I have a Lennox CH23-51-1 and the fan will shut off for about 3 seconds then comes right back on. It does this whether the thermostat is on or off, I've even completely removed the thermostat and it doesn't turn off. When I put the fan on constant on, it still turns off for 3 seconds, then comes back on. It does this every 3-4 minutes.
I had a ac repair man here today and he was in the attic and I turned off the disconnect and when I turned it back on, it didn't come on at all, so I turned it off and back on and it came on. He suggested the disconnect was bad, so I replaced it. He also said there was a high temp reset switch that will cause it to run non stop that had tripped and he reset it. None of this had any effect. The LED's on the unit are blinking fast and alternating (3 blinks per sec). The sticker on the machine says that means power is below 75v. Also, when the unit turns off, the LED's go dark, too.