Symptom guide
Heater Blowing Cold Air
What it means
Heat comes from hot coolant flowing through a small radiator (heater core) tucked inside the dash. Hot air over the core is blown into the cabin. Anything that stops coolant from reaching the core, or stops the core from passing air through, kills cabin heat.
Common causes
1. Low coolant (~30%). Air pocket at the heater core. Clue: overflow tank below MIN.
2. Stuck-open thermostat (~25%). Engine never reaches operating temp. Clue: temp gauge sits below normal at highway speed.
3. Clogged heater core (~20%). Rust or sealant deposits. Clue: one heater hose hot, the other cold.
4. Blend door actuator stuck (~10%). Clicking sound behind the dash. Clue: no temperature change between max heat and max cold.
5. Failed water control valve (~10%). On some Ford / GM / VW designs. Clue: hose to the heater core stays cool.
6. Failed water pump (~5%). Clue: engine also overheats.
How to diagnose
- Check coolant level after engine has cooled.
- Check the temp gauge after 10 min of driving.
- With engine warm and heat on max, touch both heater hoses by the firewall. Both should be hot.
- Listen behind the dash for clicking when sliding the temp knob.
Fixes
| Fix | Cost (USD) |
|---|---|
| Top up coolant | $5–$15 |
| Burp / bleed cooling system | $0–$60 |
| Replace thermostat | $20–$60 + 30 min |
| Flush heater core | $20 (DIY) – $200 (shop) |
| Replace heater core | $200–$800 part + 5–15 hr labor |
| Replace blend door actuator | $30–$150 + 1–3 hr |