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GMC Sonoma

I have a 2001 GMC Sonoma (6 cyl). Heater does not work though the hose is hot near the pump but cold at firewall. I also occasionally hear fluid behind the dash. Did replace thermostat. Do I need to replace heater core or is there a relay/servo that may not be opening to allow coolant to flow? Thanks in advance.

There may be a blockage in the line to the heater core or after it. The core itself may have a blockage in it. I assume that the coolant is at the correct level in the system. To see if the heater core is blocked you could try bypassing the heater with a connection between the ends of the two lines. If the return line is still cool then the trouble is within the supply line.

Cougar: I also occasionally hear fluid behind the dash. sounds to me like the heater core is air blocked. Did you consider that?

Yes I did JEM and was one reason I mentioned the coolant level. I suppose there could be an air bubble in the heater core.