Rinnai code 51: inlet water temperature sensor fault

To reach your set temperature quickly, a tankless heater needs to know how cold the incoming water is. The inlet water thermistor provides that reading. Code 51 means the reading is not valid.

The meaning and service steps below come from Rinnai's own diagnostic code guide for its tankless water heaters.

Common causes

• Scale on the sensor, especially with hard water
• Loose, broken or damaged wiring back to the control board
• A failed sensor

What a technician does

1. Checks the wiring harness.
2. Shuts off the water supply and drains the unit.
3. Removes the sensor, cleans off any buildup and tests its resistance, which should drop as it warms.
4. Reinstalls it or replaces it, then checks for leaks.

How to reset it

1. Close all hot water taps.
2. Turn the water heater off at the temperature controller, wait about a minute, then turn it back on.
3. Open a hot tap and watch whether the code returns.

If the same code keeps coming back, the cause is still there. Repeated resets are not a repair.

Codes 32, 33 and 51 are all temperature sensor faults. If you have seen more than one of them, scale is a likely common cause.

Before you call

Have the model and serial number from the rating label ready, along with the exact code and when it appears: at startup, during use, or only at high demand. Mention any recent work on the gas, water or electrical supply and when the heater was last descaled. This helps the technician bring the right parts and often saves a second visit.

See every code on our Rinnai error codes page.