* Blend Door Actuator: This is the most common cause. The blend door actuator is a small motor that controls the flow of hot and cold air from your heater core and AC evaporator. If it's broken or stuck, it can't switch between hot and cold, resulting in only hot air. You might hear unusual noises (clicking, whirring) from the dashboard area.
* Blend Door Cable (if applicable): Some models use cables instead of an actuator. A broken or disconnected cable would prevent the blend door from moving correctly.
* Heater Core Stuck Open: The heater core is responsible for warming the air. If it's internally bypassing its control, it will constantly deliver hot air. This is less likely than a blend door issue, but possible.
* Thermostat Stuck Open: While less likely to cause *only* hot air (as it usually leads to overheating), a malfunctioning thermostat could contribute by always allowing coolant to flow through the heater core.
* Low Coolant: While seemingly counterintuitive, extremely low coolant can prevent the system from effectively cooling, leading to hot air. Check your coolant level.
* Vacuum Leaks: Some blend door actuators are vacuum-operated. Leaks in the vacuum lines can prevent them from functioning correctly.
* Faulty Control Module/Head Unit: Although less probable, a problem with the climate control module (the part that interprets the temperature and fan speed settings) could prevent signals from reaching the blend door actuator.
Troubleshooting Steps:
1. Check Coolant Level: Make sure your coolant is at the correct level. Add coolant if needed, but only after the engine has cooled down completely.
2. Listen for Unusual Noises: Pay attention to any clicking, whirring, or grinding sounds coming from the dashboard vents area. This could indicate a faulty blend door actuator.
3. Manual Blend Door Check (Advanced, requires partial disassembly): If you're comfortable with automotive repair, you might be able to manually check the blend door's operation by accessing it (often involves removing parts of the dashboard). This will confirm if it's mechanically stuck.
4. Diagnostic Scan: A professional mechanic can use an OBD-II scanner to check for any climate control-related trouble codes. While many blend door issues don't throw codes, a scan can rule out other electronic problems.
5. Professional Inspection: If you're not comfortable tackling this yourself, take your Blazer to a qualified mechanic for diagnosis and repair. Replacing a blend door actuator is usually a fairly involved job that requires some disassembly of the dashboard.
The blend door actuator is the most likely culprit, so focusing on that first is usually a good strategy. But a thorough inspection by a professional may be necessary for a definitive diagnosis.
