My T/W/H is about two years old, and suddenly began receiving fault code # E 35, indicating irregular gas pressure. I called the tech. folks and they asked for me to confirm with a manometer that my pressure was perfect for the unit, and I determined it was. I again called the great folks at Navien, and they said it was most likely a faulty gas sensor. They asked for my address, and two days later I had a replacement sent to me via UPS. This is a write up to show the steps necessary to replace this sensor. Here is the tankless water heater in question.