USA Expert Plumbing

Rinnai 73

Rinnai code 73: burner thermocouple fault

By USA Expert Plumbing Editorial Team. Updated . 1 min read.

Short answer

Code 73 is a burner thermocouple fault on older non-SENSEI Rinnai models. Rinnai's steps are to check the thermocouple wiring, confirm the altitude settings on the control board, confirm low and high fire manifold gas pressures, and replace the thermocouple. If the code returns after that, the control board is replaced.

In this guide
  1. What a technician checks
  2. Why altitude matters
  3. Before you call

The burner thermocouple watches burner temperature. Code 73 means its signal is outside what the control board expects.

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

Rinnai lists this code for its non-SENSEI series.

If you smell gas, do not reset the unit or touch any switches. Leave the building and call your gas utility from outside.

What a technician checks

  • The wiring from the thermocouple back to the control board
  • That the settings match the altitude where the heater is installed, per the tech sheet inside the front cover
  • That low and high fire manifold gas pressures match factory specifications for that altitude

If those are right, the thermocouple is replaced. If the code still returns, Rinnai's next step is the control board.

Why altitude matters

Air is thinner at high elevation, so gas heaters are set up differently there. A unit set for the wrong altitude can burn incorrectly and trip codes like 73.

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.

Common questions

Is Rinnai code 73 a gas leak?

No. It is a sensor fault. If you smell gas, leave the building and call your gas utility from outside.

Was this article helpful?

Other Rinnai codes

CodeMeaning
02 No burner operation during freeze protection
03 Power interruption during bath fill
05 Bypass flow control device fault
10 Air supply, exhaust blockage or condensate trap full
11 No ignition
12 Flame failure
14 Thermal fuse or overheat sensor tripped
15 Venturi control fault
16 Outgoing water temperature too high (safety shutdown)
17 Venturi blockage
19 Electrical grounding fault
21 Data transfer error after a control board replacement
25 Condensate drain clogged or condensate pump error
32 Outgoing water temperature sensor (thermistor) fault
33 Heat exchanger temperature sensor (thermistor) fault
34 Combustion air temperature sensor fault
38 Exhaust gas temperature sensor fault
41 Freeze protection sensor fault
51 Inlet water temperature sensor fault
52 Gas valve or modulating solenoid valve signal fault
54 High exhaust gas temperature
57 Burner overheat switch open
58 Secondary heat exchanger overheat
61 Combustion fan motor fault
63 Recirculation low flow
65 Water flow control device (servo) fault
70 Control board (PC board) fault
71 Gas solenoid valve circuit fault
72 Flame rod (flame sensing) fault
73 Burner thermocouple fault
79 Water leak detected inside the unit
LC Scale buildup in the heat exchanger
SS Service soon (maintenance reminder)
FF Maintenance has been performed (service history marker)

All Rinnai error codes