The First Cold Morning Gives It Away
Redding doesn't get the snow Burney and Mt. Shasta do, but winter nights here run close to freezing. NOAA's climate normals for the Redding airport put the average low at 37.2°F in December and 37.7°F in January, and frost on the windshield is a regular sight on cold mornings. The first properly cold commute is when a lot of drivers find out the heater isn't pulling its weight. The fan runs, and the air coming out is barely warmer than outside.
A car heater has no heating element of its own. It borrows heat from the engine. Hot coolant runs through a small radiator behind the dash called the heater core, the blower pushes cabin air across it, and a set of doors inside the dash decides how much air goes past the core and where it comes out. When the heat quits, the cause is almost always one of three things: the coolant isn't getting hot, the hot coolant isn't reaching the heater core, or the air isn't being sent across it.
That's why a heater complaint deserves more attention than it usually gets. Several of the causes are cooling system problems first and comfort problems second.
Low Coolant
What it feels like: heat that comes and goes, heat that's fine on the freeway but fades at idle, or a gurgling sound behind the dashboard like water running through a pipe.
On many vehicles the heater core sits near the highest point of the cooling system. When coolant runs low, air collects there first, so the heater often goes cold before the temperature gauge moves at all. A heater that quits can be the earliest warning you get that the engine is short on coolant.
Coolant doesn't get used up. If it's low, it went somewhere: a weeping hose, a water pump seal, a radiator seam, the heater core itself, or internally through a head gasket. Topping it off brings the heat back for a while. Finding where it went is the actual repair, and a pressure test of the cooling system usually answers that.
Thermostat Stuck Open
What it feels like: air that's warm but never really hot, an engine that takes a long time to warm up, and heat that gets worse on the highway on a cold day. Fuel economy often slips, and eventually many vehicles set a check engine light with code P0128, which means the engine isn't reaching its normal operating temperature.
The thermostat holds coolant inside the engine until it warms up. If it sticks open, coolant flows through the radiator from the first minute and the engine runs cooler than it was designed to. The coolant never gets properly hot, so neither does the air.
Don't count on the temperature gauge to show it. On many newer vehicles the gauge is designed to sit in the middle across a wide range of actual temperatures. Live data from the engine coolant temperature sensor shows exactly how warm the engine gets and how fast, which confirms a stuck thermostat instead of guessing at one.
Heater Core: Clogged or Leaking
A heater core fails in one of two ways, and they feel very different.
Clogged: the engine is at normal temperature, the coolant is full, and the heat is still weak. The passages in a heater core are small, and years of old coolant, mixed coolant types or a bottle of stop-leak poured into the radiator can plug them. A quick check a tech makes: with the engine warm, both heater hoses at the firewall should be hot. One hot and one much cooler means coolant isn't flowing through the core.
Leaking: this is the one to take seriously. The signs are a sweet, syrupy smell inside the car, a greasy film on the inside of the windshield that smears when you wipe it, fog that comes right back when the heat is on, and damp carpet on the passenger side.
Blend Door or Actuator
What it feels like: the engine is warm and the coolant is full, but turning the temperature control barely changes anything, or the air is hot on one side of the car and cold on the other. There's often a clicking or tapping from behind the dash, especially right after startup.
On most modern vehicles the blend doors are moved by small electric motors called actuators. Their plastic gears strip, or they lose track of where they are. Sometimes the fix is a recalibration with a scan tool. Sometimes it's a new actuator, and on some vehicles that actuator is buried deep enough in the dash that the labor is most of the job. Commanding it with a scan tool and watching whether the door actually moves tells us which one you've got.
Blower Motor or Resistor
What it feels like: the air may be hot, but there isn't much of it.
The classic version is a fan that only works on the highest setting. On vehicles that use a blower resistor, the lower speeds run through the resistor and the top speed bypasses it, so high-only almost always points to the resistor or its connector. A fan that doesn't run at all could be the motor, a fuse, a relay, the wiring or the climate control module, and that calls for circuit testing, not a guess. A blower that squeals, rattles or only starts after a thump on the dash is a motor on its way out.
Weak airflow at every speed with a healthy-sounding fan is often just a cabin air filter packed solid. Check that first. This time of year it's also worth knowing that mice love to nest in the cabin air intake. More on that in rodent damage to car wiring.
When It's a Safety Problem
A cold cabin is miserable. These are actually unsafe:
- A windshield you can't clear. The defroster runs through the same system. On most vehicles, defrost mode also runs the A/C compressor to dry the air, which is why a weak A/C can make defrost slow too. If you can't keep the glass clear on a wet morning, you can't see. More in foggy windows and rainy-season driving.
- A sweet coolant smell in the cabin. That's coolant getting into the air you're breathing, almost always from a leaking heater core. Engine coolant is toxic, and the vapor leaves a film on the glass that makes fogging worse.
- The heat suddenly goes cold while the temperature gauge climbs. The engine may be losing coolant fast or overheating. Pull over and shut it off. Driving on an overheating engine is how a hose replacement turns into an engine replacement.
- Steam or wet floors inside the car. A heater core that has let go can put hot coolant into the footwell.
What We Check
When a car comes in with no heat, the diagnosis runs in roughly this order: coolant level and condition with the engine cold, a cooling system pressure test, engine temperature in live data to see whether the thermostat is doing its job, heater hose temperatures going in and out of the core, blend door operation with a scan tool, and blower circuit testing if airflow is the complaint.
Usually the cause is clear long before anyone takes the dash apart. Replacing a heater core on many vehicles means removing much of the dashboard, so you want it confirmed before anyone starts. You get a written estimate before any repair begins, and the work is backed by our 12-month/12,000-mile warranty on parts and labor.
More on our heater repair service. Heading up the hill? Read winter car prep for Redding drivers.
Heater blowing cold, or a sweet smell in the cabin? Call (530) 785-9900. 5490 Churn Creek Rd, Redding, CA 96002. Mon–Fri 8AM–5PM.