I am not all that familiar with the model you have stated but the description of the problem you are having was the same I had with what I beleive is a similar model to yours, after a whole lot of mucking about trying to identify the fault it turned out to be a combination of two things – one being a faulty pressure switch from the fan to the controller switch and the other was the circuit board.
Rheem should be able to send you or your service person a manual on the fault finding tips to follow for this model.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians