigger a demand. The unit should now sequence through ignition without displaying the Error 33 code. If the code persists, the issue may lie in the control board itself.
How to Prevent Error 33
To avoid a recurrence of this error and extend the life of your Rheem tankless unit, follow these professional maintenance tips:
- Annual Descaling Flush: Mineral buildup is the leading indirect cause of thermistor errors. Flush your unit with food-grade white vinegar or a professional descaling solution every 12 months to keep the heat exchanger surfaces clean and temperatures stable.
- Install a Surge Protector: Since the control board interprets the thermistor’s signals in millivolts, even a small power surge can damage the sensitive logic gates. Use a high-quality surge protector designed for appliances to shield the PCB.
- Check Cabinet Seals: Ensure the front cover gasket is intact. If moisture or steam from a nearby leak enters the cabinet, it can corrode the thermistor connections, leading to the false resistance readings that trigger Error 33.
FAQ
Frequently Asked Questions
Q: Can I bypass the thermistor just to get a quick shower?
A: Absolutely not. The heat exchanger thermistor is a critical safety component. Bypassing it could allow the heat exchanger to reach temperatures high enough to melt the copper or cause a fire. The control board is designed to detect “jumpered” wires and will usually throw a different error code anyway.
Q: Does Error 33 mean I need a whole new heat exchanger?
A: Rarely. In 90% of cases, Error 33 is an electrical issue with the sensor or the wiring. You only need to replace the heat exchanger if it is physically leaking or if it is so clogged with scale that the new sensor continues to detect legitimate overheating.
Q: My multimeter readings are slightly off from the manual; is that okay?
A: Thermistor resistance is highly dependent on temperature. If the water inside the unit is still warm from a previous attempt to run, the resistance will be lower than the “room temperature” spec. However, if the reading is “infinite” or “zero,” the part is definitely failed.