Troubleshooting Rinnai Error Code 33: What It Means & How to Fix

Error Code 33 on a Rinnai tankless water heater specifically indicates a Heat Exchanger Thermistor Failure. This diagnostic code triggers when the unit’s control board detects an “open” or “shorted” circuit within the thermistor circuit, or if the temperature readings provided by the sensor fall outside the pre-programmed operating parameters required for safe ignition and modulation.

If you are seeing this code, your unit has likely entered a safety lockout mode. You might experience a total lack of hot water, or perhaps the unit begins to fire but shuts down almost immediately with a persistent beep. While a technical fault like this can be frustrating—especially when it interrupts your morning shower—it is a common issue that is entirely resolvable through systematic diagnosis and component replacement. As an engineer, I can assure you that once the sensor’s integrity is restored, your Rinnai unit will return to peak performance.

• Technical Difficulty: Intermediate (Requires use of a digital multimeter and basic hand tools)
• Estimated Time: 45 to 75 minutes
• Tools Needed:
  • Phillips Head #2 Screwdriver
  • Digital Multimeter (DMM) with fine probes
  • Needle-nose pliers
  • Clean rag or abrasive pad (for cleaning connections)
• Estimated Cost: $25 – $65 (Self-repair part cost) or $200 – $400 (Professional service call)

Symptoms of Rinnai Error 33

When the heat exchanger thermistor fails, the system provides several physical and visual indicators before and during the lockout phase:

• The Digital Display Flash: The most obvious sign is the number “33” blinking on the temperature controller or the internal PC board LED.
• The “Cold Water Sandwich”: You may notice the water starts to get warm but then suddenly goes cold as the PCB detects an irrational temperature spike or drop and cuts the gas valve for safety.
• Audible Alarm: Many Rinnai models will emit a continuous or intermittent beeping sound to alert the user that the system has ceased operation due to a hardware fault.
• Fan Operation without Combustion: You might hear the combustion fan engage and spin up (purging the chamber), but the igniter never clicks, and the unit never produces a flame.

What Triggers this Code?

In my years of field engineering, I’ve found that Error 33 is rarely a “ghost” code; it usually points to a tangible failure in the thermistor or its communicative path. Here are the primary catalysts:

1. Thermal Fatigue and Resistance Drift: The thermistor is an NTC (Negative Temperature Coefficient) resistor. This means its resistance decreases as the temperature increases. Over years of constant expansion and contraction—heating up to 140°F and cooling down to 40°F—the internal semi-conductor material can crack or degrade. This causes the resistance to “drift” out of the range the PCB expects, leading to a “short” or “open” circuit error.

2. Mineral Scale Insulation: If your area has hard water, calcium and magnesium can coat the thermistor well or the heat exchanger fins. This scale acts as an insulator. The heat exchanger might be getting dangerously hot, but the thermistor is shielded by scale, or conversely, the scale causes local “hot spots” that trick the thermistor into reporting a temperature that the PCB deems impossible, triggering the safety shut-off.

3. Connection Corrosion: Rinnai units are often installed in damp environments (garages, outdoor recesses). Moisture can penetrate the Molex connectors that link the thermistor to the wiring harness. Even a small amount of oxidation (verdigris) increases electrical resistance, which the PCB interprets as a temperature change, eventually resulting in Error 33.

4. Voltage Spikes: While rarer, a significant power surge can damage the sensitive circuitry on the PC board that interprets the thermistor’s signal. If the board’s “reading” logic is fried, it may display Error 33 even if the sensor itself is perfectly healthy.

The Complete Solution: Troubleshooting and Repair

Follow these steps in order to diagnose and resolve the fault. Warning: Turn off the electrical power supply and the gas supply to the unit before removing the front cover.

1. Access the Internal Components: Use your Phillips head screwdriver to remove the four screws holding the front access panel in place. Lift the panel up and away. Locate the Heat Exchanger (the large copper component at the top). The thermistor is typically clipped into the copper piping exiting the heat exchanger.
2. Visual Inspection: Look at the two wires (usually white or black) leading to the thermistor. Check for signs of melting, rodent damage (chewed wires), or loose pins at the connector. If the wires are frayed, the circuit is broken, which is a guaranteed Error 33.
3. Resistance Testing (The Engineering Gold Standard): Unplug the thermistor from the wiring harness. Set your Digital Multimeter to the Ohms (Ω) setting. Place your probes on the two pins of the thermistor connector. At room temperature (approx. 68°F), you should see a reading between 10kΩ and 15kΩ. If the meter reads “OL” (Open Loop) or “0.00” (Short Circuit), the thermistor is defective and must be replaced.
4. Cleaning the Connection: If the resistance reading is within spec, the issue might be high-resistance contact at the plug. Use a clean rag or electronic contact cleaner to wipe the pins. Reconnect it firmly and attempt a restart.
5. Replacing the Thermistor: If the sensor failed the resistance test, remove the retaining clip or screw holding the sensor bulb against the pipe. Be careful not to drop the clip into the bottom of the unit. Apply a small amount of thermal paste (if provided with the new part) to the tip of the new thermistor, seat it firmly against the pipe, and re-secure the clip.
6. System Reset: Once the new part is installed, re-attach the front cover. Turn the gas and power back on. Power the unit off and on at the controller to clear the stored code. Run a hot water tap to verify the burner stays ignited and the code does not return.

How to Prevent Error 33

To ensure you don’t face a repeat of this failure, implement the following maintenance protocols:

Annual Descaling Flush: Scale buildup is the #1 silent killer of tankless sensors. Flush your unit once a year with food-grade white vinegar or a professional descaling solution. This removes the mineral “blanket” that causes thermistors to overheat and fail prematurely.

Install a Dedicated Surge Protector: The control boards on Rinnai units are highly sensitive to “dirty” power. A simple plug-in surge protector designed for appliances can prevent voltage spikes from damaging the thermistor’s reporting circuit on the PCB.

Seal External Entry Points: If your unit is outdoors, ensure the venting and pipe covers are sealed against insects and rodents. Mice are notorious for nesting inside the warm cabinet and chewing through the thin thermistor wires, which will immediately trigger an Error 33.

Frequently Asked Questions

Q: Can I run the unit temporarily while waiting for the part?
A: No. Error 33 is a “hard lockout.” The system cannot safely monitor the temperature of the water leaving the heat exchanger. Without this data, the unit risks an “over-temp” condition which could melt internal components or cause the heat exchanger to burst. The system will not allow ignition until a valid resistance signal is received.

Q: Is Error 33 the same as Error 32?
A: Close, but no. Error 32 refers to the Outbound Water Thermistor (the one at the very bottom of the unit where water exits to your house). Error 33 refers specifically to the Heat Exchanger Thermistor (located at the top of the unit on the copper coil). They are different parts and are not usually interchangeable due to wire length and connector shapes.

Q: What if I replaced the thermistor and I still get Error 33?
A: If a new sensor doesn’t fix it, you likely have a “failed high” circuit on the PC Board itself. Use your multimeter to check for 5V DC coming from the board at the thermistor plug (with the sensor unplugged and power on). If there is no voltage being sent to the sensor, the PC board’s internal transformer or logic chip has failed and the board must be replaced.