Rinnai SENSEI RX Code FF
Maintenance performed marker. Not a fault at all. FF is a placeholder a service provider enters into the diagnostic code history to record that maintenance or service was carried out, by pressing the UP, DOWN and ON/OFF buttons together. Seeing it in the code history means someone serviced the unit — it does not mean anything is wrong.
DIY fixThe verdict
No parts needed — reset or re-check a setting
Nothing to fix and nothing to pay for. If FF is in your code history, it is a bookmark left by whoever last serviced the heater. It is only worth a second look if you have no record of any service — that is a question for the previous owner or your installer.
Typical cost: $0–$0
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the SENSEI RX — not averaged across the brand.
Source: Rinnai Troubleshooting Tankless Water Heater Diagnostic Codes (100000652) (p. 64), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Nothing to do. FF marks service, not a fault.
- If it appears alongside a real code, deal with that code — FF is not the cause.
Frequently asked questions
How do I see the code history on a SENSEI RX?
With the heater off, press and hold the ON/OFF button for 2 seconds and then press the up arrow at the same time. The last nine maintenance codes display in sequence. Repeat to exit.
Should I enter FF myself after flushing the unit?
It is what the code is for — Rinnai describes it as a placeholder entered after performing service, using UP, DOWN and ON/OFF together. Keeping the history honest helps whoever works on it next.
Code FF on other Rinnai series
Other Rinnai series report code FF under the same fault name, but each manual is transcribed separately and they do not read alike: