Cooling System Issues (Most Likely):
* Low Coolant: This is the simplest and most common cause. Check your coolant level in the overflow reservoir (when the engine is cool!). If it's low, you have a leak somewhere in the system (radiator, hoses, water pump, etc.). Low coolant prevents proper heat transfer, leading to overheating and no heat in the cabin.
* Water Pump Failure: The water pump circulates coolant throughout the engine and heater core. A failing water pump won't circulate coolant effectively, causing overheating and no heat. Listen for unusual noises (whining, grinding) from the pump area.
* Thermostat Stuck Closed: The thermostat regulates coolant flow. If it's stuck closed, coolant won't circulate properly, leading to overheating and no heat. A stuck-open thermostat *could* cause the engine to run cool, but wouldn't cause a no-heat problem.
* Radiator Issues: A clogged radiator or damaged radiator fins prevent efficient heat dissipation, leading to overheating. A leak in the radiator would also cause low coolant.
* Hoses Clogged or Damaged: Blocked or cracked hoses prevent coolant flow, leading to both overheating and lack of heat.
* Head Gasket Failure: A blown head gasket allows coolant to mix with engine oil or enter the combustion chamber. This would likely show up as white smoke from the exhaust, milky oil, and potentially low coolant. This is a serious problem.
Heater Core Issues (Specific to Heat):
* Clogged Heater Core: The heater core is a small radiator inside the car that warms the air. If it's clogged with debris, it won't heat the air properly.
* Heater Core Leak: A leak in the heater core would lead to coolant leaking into the passenger compartment (you'll likely smell sweet coolant). This would also cause low coolant and eventually overheating.
* Heater Control Valve Problem: This valve controls coolant flow to the heater core. If it's stuck closed, no hot coolant will reach the heater core.
Other Less Likely Possibilities:
* Air in the Cooling System: Air pockets can prevent proper coolant flow. This is often resolved by bleeding the cooling system.
* Faulty Cooling Fan: The cooling fan helps dissipate heat, especially when idling or at low speeds. A malfunctioning fan can lead to overheating.
Troubleshooting Steps:
1. Check Coolant Level: This is the FIRST thing to do.
2. Inspect Hoses and Radiator: Look for leaks, cracks, or bulges.
3. Feel the Radiator and Upper/Lower Radiator Hoses: Are they hot? If not, the thermostat might be stuck closed or there's a major circulation problem.
4. Listen for Unusual Noises: Pay attention to sounds from the water pump area.
5. Check for Leaks: Look under the car for any signs of coolant leaks.
6. Check Engine Oil: Is it milky or frothy? (Indicates a potential head gasket issue)
If you're not comfortable working on your car, it's best to take it to a mechanic for diagnosis and repair. Ignoring overheating can cause serious and expensive engine damage.
