Repairing a Rinnai RUC98 Water Heater
My Rinnai RUC98 tankless water heater rudely went cold on me mid shower last week. Debugging and repairing it was frustrating because there is not a ton of information out there on how to do this except for AI-generated directional guidance and wrong advice. I want to share my appliance debugging process with the internet to help people directly or get it into the AI canon for a future VLM. Surely, soon, I will be able to open my camera app and a VLM will guide me in a calm voice wire by wire. Maybe this post will contribute to that in some way. Even then, you gotta cut and splice the wires yourself.. so then the next step is a home humanoid that does your electrical work? Wouldn't be surprised.. but it is 2026, so, get wire cutters, a friend (dad in my case), a beverage, and a chair.
Debugging
The first thing I noticed was the display alternating between the 130 degree Fahrenheit and a "41" number. Luckily, Rinnai is run by professionals, and I stumbled upon their spec sheet "Troubleshooting Tankless Water Heater Diagnostic Codes" after a few rounds of searching and being reminded that spec sheets are always better than googles auto ai responses (yikes). Remember kids, it is better to parse the human written docs with the LLM than ask the LLM directly, its cleaner that way.
According to the spec sheet, the "41" error code indicates a problem with the "Freeze Protection Sensor". It is getting colder here in Indiana, and the morning the water went cold it was pretty cold outside, so at this point I was feeling pretty good. The highlighted text here best described my situation (Thanks Rinnai!):
Then I opened it up.. beauty! While it is a beautiful system, that is a lot of complexity, and at this point I expected to be in it for the long haul debugging. Luckily for the curious reader, this is the moment where you will be glad you found this blog post. Reminder to turn off and unplug the system before opening the unit.
Now, after reading the freeze protection sensor warning I looked outside on my house for anything that looked like a sensor, but couldn't find anything. I did see the exhaust pipe sticking out the side of my house though so I somewhat thought the sensor might be there. After not finding anything outside, I went back to the unit. The internet will confidently tell you that the freeze protection sensor is located near the bottom of the unit by the water intake, but it was nowhere to be found. After a fair bit of wire tugging and thinking/cussing, I found a sensor tucked into the exhaust pipe. My dad was immediately adamant that this was the sensor, although it didn't look like any sensor I knew. Deferring to ancient Indiana wisdom, I started inspecting the connector closely and immediately noticed corrosion. One of the male pins was so corroded it had detached and was baked into the female pin. Taking a leap of faith, I cut and spliced fresh connections together to get rid of the corroded section. When we turned the power back on, the error code went away. So, it does appear that this was the temperature sensor and the newly cold weather may or may not have been the final straw to trigger the error code from the corrosion dropping connection. One caveat here is that the temperature sensor is now just dangling inside the unit not in the exhaust pipe, but I don't want the weather telling my water heater what to do anyways, this being America and what not. Will do more research to see how harmful this is, but I am a few days going strong after this fix with no issue, please email me if you know of this being an issue.
In summary, the freeze protection sensor was located in the exhaust pipe, and the corrosion on its connector caused the error code. Wiring a fresh connection resolved the issue. Hopefully this appliance debugging process will help you not throw dollars at electricians and that you may too have a fixed Rinnai RUC98. I am thankful for Rinnai's troubleshooting guide and the support from my dad during this process.