I have a Dayton olutdoor patio heater (box shaped, hung under eave). When I turn it on it may or may not ignite. If it ignites, it only burns for a short time then goes out. Can you help me?

I suspect, that since it's an outdoor, eave mount patio heater, that it may have an insect infestation.
Insects love to get into these heaters when they are not in operation and build their nests, Spiders are a big offender. In which case, the heater may just need a thorough cleaning. Particularly, in and around any electrical connects inside the heater. Especially, the infrared bulb connections. The other issue may be that the infrared bulb may be close to it's life span and needs to be replaced. Before you attempt any cleaning or troubleshooting, make sure the circuit breaker that feeds this heater is turned OFF.

Hope this helped to solve the problem.