Short answer: Your car heater usually stops blowing warm air because something’s off with coolant flow, temperature control, or the fan that pushes air through the vents. Fixing it can be as simple as topping up coolant—or as complex as replacing a stubborn blend-door motor. Below, we break down every culprit, show you quick checks you can safely do at home, and explain when it’s time for a car aircon service.
How Does a Heater in a Car Work?
The heater is a mini radiator that sits inside the dashboard. Hot engine coolant flows through its core; a fan (blower motor) pushes cabin air across the hot fins; and a series of doors channel that warm air to your vents. When any link in that chain fails—coolant, heater core, thermostat, blower, doors, or the electrics that run them—you get an icy blast instead of cosy comfort.
Quick DIY Car Heater Checks
- Temperature gauge reading low? Could be a stuck-open thermostat.
- Coolant level low? Top it up (with the correct mix) only when the engine is cold.
- Blower fan silent? Dial through all fan speeds. No noise = fan or resistor fault.
- Mist on windscreen with sweet smell? Possible heater-core leak.
- Dual-zone climate: If one side is hot and the other cold, suspect a blend-door motor.
Why Your Car Heater Stops Working
1. Low or Contaminated Coolant
Heaters rely on a steady flow of hot coolant. Low coolant levels (often from small leaks) or sludge-filled coolant can’t transfer enough heat. Check the overflow bottle markings and look for rusty, brown, or milky fluid.
2. Faulty Thermostat
The thermostat regulates engine temperature. If it sticks open, the engine runs too cool and the heater never gets above lukewarm. A stuck-closed thermostat, on the other hand, causes overheating and can damage the heater core.
3. Blocked or Leaking Heater Core
Old coolant, tap water, or stop-leak additives can clog the tiny passages inside the heater core. Leaks leave tell-tale damp carpet or foggy cabin windows. A core flush sometimes saves it; severe cases need replacement.
4. Airlocks in the Cooling System
Air trapped during a hurried coolant top-up blocks flow. You end up with a gurgling noise behind the dash and an unpredictable cabin temperature. A proper bleed with the engine at operating temp usually sorts it.
5. Blower Fan or Resistor Failure
No fan, no airflow. If the blower works only on its highest speed, the resistor pack is cooked. If there’s no action at any speed, suspect a blown fuse, bad relay, or a worn-out motor.
6. Blend-Door or Climate-Control Faults
Modern HVAC systems use small electric motors to swing blend doors that mix hot and cold air. When a motor fails—or its plastic gear strips—you get the wrong temperature no matter what you dial in. Climate-control panels themselves can fail too.
7. Electrical and Sensor Gremlins
Temperature sensors, wiring faults, or a faulty body control module can send mixed messages to the HVAC system. Dual-trade auto electricians (like our team at Voltaic) use scan tools to pinpoint these glitches quickly.
Troubleshooting Tips You Can Do at Home
- Check coolant level in the overflow bottle with the engine stone-cold.
- Inspect the radiator cap seal for cracks—poor sealing drops system pressure.
- Listen for the blower at each fan speed; note any squeals or intermittent stops.
- Feel both heater-core hoses with the engine warm—both should be hot. One cold hose screams blockage.
- Look for damp foot-well carpet or a sticky film on the inside of the windscreen.
If anything above looks off—or you simply don’t have time—our auto-diagnostics team can run a full cooling-system pressure test and scan for fault codes while you enjoy a coffee in the waiting room.
When to Call a Car Heating Technician
DIY coolant flushes or thermostat swaps are doable if you’re mechanically minded. But once you’re wrestling with blend doors buried deep in the dash, electronic climate control faults, or recurring coolant loss, professional help saves hours—and your sanity.
At Voltaic, we combine 20+ years of hands-on experience with factory-grade scan tools. We’ll pinpoint the failure, whether it’s mechanical or electrical, and get your heater blowing toasty in one visit. Plus, our $199 air-conditioning service covers the other half of your climate system, so you stay comfy year-round.
Preventive Maintenance Tips
- Replace coolant every two years with the right spec—avoid tap water top-ups.
- Have the cooling system pressure-tested at each service to catch small leaks early.
- Run the heater for five minutes every month, even in summer, to keep doors moving and seals lubricated.
- Book an annual HVAC check; our techs inspect hoses, belts, and cabin filters in one hit.
- Address engine overheating immediately—excess heat is heater-core kryptonite.
FAQs
Why does my heater blow cold at idle but warm up when driving?
Low coolant flow or a weak water pump lets coolant cool down at idle. Once engine speed rises, flow improves and the heater core warms up.
Can low battery voltage affect my heater?
Yes. Modern climate systems rely on sensors and electric motors. Under-voltage from a tired battery can limit blower speed and door movement. Our battery experts can test and replace weak batteries on the spot.
Is a heater problem an MOT (road-worthy) fail?
In WA a non-functional heater isn’t a direct fail, but fogged windows from a leaking core or fan failure can impair visibility—and that’s a defect.
Does heater failure mean my engine is about to overheat?
Not always, but the two are linked. A cooling-system issue that starves the heater core can eventually overheat the engine, so don’t ignore it.
How much does heater repair cost?
Minor fixes such as thermostat or fuse replacement usually start around $150–$250. Heater-core replacement can run into four figures because of the labour to remove the dash. We’ll quote upfront before any work begins.
Contact a Car Heating Expert
If your heater is blowing colder than a Fremantle sea breeze in July, don’t tough it out. Book online or call our Welshpool or Wangara workshops to book your car aircon service or car diagnostics test, and we’ll get you back to toasty drives with reliable, expert service.