Rinnai Tankless Error Code 73: What a Burner Sensor Circuit Fault Means for East Cooper Homes
If your Rinnai tankless is flashing a 73 and the hot water quit on you in Charleston, Daniel Island, Mount Pleasant, or anywhere else in East Cooper, here's the straight story. Code 73 isn't something you'll fix with a broom or a garden hose, but it's worth knowing what it means before you call anybody.
I'm Christian with Avery & Sons Plumbing + Tankless. Here's what Code 73 is, what usually causes it, what you can safely check, and when it's time for a pro.
What Code 73 actually means
Rinnai lists Code 73 as a burner sensor circuit fault. The burner sensor is a thermocouple that sits at the burner and helps the heater's control board keep tabs on what's happening during combustion. When the board sees a problem in that sensor's circuit, it throws a 73 and stops the heater rather than run without good information.
A few things to know:
It's on non-SENSEI models. Rinnai's current troubleshooting manual lists Code 73 under its non-SENSEI series. Rinnai's older service manual notes it applies only to certain models, specifically the VB and condensing indoor units. If you've got a newer SENSEI heater, you're less likely to ever see this one.
It's not the same as Code 31. Rinnai has a separate burner sensor code, 31, which deals with the sensor's readings. Code 73 points at the circuit itself, meaning the wiring, the connections, or the parts on either end of it.
What usually causes it
Rinnai's troubleshooting steps for Code 73 point to these causes, roughly in the order a tech checks them:
Wiring or connections. Loose, broken, or damaged wiring between the burner thermocouple and the PC board (the heater's control board).
Altitude settings. The DIP switches on the PC board need to be set for the altitude where the heater is installed. Around here we're at sea level, so this mostly matters if someone set it up wrong or swapped a board.
Gas pressure settings. The low-fire and high-fire manifold gas pressures need to match factory specs. That's gas work, and Rinnai says it's for someone qualified to service gas systems.
The thermocouple itself. If everything above checks out, Rinnai says to replace the burner thermocouple.
The PC board. If the code comes back after a new thermocouple, the board is next.
What you can safely do yourself
Write the code down. Note "73" and anything else you've seen on the controller lately. That saves time on the service call.
Try one reset. Turn the heater off and back on at the controller, or cut the power at the breaker for a minute. If the 73 comes right back, stop resetting it. Repeated resets won't fix a circuit fault.
Make sure gas and power are on. Check that your other gas appliances are working and the heater's breaker hasn't tripped. That rules out the simple stuff.
Leave the inside alone. Everything on Rinnai's list for Code 73 happens behind the front panel: wiring, DIP switches, gas pressures, and parts. That's not homeowner territory, especially on a gas appliance.
And as always, if you ever smell gas, get out of the house and call your gas utility or 911 from outside.
When to call a pro
Call if the 73 comes back after one reset, if you're out of hot water, or if you've had other codes show up recently. A good tech will open the heater up, check the thermocouple wiring and connections back to the board, confirm the DIP switches and gas pressures, and only then start replacing parts. The order matters, because a loose connector is a lot cheaper than a new control board.
A local note
The Lowcountry is rough on electrical connections. Salt air and humidity work on terminals over time, which is a big reason we look hard at wiring and connectors first, especially on heaters in garages and on exterior walls near the water on Daniel Island, Mount Pleasant, Sullivan's Island, and the Isle of Palms. A lot of the older non-SENSEI Rinnais around Charleston are getting some age on them now, too, and that's when circuit codes like this tend to show up.
Soft next step
If your Rinnai is showing Code 73, it's telling you something in the burner sensor circuit needs a look. Avery & Sons Plumbing + Tankless works on these heaters all the time. Call (843) 305-5555 or visit averysplumbing.com. We'll find out what's actually wrong and tell you straight what it needs and what it doesn't.
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