* Blend Door Actuator: This is a small motor that controls the mix of hot and cold air. If it's malfunctioning, it might be stuck in the "cold" position even though the coolant is hot. This is a very common problem in older vehicles.
* Low Refrigerant: Even though you're experiencing a heating issue, low refrigerant in the AC system can affect the blend door's ability to properly mix hot and cold air. A leak somewhere in the AC system is possible. A mechanic will need to check refrigerant levels and pressure.
* Heater Core Restriction: While you checked for flow, a partial blockage within the heater core itself is possible. It might be partially clogged, restricting hot coolant flow even though some gets through.
* Stuck Blend Door: The blend door itself (the physical door that mixes air) could be mechanically stuck in the cold position. This wouldn't necessarily be detected by simply checking for hot coolant flow through the hoses.
* Faulty Blower Motor Resistor: While this usually causes varying fan speeds or a complete fan failure, a problematic resistor could indirectly affect the temperature control in some unusual circumstances. It's less likely, but worth considering.
* Wiring/Vacuum Leaks: Malfunctioning wiring or a vacuum leak affecting the HVAC system's controls could also be the cause. These are harder to diagnose.
Troubleshooting Steps (in order of easiest to hardest):
1. Check the Blend Door Actuator: This is often the easiest and cheapest fix. Look up a video specific to your 2001 Chevy model on YouTube showing how to access and test this. Many times it can be accessed from inside the car's cabin. It often makes a clicking sound when it operates. If it's not clicking or moving, it needs to be replaced.
2. Have the AC system checked for refrigerant: This should be done by a professional. Even a small leak can have a significant effect on the system's ability to control temperature.
3. Consider a heater core flush or replacement: If the above steps don't solve the problem, the heater core might be partially blocked, despite your water hose test. This requires more intensive work.
4. Consult a repair manual specific to your 2001 Chevy model: This will give detailed diagrams and troubleshooting steps for your vehicle's HVAC system.
Don't attempt complex repairs yourself unless you are comfortable working on vehicles. Taking it to a qualified mechanic is the safest and most efficient way to resolve the issue. The mechanic can use diagnostic tools and their experience to pinpoint the exact cause more quickly and accurately than someone trying to troubleshoot based on general advice.
