" wrote:

My water heater burner won't stay lit. Here's what I know:

1. If I light the pilot, it can stay on indefinitely. So I think the
thermocouple is actually functioning.
2. If I then turn on the burner, after a few seconds there will be a
*click*, and the whole thing shuts down. This takes less than a
minute.

Gas water heaters built in the past 5 years or so in the US are required to have
sealed burners. Since you can't have a flame without oxygen, they have what's
called a flame arrestor, which is a honeycomb like filter that allows air to
pass through, but won't allow a flame to go the other direction. Flame arrestors
are notoriously susceptable to plugging up with dust, so the burner also has to
be protected with an oxygen sensor.

It sounds like either your oxygen sensor is bad, or more likely, that there
isn't enough air getting to the burner to support a full on flame.