* Blend Door Actuator Problems: This small motor controls the blend door, which mixes hot and cold air. If it's faulty, it might be stuck in the "cold" position, regardless of the temperature setting. This is a very common cause of this issue.
* Blend Door Cable (if applicable): Some older Mazda 626s might use a cable instead of an actuator. This cable could be broken, frayed, or disconnected, preventing proper operation of the blend door.
* Vacuum Leak: The blend door is often controlled by vacuum. A leak in the vacuum lines can prevent the door from moving correctly.
* Stuck Blend Door: The blend door itself might be physically stuck in the cold air position due to wear, debris, or damage.
* Heater Core Issues (less likely): While less common, a clogged or partially blocked heater core can restrict hot coolant flow. However, if the core was completely blocked, you'd likely get *no* heat at all, not just cold air. A leak in the heater core would cause coolant loss, so check for that as well.
* Coolant Level and Thermostat: While the engine is warm, ensure the coolant level is correct and the thermostat is opening properly. A low coolant level or a malfunctioning thermostat will reduce the amount of heat available to the heater core, but usually wouldn't result in *only* cold air.
* Wiring Issues (less likely): A problem with the wiring to the blend door actuator could prevent it from functioning correctly.
Troubleshooting Steps:
1. Check coolant level: Ensure the coolant reservoir is full.
2. Check the heater control: Try different temperature settings to see if there's any variation in air temperature. Even slight changes suggest the actuator might be working, just weakly.
3. Listen near the dashboard: With the heater on, you might hear a clicking or whirring sound from the blend door actuator. A lack of sound suggests a failure.
4. Inspect vacuum lines (if applicable): Visually check the vacuum lines for any cracks, holes, or disconnections.
5. Consult a repair manual: A repair manual specific to your Mazda 626 year and model will provide detailed diagrams and troubleshooting steps.
It's highly recommended to have a mechanic diagnose the problem, as pinpointing the exact cause can be tricky without the proper tools and experience. The blend door actuator is the most likely culprit and usually a relatively inexpensive part to replace.
