Rinnai code 03: power interruption during bath fill

Some Rinnai systems include a bath fill controller that fills the tub to a set level and temperature. Code 03 tells you the power dropped while that was happening, so the heater stopped the fill for safety.

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

How to clear it

1. Turn off all hot water fixtures.
2. Press the ON/OFF button on the controller twice. This resets the code.
3. Check the tub level before using bath fill again.

If it keeps coming back

A single code 03 after a storm or a tripped breaker is normal. Rinnai's guide says that if the code reappears after the heater has been reset three times, the control board should be replaced. At that point, have a technician confirm the power supply to the unit is stable before a new board goes in.

Protecting the unit from power problems

• Plug the heater into a properly grounded outlet on a dedicated circuit where possible.
• Consider a surge protector rated for appliances.
• Have an electrician look at breakers that trip often.

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.