The 59 code is typically caused by insufficient gas supply to the unit. It can also be caused by obstructed exhaust flow, stuck exhaust damper (located directly on top of the unit's exhaust), insufficient makeup air, or improperly installed venting (i.e. too long or too many elbows).
I would recommend initially checking the incoming gas pressure at the gas connection on the unit. The gas pressure should not drop below 5" water column for a natural gas unit, 8" for a propane system (the unit need to be operating at its maximum output).
59 is a combustion abnormality check the flue for a blockage
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