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Hello and thank you for your question,If you warm the engine up to normal operating temp and turn the heater on and then go out under the hood and place your hands on both heater hoses at the firewall , are both hoses getting nice and hot?
Luke warms means you do not have good flow through the heater core which can be caused by either the cooling system being low on coolant or the heater core is restricted. You need to start first with making sure the cooling system is full with no trapped air and if that checks okay then the next steps would be to back flush the heater core. Check this an post back if you need more help and or have more questions.