* Blend Door Actuator: This is a small motor that controls the blend of hot and cold air. If it's malfunctioning, it might be stuck in the "cold" position, even when you select hot. This is a common issue in many vehicles, including Previas.
* Faulty Heater Core: The heater core is a small radiator that warms the air. If it's clogged, leaking (causing coolant loss), or otherwise damaged, it won't be able to heat the air effectively. Leaking coolant would also be indicated by low coolant level and potentially a sweet smell in the cabin.
* Coolant Level: Insufficient coolant in the system means there's not enough fluid to be heated, resulting in weak or no heat. Check the coolant level in the expansion tank (often a translucent reservoir).
* Thermostat: While less likely to result in *only* cold air (it usually leads to inconsistent or poor engine temperature), a faulty thermostat stuck in the "open" position can prevent the engine from reaching its optimal temperature, thus not providing sufficient heat to the heater core.
* Water Pump: A failing water pump won't circulate coolant properly, preventing heat from reaching the heater core. This would usually be accompanied by overheating of the engine, but a weak pump *might* only show up as poor heating.
* Vacuum Leaks: Some heating systems rely on vacuum to control the blend doors. Leaks in the vacuum lines can prevent proper operation.
Troubleshooting Steps:
1. Check the coolant level: This is the easiest and quickest check. Low coolant is a major red flag.
2. Check the coolant temperature: Ensure the engine is reaching its normal operating temperature. A slow-to-warm engine suggests a thermostat problem.
3. Inspect the heater controls: Make sure the fan is on and the temperature dial is set to hot.
4. Listen for any unusual noises: The blend door actuator might make clicking or grinding sounds if it's failing.
5. Check for leaks under the dashboard: A leaking heater core will often leave evidence of coolant drips or stains.
If you're not comfortable performing these checks yourself, take your Previa to a qualified mechanic for diagnosis and repair. They have the tools and expertise to pinpoint the exact cause of the problem. Mentioning the specific symptoms (cold air blowing when heat is selected) will greatly assist them in their diagnosis.
