Interior heater does not work?

The issue you're experiencing with your 2001 Jeep Grand Cherokee, where the interior heater does not work despite feeling heat at the control panel, could be caused by a few different factors. One common problem in this model year is related to the blend door and the blend door actuator. The blend door is responsible for directing the flow of air from the heater core to the vents to control the temperature inside the vehicle. If the blend door is stuck or the blend door actuator is malfunctioning, it can result in a lack of hot air coming out of the vents.

Here are some steps you can take to diagnose and potentially fix the issue:

Check the Blend Door Actuator: The blend door actuator is an electric motor that controls the movement of the blend door. Listen for any clicking or grinding noises coming from the dash when you adjust the temperature settings on the control panel. If you hear unusual noises, it may indicate a faulty blend door actuator.

Inspect the Blend Door: If the blend door actuator is working correctly, check the blend door itself to ensure it moves freely and is not obstructed or broken. Sometimes, the blend door can get stuck due to debris or other issues.

Perform a Heater Core Flush: A clogged or restricted heater core can also result in poor heating performance. Consider performing a heater core flush to remove any debris or sediment that might be blocking the flow of hot coolant.

Check the Heater Control Valve: The heater control valve is responsible for regulating the flow of hot coolant to the heater core. Ensure it opens and closes correctly when you adjust the temperature settings on the control panel.

Inspect the Vacuum Lines: Some HVAC systems use vacuum-operated components to control the blend door and other functions. Check for any disconnected or damaged vacuum lines that may be affecting the heating system.

Scan for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to check for any stored diagnostic trouble codes related to the HVAC system. DTCs can provide valuable information about the specific issue.

Refer to Service Bulletins or Technical Service Manuals: As the dealer mentioned a "modification, " there might be relevant service bulletins or technical service manuals that address this issue for your specific model year. You can search for these resources online or at automotive repair information websites.

Flushing is when they force water through the system to remove any sludge or blockage in the system.
Does the engine heat up normally?

Could be the heater bypass valve is stuck or the heater core is blocked.
I'd check the heater core first.