Why is the heater not working in my Series 60 Detroit engine?

The heater in your Series 60 Detroit engine may not be working due to several potential issues. First, check for coolant flow; a clogged heater core or low coolant levels can prevent proper heating. Additionally, ensure that the heater control valve is functioning correctly and that there are no blown fuses or electrical issues affecting the heater's operation. If these components are in working order, further diagnostics may be needed to identify any underlying problems.