Plumbing Questions? Ask a Plumbing Expert.
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The first thing to try is to clean the filter on the cold water inlet connection at the bottom of the unit. If that doesn't work they you could have a bad flow control/sensor. The latter would require a Rinnai trained plumber to replace. You can probably clean out the filter.
Then you'll probably have to call a plumber since the remaining possibilities require some training to fix.