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Error code reference

Rheem Water Heater Error Code 31

Faulty inlet thermistor.

Rheem codesRheem water heater codes

Faulty inlet thermistor.

First step from the service manual

Contact a dealer or qualified service technician.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

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What 31 means on a Rheem water heater

Error code 31 on a Rheem water heater indicates a fault with the inlet thermistor, which is a temperature-sensing resistor located at the cold water inlet of the unit. The thermistor continuously monitors incoming water temperature and sends resistance-based signals to the control board. As water temperature changes, the thermistor's resistance value shifts in a predictable curve, allowing the control board to regulate heating accurately. When the control board receives a resistance reading outside the expected range, either too high, too low, or erratic, it flags code 31 and typically suspends normal operation to prevent unsafe heating conditions.

The inlet thermistor is a critical component in tankless and hybrid water heater systems because the control board uses inlet temperature data to calculate the required energy output to reach the target outlet temperature. A faulty or disconnected thermistor breaks this feedback loop entirely. Common failure modes include thermistor degradation over time, corrosion at the connector pins, a short circuit in the wiring harness, or physical damage from scale buildup or handling.

Because diagnosing and replacing the inlet thermistor involves accessing internal components and verifying resistance values against the unit's temperature-resistance specifications, Rheem recommends contacting a qualified service technician for this repair. Attempting the repair without proper tools and reference data can lead to misdiagnosis or further damage to the control board or associated wiring.

Source: manufacturer service documentation.

Common causes of 31

  1. 01

    Failed or degraded inlet thermistor

    Part replacement

    The thermistor itself can fail over time due to heat cycling, age, or internal degradation, causing its resistance output to fall outside the acceptable range for the control board. This is the most common root cause of code 31.

  2. 02

    Corroded or loose wiring connector

    Wiring / connection

    Corrosion or a loose connection at the thermistor's wiring harness connector can interrupt the signal to the control board, mimicking a thermistor failure. Inspecting the connector for oxidation, moisture intrusion, or a poor seating can reveal this issue.

  3. 03

    Damaged wiring harness

    Part replacement

    A short circuit, pinched wire, or broken conductor in the harness running from the inlet thermistor to the control board can produce an out-of-range resistance reading. Visual inspection of the harness routing may reveal damage.

  4. 04

    Scale or debris buildup on the thermistor

    Wiring / connection

    In areas with hard water, mineral scale can accumulate on the thermistor body, affecting its thermal contact with the water and causing inaccurate resistance readings. This is more likely in units that have not been flushed regularly.

  5. 05

    Faulty control board

    Part replacement

    If the thermistor and all associated wiring test within specification, a defective control board that cannot properly interpret the thermistor signal may be generating a false code 31. This is the least common cause and is typically diagnosed after ruling out all other components.

See the test procedure for each cause

Frequently asked questions about 31

What does 31 mean on a Rheem water_heater?
Code 31 on a Rheem water heater indicates a fault with the inlet thermistor, the sensor that measures the temperature of cold water entering the unit. The control board has detected a resistance value from the thermistor that falls outside the expected range. This can be caused by a failed thermistor, damaged wiring, or a corroded connector. The unit will not operate normally until the issue is resolved.
Can I fix error code 31 on my Rheem water heater myself?
Rheem advises contacting a dealer or qualified service technician to address code 31. Diagnosing the inlet thermistor requires checking resistance values against specific temperature-resistance specifications and accessing internal components safely. A knowledgeable technician can differentiate between a bad thermistor, wiring fault, or control board issue. Attempting the repair without the proper reference data and tools risks misdiagnosis or additional damage.
Is it safe to use my Rheem water heater while code 31 is active?
Operating the unit with an active code 31 is not recommended. Without accurate inlet temperature data, the control board cannot properly regulate the heating cycle, which may lead to inconsistent water temperatures or unsafe operating conditions. It is best to shut the unit down and have it inspected before resuming use.
Will resetting my Rheem water heater clear error code 31?
A reset may temporarily clear the code display, but if the inlet thermistor or its associated wiring is genuinely faulty, the code will return once the unit attempts to operate and reads the out-of-range signal again. A reset does not fix the underlying hardware problem and is not a substitute for proper diagnosis and repair.
How much does it cost to repair error code 31 on a Rheem water heater?
Repair costs vary depending on the specific cause. If only the inlet thermistor needs replacement, parts and labor typically range from roughly 100 to 300 dollars. If the wiring harness or control board is also involved, costs can be higher, potentially ranging from 200 to 600 dollars or more depending on parts pricing and local labor rates. Getting a diagnostic assessment from a qualified technician will provide a more accurate estimate for your specific unit.

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