Rinnai SENSEI RX Code LC
Scale build-up lockout. The heater has detected lime scale in the heat exchanger and is counting down to protecting itself. Rinnai displays this as LC, as LC0 through LC9, or as 00 when you read it back in the maintenance code history. The digit is a counter: each temporary reset buys 70 hours of operation, and LC2 means the unit has been reset for three of those 70-hour periods without an actual flush.
Also displayed as: 00, LC0, LC1, LC2, LC3, LC4, LC5, LC6, LC7, LC8, LC9
DIY fixThe verdict
Descale the unit
Flush the heat exchanger. This is the single most common tankless fault that is genuinely a homeowner job, and on this generation the unit will lock out until it is done. The temporary reset exists so you are not without hot water — it is not a fix, and Rinnai is clear that damage from scale is not covered by the warranty.
Typical cost: $25–$300
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. 67), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Flush the heat exchanger with four gallons of undiluted white vinegar — see our descaling guide for the full SENSEI RX procedure, which uses the unit's own dES descaling mode.
- If you need hot water before you can flush it, press the On/Off button on the controller five times within five seconds. That buys 70 hours of operation and increments the LC counter.
- After a successful flush in dES mode the LC code clears itself. If it does not, the hard reset is pressing the A and B buttons on the PC board together for five seconds.
- If your water is hard, treat it. Rinnai considers a softener or conditioner required above roughly 12 grains per gallon.
Frequently asked questions
What is the difference between LC, LC1, LC5 and 00?
They are the same fault. LC on its own is the scale lockout; the digit counts how many 70-hour temporary resets have been used without a real flush; and 00 is how the same event is written in the maintenance code history.
Can I keep using the five-press reset instead of flushing?
You can, for 70 hours at a time, and the counter keeps climbing. What you are doing is running a heat exchanger that Rinnai has already detected scale in — and heat exchanger damage from scale is excluded from the warranty.
Does the LC code clear itself after a flush?
On a SENSEI RX, yes, if the flush is done properly in the unit's dES descaling mode. If it does not clear, Rinnai's hard reset is the A and B buttons on the PC board held together for five seconds.
How often should a SENSEI RX be flushed?
Rinnai's own table ties it to hardness: nothing required on soft water, once a year for residential installs on moderately hard water and up, and twice a year for commercial. On very hard water, treatment matters more than frequency.