My heater isn't get hot new heater core all knobs
are working correctly but even the heater core isn't
getting hot what could be the problem
If the heater core is not getting hot then coolant isn't reaching it. It may be a faulty heater control valve (a solenoid activated switch on the heater hose, see: http://www.autozone.com/cooling-heating-and-climate-control/heater-control-valve/chevrolet/suburban-1500), or it may be restricted coolant flow in the engine/radiator (caused by a weak water pump for instance). I would check the heater control valve first.
Search Maintenance & Repair Questions
Maintenance & Repair Experts