Rinnai Tankless Water Heater Error 32 Solved: Detailed DIY Repair

If you are seeing this code, you are likely experiencing a complete lack of hot water, or perhaps the unit begins to fire and then abruptly shuts down. You might hear the fan engage, but the ignition sequence fails to complete. While a fault code can be frustrating, Error 32 is a diagnostic signal designed to protect the system from overheating; with the right technical approach, it is a straightforward component to test and replace.

Symptoms of Rinnai Error 32

As a senior engineer, I categorize the symptoms of Error 32 into three distinct operational failures. Recognizing these will help you confirm the diagnostic code is accurate to the mechanical state of the machine:

• Immediate Lockout: Upon a call for hot water (opening a tap), the digital controller immediately flashes “32” and the unit emits an audible beep. In this scenario, the burner never ignites because the PCB (Printed Circuit Board) cannot verify the safety of the output temperature.
• Thermal Fluctuations: In the early stages of thermistor degradation, you may experience “hunting,” where the water temperature swings wildly from hot to cold before the unit eventually gives up and throws the error code. This is caused by the thermistor sending erratic resistance signals to the modulating gas valve.
• System Post-Purge Cycle: You may hear the combustion fan running at high RPMs for several minutes after the error appears. This is a safety protocol where the unit attempts to shed any residual heat from the heat exchanger because it lacks the data to confirm the actual water temperature.

Technical Explanation of the Fault

The outgoing water thermistor is a Negative Temperature Coefficient (NTC) resistor. In technical terms, as the temperature of the water increases, the electrical resistance of the thermistor decreases. The Rinnai PC board sends a low-voltage reference signal through this component and measures the return voltage to calculate precise temperature. Error 32 triggers when that return signal falls outside of the programmed parameters (typically 0.3V to 4.8V).

Common Root Causes:

1. Component Aging and Resistance Drift: Over thousands of thermal cycles, the semi-conductive material inside the thermistor can degrade. This “drift” causes the sensor to report temperatures that are physically impossible, leading the PCB to reject the data and trigger a fault.
2. Scale and Mineral Encapsulation: If you live in a hard water area, calcium carbonate can accumulate on the thermistor bulb, which sits directly in the water flow. This scale acts as an insulator, slowing the response time of the sensor. If the sensor doesn’t react fast enough to a rise in burner intensity, the PCB assumes the sensor is frozen or faulty.
3. Wiring Harness Corrosion: Tankless units produce condensation. If there is a slight leak at a connection or high humidity, the Molex connectors joining the thermistor to the main wiring harness can develop oxidation. This added resistance is interpreted by the computer as a hardware failure.
4. Electrical Surges: Because the thermistor is a sensitive electronic component, a power surge can “pop” the internal resistor, leading to an immediate “open circuit” (infinite resistance) condition.

Troubleshooting & Replacement Instructions

Before proceeding, ensure you have disconnected the 120V power supply and closed the gas inlet valve. Safety is paramount when working with gas-fired appliances.

1. Accessing the Internal Components: Use your Phillips #2 screwdriver to remove the four screws securing the front access panel. Carefully lift the panel up and off the hooks. Locate the “Outgoing Water Thermistor”—it is typically found on the copper piping near the bottom right of the heat exchanger, where the hot water exits the unit. It will have two wires (usually white or grey) leading to a plastic connector.
2. Visual and Mechanical Inspection: Check the wiring for any signs of singeing, rodent damage, or moisture. Disconnect the plastic Molex connector. Inspect the pins for green or white oxidation. If oxidation is present, cleaning the contacts with electronic cleaner may resolve the issue without part replacement.
3. The Multimeter Resistance Test: Set your multimeter to the Ohms (Ω) setting (20k scale). Insert your probes into the terminals of the thermistor side of the plug. At room temperature (approx. 68°F / 20°C), you should see a reading between 11.5kΩ and 15kΩ. If the meter reads “OL” (Open Loop) or “0.00,” the thermistor is definitively dead and must be replaced.
4. Draining the Unit (Crucial Step): You cannot remove the thermistor without draining the unit, as it is a “wet” sensor. Close the cold water inlet valve. Open the hot water drain port (or a nearby tap) to relieve pressure. Use a bucket to catch the roughly half-gallon of water remaining in the heat exchanger.
5. Removing the Thermistor: The thermistor is held in place by a metal clip or a single screw. Carefully remove the fastener. Pull the thermistor straight out of the brass housing. Be prepared for a small amount of residual water. Ensure the old O-ring comes out with the sensor; if it stays stuck inside, use your needle-nose pliers to remove it.
6. Installation of the New Sensor: Lubricate the O-ring on the new thermistor with a tiny amount of silicone grease (do not use petroleum jelly). Press it firmly into the port, re-secure the clip or screw, and reconnect the wiring harness.
7. Re-pressurizing and Testing: Close the drain ports and slowly open the cold water inlet. Check for leaks around the new sensor. Plug the unit back in and restore gas. Fire a high-demand fixture (like a bathtub) to ensure the unit modulates through its temperature range without triggering Error 32.

How to Prevent Error 32

While electronic components eventually fail, you can significantly extend the lifespan of your Rinnai’s sensors by following these engineering best practices:

• Annual Descaling (Flushing): Mineral buildup is the primary enemy of “wet” sensors. By flushing your unit with food-grade white vinegar or a professional descaling solution once a year, you prevent the calcification that causes thermistor lag and overheating.
• Install a Dedicated Surge Protector: The control board and sensors in a Rinnai unit are sensitive to “dirty” power. A high-quality surge protector designed for appliances will prevent voltage spikes from frying the delicate resistors inside the thermistors.
• Maintain Internal Dryness: Check your venting for proper slope. If condensation from the exhaust pipe drips back into the cabinet, it will eventually find its way into the thermistor’s electrical connections. Ensure the cabinet remains bone-dry.

FAQ

Frequently Asked Questions

Can I bypass the thermistor to get hot water temporarily?
Absolutely not. The thermistor is a critical safety component. Bypassing it would allow the burner to fire without any temperature regulation, potentially leading to a catastrophic heat exchanger failure or scalding water temperatures. The PCB is designed to detect “jumpered” wires and will refuse to fire.

Does Error 32 mean my Heat Exchanger is cracked?
Not necessarily. While a leak from the heat exchanger could drip onto the thermistor and cause it to short out, Error 32 itself is specifically an electrical circuit code for the sensor. If the unit is dry inside, the heat exchanger is likely fine, and you are simply dealing with a failed $50 sensor.

Is the “Inlet” thermistor the same as the “Outgoing” thermistor?
On most Rinnai models, the physical sensors are very similar, but they often have different wire lengths or connector shapes. You must ensure you purchase the “Outgoing” (Hot Water) thermistor, as its resistance curve is calibrated for the higher temperature ranges seen at the exit of the unit.