Air is a very important part of combustion and without it you have none.
There is as you can see from this video many ways to block or keep the air from entering the combustion chamber.
Something in the fan or a bad fan is the easiest fix on a code 10 but after that is in my opinion one of the hardest to fix especially on a interior unit.

If you have a question or would like a Rinnai guy stickle please E-mail me at [email protected]
I hope you enjoy this video,
Mike

PLEASE USE A LICENSEB PLUMBER TRAINED IN LEVEL 3 RINNAI TRAINING.
FOLLOW ALL NATIONAL AND LOCAL PLUMBING AND GAS CODES AND TO FILE A PERMIT WITH YOUR LOCAL BUILDING DEPARTMENT.