Insufficient, Erratic, or No Heat
- Low coolant level.
- Incorrect thermostat.
- Restricted coolant flow through heater core.
- Heater hoses plugged.
- Misadjusted control cable.
- Sticking heater control valve.
- Vacuum hose leaking.
- Vacuum hose blocked.
- Vacuum motors inoperative.
- Blocked air inlet.
- Inoperative heater blower motor.
- Oil residue on heater core fins.
- Dirt on heater core fins.