Rinnai code 33: heat exchanger sensor fault

Code 33 is a close relative of code 32. Both are temperature sensor faults; code 33 is the sensor on the heat exchanger rather than on the outgoing water line.

Rinnai makes several generations of tankless heaters (SENSEI, SENSEI RX/CX and older non-SENSEI models). The meaning below comes from Rinnai's own diagnostic code guide; the exact service steps differ slightly between generations.

What causes code 33

• Scale or deposits on the sensor that give false readings
• A loose, corroded or damaged wiring connection to the control board
• A sensor that has simply failed

What a technician does

1. Inspects the wiring harness from the sensor back to the board.
2. Turns off the water supply, drains the unit and removes the sensor.
3. Cleans off any scale and measures the sensor's resistance, which should fall as it warms up.
4. Reinstalls it with the O-ring in place and checks for leaks. If the code returns, the sensor is replaced.

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 comes back, the cause is still there. Resetting over and over is not a repair.

Is it a DIY job?

Removing a sensor means draining a pressurized water heater and working inside the cabinet near the gas and electrical parts. It is best left to a qualified technician. What you can do is keep up with annual maintenance, including cleaning the inlet filter and scheduling a descale in hard water areas.

See the full list on our Rinnai error codes page.