C4 13 error flashing on water heater, shuts off after a minute

Our water heater was shutting off after about a minute of use. I called the Rheem support number and they told me:

1. They wouldn't support me because I used flexible pipe for the gas line -- it had to be black pipe. The fact that the manual said flexible pipe was OK for lengths under 36 inches was irrelevant to them.

2. The error code represented some sort of blockage or gas flow problem. They sent me a 10-page PDF file detailing the steps necessary to take apart the unit, remove the burner, and clean everything.

After spending about four hours disassembling the unit I came across the ODS (Oxygen Depletion Sensor) and found a cocoon of some sort inside. Some critter had crawled up there and made a nest inside the 3/8" pipe, preventing the system from properly detecting oxygen levels and causing it to shut down.

I scraped the cocoon out with a small tool, then ran a ribbon of steel wool through the pipe to clean it thorougly. After reassembly everything worked perfectly, and now we get hot water even faster than ever!

The increased performance me think that the cocoon may have been in there before we even purchased it, but might have shifted or worked loose and finally caused an error.