Burner thermocouple
A millivolt sensor that reads how hot the burner actually is. It exists only on Rinnai's older non-SENSEI generation, and its reading points at two opposite problems depending on which way it is wrong.
What it does
A thermocouple generates a small voltage when heated — no power supply, just physics. On Rinnai’s non-SENSEI units one sits at the burner and reports combustion temperature to the control board in millivolts.
Rinnai publishes the numbers, which is unusually specific and makes this one of the more diagnosable components on the machine. Normal combustion reads 20 to 27 mV. The unit sets code 31 above 30 mV or below 8 mV.
Reading the direction
This is the useful part, and it is why the code is worth understanding rather than just replacing the sensor:
- Above 30 mV suggests the unit is over-firing, or that blocked heat exchanger fins are trapping heat where it should be moving into the water. Rinnai’s checks go to manifold gas pressure first, then to inspecting the exchanger for blockage.
- Below 8 mV suggests the opposite: low gas pressure or low-BTU gas. The thermocouple is not seeing a hot enough chamber because there is not enough heat being made.
So the same code number, on the same part, sends a technician in two completely different directions. In neither case is replacing the thermocouple the first move.
Two codes, one component
Rinnai splits it. Code 31 is the reading being out of range, which points at combustion. Code 73 is a fault at the component or its wiring — the harness back to the control board, the altitude settings, and the manifold pressures, and only then the part.
Neither number exists on the SENSEI or SENSEI RX generations, which do not use a burner thermocouple this way. If you are looking at a code 31 or 73, you have an older Rinnai.
Why this is not a DIY part
The reading has to be taken while the burner is firing, which means a meter on a live gas appliance, and Rinnai’s own text carries a warning that you must be qualified to service gas systems before the pressure checks that follow. The part is cheap; the diagnosis is the job.
What a replacement usually costs
$20-90 for the thermocouple. $150-900 all-in, and the spread is honest — if the reading was high because the heat exchanger is blocked, you are paying for that instead.