Rinnai tankless water heater code 73
Your series decides which reading applies. Across the 2 Rinnai series documented here, code 73 is listed as burner thermocouple fault (1 series) and burner sensor circuit fault (1 series). Find your series in the table below, where those readings route to a professional.
What code 73 means on each Rinnai series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| Non-SENSEI | Burner thermocouple fault | Call a pro | $150–$900 |
| RH180 Hybrid | Burner sensor circuit fault | Call a pro | $150–$900 |
Each row opens the full page for that generation. We hold manuals for Non-SENSEI and RH180 Hybrid, and each one gets its own reading of code 73 — what Rinnai documents, what you can safely check yourself, and what the repair should cost.
What each manual actually says
The table above compares 2 series in four columns, which is enough to route you and not enough to explain anything. This is the Rinnai wording behind each row, one entry per manual, so you can see where the 2 readings of code 73 agree and where they part company.
Non-SENSEI — Burner thermocouple fault
A wiring or component fault at the burner thermocouple, as opposed to code 31, which is the same component reading out of range. This code exists only on the Non-SENSEI generation. Rinnai checks the harness back to the PC board and the altitude settings, confirms manifold gas pressures against factory specification for the altitude, and then replaces the thermocouple — and the PC board if the code returns.
RH180 Hybrid — Burner sensor circuit fault
The circuit between the burner sensor and the board is faulty, as distinct from the sensor being out of range. Rinnai's remedy on the RH180 is two lines: check the sensor wiring and the PCB for damage, and replace the sensor.
Does it matter which Rinnai you have?
Non-SENSEI and RH180 Hybrid all route code 73 the same way: a professional, at $150–$900. That makes it one of the Rinnai numbers you can act on before working out which unit you own.
Which Rinnai do you have?
On the 2 series that report code 73, the model number on the rating label starts C, RC, RU, RUC and RUR — match yours against the 64 listed below. You can also use the full model-number decoder.
Where this comes from
Source: Rinnai Troubleshooting Tankless Water Heater Diagnostic Codes (100000652) (p. 61), read Aug 2026 — the manufacturer's own documentation for this Rinnai range.
Source: Rinnai RH180 Operation and Installation Manual (100000319) (p. 36), read Sep 2026 — the manufacturer's own documentation for this Rinnai range.
The same number on other brands
Code 73 is not a standard across the industry — every manufacturer publishes its own table. If your unit is not a Rinnai, this number means something else entirely:
Parts, guides and symptoms behind code 73
Code 73 on these 2 Rinnai series points at 4 parts and 2 symptoms, each named by the manufacturer's own procedure for this code.