Rinnai code 34: combustion air sensor fault

Code 34 means one of the heater's temperature sensors, called thermistors, is giving an invalid reading. Which sensor depends on your model.

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

Which sensor it is

What a technician does

1. Checks the wiring from the sensor back to the control board.
2. Removes the sensor and tests its resistance, which should drop as it warms.
3. Replaces the sensor if it does not respond correctly.

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.

If you have an RX/CX model

Because the sensor on these models relates to scale detection, ask the technician whether the heat exchanger is due for a flush. In hard water areas, plan to descale a tankless water heater about once a year.

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.