Rinnai Energysaver 1001F - Turn on, ignites, blows + powers off

We never had trouble w/ our heater - but now we are selling our condo & during the buyers inspection we learned our rinnai woudn't work. We checked it out, turned it on and it powers on - clicks to ignite, blows a little bit. Once it warms up or we turn it up (less than a minute or so), it powers down on it's own (lights turn off) just as it really gets going. It then fans out the heat until it cools down. Is it overheating? How do we fix this? We dd take off the top & sides & clean it all out. Any help is appreciated - thank you!
-Erin