* Low Coolant: This is the *most common* and easiest thing to check first. Open the coolant reservoir (when the engine is cool!) and check the coolant level. If it's low, you've found your problem – you need to add coolant (a 50/50 mix of coolant and distilled water). A low coolant level prevents proper heat transfer. However, a leak is likely the *cause* of the low level, so you need to find and fix that.
* Thermostat Stuck Closed: The thermostat regulates coolant flow. If it's stuck closed, coolant won't circulate through the heater core, preventing heat from reaching the cabin. A faulty thermostat will also prevent the engine from reaching operating temperature, explaining the cold temperature gauge.
* Water Pump Failure: The water pump circulates coolant throughout the engine and heater core. If it's failing, coolant won't circulate properly, resulting in no heat and a cold gauge. Listen for unusual noises from the water pump (whining or squealing).
* Heater Core Issues: The heater core is a small radiator inside the cabin that warms the air. It could be clogged, leaking (leading to a sweet smell in the cabin), or have a restricted flow.
* Bleeding the Cooling System: Air pockets in the cooling system can prevent proper coolant flow. The system might need to be bled (removing trapped air) to ensure proper circulation.
* Faulty Temperature Sensor: The sensor that feeds the temperature gauge is separate from the system that controls the heater. A faulty sensor will give a false reading on the gauge, but might not directly cause the heating problem. However, it's worth investigating if the other problems are ruled out.
Troubleshooting Steps:
1. Check Coolant Level: This is your absolute first step.
2. Inspect for Leaks: Look carefully around the radiator, hoses, water pump, and engine block for any signs of coolant leaks.
3. Check Thermostat: This often requires removing it. If it's stuck closed, it will not open when immersed in hot water. Replacing it is relatively inexpensive.
4. Listen for Water Pump Noise: A failing water pump usually makes noise.
5. Check Heater Core Hoses: Feel the hoses leading to and from the heater core (under the dashboard). If one is significantly colder than the other when the engine is warm, that points to a problem with the heater core or its circulation. (Caution: Hot hoses!)
6. Bleed the Cooling System (If necessary): Consult your owner's manual for instructions.
If you're not comfortable working on your vehicle yourself, take it to a qualified mechanic for diagnosis and repair. Ignoring a cooling system problem can lead to serious engine damage.
