1. Cooling System Issues:
* Low Coolant: Check your coolant level in the overflow reservoir. Low coolant prevents proper heat transfer. Top it off with the correct 50/50 mix of coolant and distilled water if necessary. If it's consistently low, you might have a leak somewhere in the system.
* Air in the Cooling System: Air pockets can prevent coolant from circulating efficiently. Properly bleeding the cooling system is crucial. This process varies slightly depending on the engine, so consult your owner's manual or look up a video specific to your 2000 Sierra's engine (likely a 4.3L, 4.8L, 5.3L, or 6.0L Vortec).
* Water Pump: A failing water pump won't circulate coolant effectively. Check for leaks around the pump and listen for unusual noises (whining or grinding) while the engine is running.
* Radiator: A clogged radiator restricts coolant flow. You might need to flush the radiator or replace it if it's severely clogged.
* Hoses: Check all hoses for cracks, kinks, or collapses. These can restrict flow.
2. Heater Core Issues:
* Clogged Heater Core: This is a common cause of poor heat. The heater core is a small radiator inside the cab that warms the air. It can become clogged with debris or rust, reducing its efficiency. This often requires replacing the heater core, a relatively involved job.
* Heater Core Leaks: A leaking heater core can lead to coolant loss and poor heating, or even a foggy interior. Look for wet spots under the carpet on the passenger side. If found, replacing the heater core is required.
* Heater Control Valve: This valve controls the flow of coolant to the heater core. If it's stuck closed or malfunctioning, you won't get heat. Testing it requires checking for coolant flow when the heater is turned on.
3. Other Potential Issues:
* Blend Door Actuator: This controls the air flow between hot and cold air. If it malfunctions, you may only get cold air, even if the heater core is working.
* Wiring/Electrical Issues: Check the fuses and relays related to the heater system. A faulty blower motor resistor can also cause issues.
* Thermostat (Again): While you've replaced it, it's possible you got a faulty replacement. Although unlikely if you did not have issues immediately following the replacement.
Troubleshooting Steps:
1. Check Coolant Level: The easiest and most important first step.
2. Bleed the Cooling System: If coolant is low or you've recently worked on the system.
3. Feel the Heater Hoses: With the engine warmed up, feel the hoses leading to and from the heater core. They should both be hot. If one is cold, there's a restriction in the system.
4. Inspect the Heater Core: Look for leaks under the carpet or near the firewall on the passenger side.
If you're not comfortable tackling these repairs yourself, it's best to take your Sierra to a qualified mechanic for diagnosis and repair. They have the tools and expertise to pinpoint the exact cause of your heating problem. Be sure to mention you've already replaced the thermostat.
