Rinnai Error Code 52 — Outlet Water Temperature Sensor Fault
Error Code 52 on Rinnai tankless water heaters indicates a fault with the outlet water temperature sensor (also called the outgoing hot water thermistor). The sensor has failed or is reading an out-of-range value, causing the unit to shut down to prevent scalding or heat exchanger damage.
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Role of the Outlet Temperature Sensor
The outlet temperature sensor monitors the temperature of the water leaving the unit. This is the primary feedback for the modulating burner — if the sensor fails, the unit cannot regulate water temperature and shuts down safely with Error 52.
Rinnai Temperature Sensor Codes
| Code | Sensor | Location |
|---|---|---|
| 52 | Outlet water temperature sensor | Outgoing hot water side |
| 53 | Inlet water temperature sensor | Incoming cold water side |
| 61 | Gas combustion fault | Not a sensor — burner issue |
Common Causes {#most-likely-cause}
| Cause | Likelihood |
|---|---|
| Failed outlet temperature sensor | Very High |
| Corroded or loose sensor connector | High |
| Damaged sensor wire (water damage, pinching) | Medium |
| Control board analog input failure | Low |
Step-by-Step Diagnosis {#diagnosis}
Step 1 — Locate the outlet temperature sensor
- The sensor is mounted on or near the outgoing water connection on the heat exchanger
- It is a small thermistor in a metal probe or clip
- Visually inspect for obvious damage, corrosion, or disconnected wires
Step 2 — Check the wiring connector
- Disconnect and re-seat the sensor connector at the control board
- Inspect for corrosion from condensate or water infiltration
- Clean contacts with electrical contact cleaner
Step 3 — Measure sensor resistance Rinnai outlet temperature sensors are NTC thermistors:
- At 77°F (25°C): typically 10K–15K ohms (model-specific)
- Infinite resistance: sensor is open — replace
- Near-zero resistance: sensor is shorted — replace
- Resistance should decrease as temperature increases
Step 4 — Compare to a reference
- Take the sensor resistance reading and compare to the service manual resistance table
- Hold the sensor in your hand (roughly body temperature, ~98°F) and confirm resistance drops accordingly — confirms the sensor tracks temperature
Step 5 — Check the control board
- If the sensor resistance is correct but Error 52 persists, the board analog input may have failed
- This is uncommon — try a known-good replacement sensor first
Replacement Parts {#parts}
| Part | Notes |
|---|---|
| Outlet temperature sensor | Amazon | Rinnai part 100001223 or model-specific equivalent |
| Inlet temperature sensor (if confused with 53) | Amazon | Rinnai part 100001222 |
| Control board | Amazon | Last resort — verify sensor and wiring first |
Reset Procedure
- Replace the sensor
- Reconnect all wiring
- Press ON/OFF to power cycle the unit
- Error 52 should clear on startup
- Open a hot water fixture and verify stable outlet temperature
Note: Rinnai Error 52 and Error 53 are often confused. Error 52 = outlet (hot water leaving the unit); Error 53 = inlet (cold water entering the unit). Both are fixed the same way — replace the failed sensor.