Car heater blowing warm, but not hot air?

Chevrolet Silverado 2500 blows warn air on both drivers and passenger side.But this air is not very hot.Previously it was very hot air.
In this case first inspect the Antifreeze level.If its low,then get it refilled to required level.
Other possibility can be faulty thermostat,there are two check both thermostats.
But if this possibilities are already been inspected or replaced.
Then try this,Take the heater hoses off at the firewall, use a pressure washer to flush both ways to the heater core to remove any coating on the inside left by the dexcool.

But if still the problem is same,then heater core is faulty and needs to be replaced.

This will help.Thanks.

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