My hot water heater is an American brand Powerflex. I have plenty of hot water and my pilot light is on but when the unit tries to heat up after a shower or something, the burner will kick on for about a minute or more then make a loud clicking sound and kick off. It will then immediately turn back on and then click off again several times before clicking off altogether. The pilot light stays on the whole time. We cleaned the air tube out on the power vent, cleaned the metal tube on the vent line, vacuumed real good around and in everything but it still did it. Here's the kicker... when we took the lid off the power vent unit itself, it worked perfectly. Any ideas? I'm thinking some kind of sensor?? As soon as you put the lid back on it acts up again.