1. Check for coolant flow:
* Feel the heater hoses: With the engine warmed up and the heater fan on high, feel the two heater hoses (the ones going to the firewall near the heater core). They should both be hot. If one is cold and the other hot, there's a blockage or problem in the heater core itself. If both are cold, the problem is further upstream in the cooling system.
* Check coolant level: Make sure your coolant level is adequate in the radiator and overflow tank. Low coolant will restrict heat transfer.
* Check for leaks: Inspect the heater hoses, connections, and the heater core itself for leaks (look for wet spots or corrosion).
2. Check the blend door actuator:
* This controls the flow of air through the heater core. If it's malfunctioning, it might be directing all the air away from the heater core, even if the coolant is flowing. This usually requires accessing the control panel beneath the dashboard, potentially involving removing some trim pieces. It's sometimes possible to feel or hear this actuator functioning if you listen carefully while you adjust the temperature controls. A malfunctioning blend door will often result in inconsistent heating/cooling and might be accompanied by unusual noises from the dashboard.
3. Check the heater core itself:
* This is the most difficult to access and is generally best done if the other steps don't resolve the issue. It's located within the firewall and is often difficult to reach. A clogged or damaged heater core needs replacement.
4. Consider the water valve:
* Some vehicles have a water valve (or thermostatically controlled valve) that regulates coolant flow to the heater core. If this valve fails, coolant flow will be restricted or stopped entirely.
5. Check the temperature controls:
* Ensure your temperature control is properly set to "hot" and is functioning correctly. A faulty control or wiring could be interfering with the system.
In summary: The problem is almost certainly not a simple fuse or resistor issue given that the fan works. Focus your efforts on checking coolant flow, the blend door actuator, and the heater core/water valve. If you're uncomfortable working on these components yourself, consider taking your truck to a mechanic. They have the tools and experience to diagnose and repair the problem efficiently.
