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No Heat At Idle? Top 5 causes

Heat then no heat… aargh!! Flip-flopping car heater makes driving to work, especially in heavy traffic, a miserable experience. Your car has a ton of heat available; we just need to unlock it. I’m John Cunningham experienced mechanic; you are in the right place; we’ll take care of this heating problem right now.

Five common causes of no heat from the car heater at idle:

  1. Low Coolant level
  2. Air in the system
  3. Stuck open thermostat
  4. Partially blocked heater core
  5. Faulty heater valve

By the end of this post, you’ll learn how a heater system works, why your car has no heat at idle, how to diagnose it, and what you can do to fix it. You’ll also learn a ton of coolant system maintenance pro tips.

No Heat at Idle

System Overview

Your car’s heating system, also known as HVAC (Heating Ventilation Air Conditioning), doesn’t actually create any heat. Instead, it scavenges heat from the engine’s coolant system. And so, more often than not, the root cause of heating system issues is rooted in the coolant system.

Heating system

At its most basic, the heating system diverts some hot coolant from the engine into a heater core behind the dashboard. A heater fan blows across the heater core sending the available heat into the cabin.

Engine coolant system

The whole system is housed inside a heater matrix that employs various passageways, blend doors, sensors, and stepper motors to direct airflow to different cabin areas. For example, air may be directed to the windshield, face, or feet of passengers or any combination thereof.

The whole system is managed by a control panel on the dashboard, which is also a control module (computer) that communicates with other control modules in the car over a communication network. Your heating system is quite a sophisticated system.

Coolant system

As said, we can’t fully diagnose the heating issue without also examining the engine’s cooling system. Very often, paying attention to seemingly trivial problems such as poor cabin heat can save you some large bills later. Problems with the heating systems and, as you already know, likely coolant related. Coolant systems must work efficiently, or engine failure is a real risk.

1 Low Coolant Level

Low coolant is number one for a good reason; it’s the most common root cause of heater problems. Proper coolant fluid level is important, as low coolant creates a void inside the system, and the void creates air pockets. More on this later.

Blocked car rad

The engine core is the source of all heat and is the area most at risk if the system runs low on fluid. Your heater system is least likely to be damaged by a lack of coolant and therefore is designed to allow heater core coolant to fill the engine jackets during a low coolant level event. And that’s why it’s important to investigate a lack of heat from the heater; it’s a warning sign.


Using your senses works great; listen to your heater. Can you hear coolant sloshing around behind the dashboard? That’s a good indication your car is low on coolant. Engine, is the temperature higher than normal?

Car coolant

For most cars, using a little coolant is normal, but if you are consistently topping up the reservoir, you’ve likely got one of two problems. Your engine has a coolant leak or a failing head gasket.

Common places coolant systems leak include:

  • Heater core – Check the carpet in the front passenger area for dampness and a sweet smell.
  • Pipe fittings – Check hoses for splits and loose or faulty clamps.
  • Water Pump – Check for a leak at the water pump or high-pitched squeal.
  • Rad – Check the rad for damp patches
  • Rad cap – Check for staining around the rad cap area, indicating a worn cap seal.

A faulty head gasket is common and has some symptoms depending on where it fails. Here are a few of the tell tails.

Head gasket failure chart
  • White smoke – White smoke at the tailpipe is a sign the coolant is leaking into the combustion chamber.
  • Head leak – Check for signs of a coolant leak around the cylinder head, which is a sign of a faulty head gasket.
  • Oil level – Coolant in the oil is a sign of a faulty head gasket

How To Fix it:

For most, topping up the system solves the problem. If you suspect a more serious problem, check for exhaust gases in the coolant system or run a leak-down test. I’ve listed a chemical coolant system test kit here on the Coolant system tools page.

I covered checking for a head-gasket fault in this post, “Better to replace head-gasket or engine”.

2 Air Locked Coolant System

Low coolant causes air locking. Air locking simply means the coolant system contains pockets of air that prevent the coolant from moving inside the system. Air locking happens for a few reasons, common among them including:

  • Bad rad cap
  • Incorrectly bled system after repair work
  • Leaking hose clamp
  • Leaking hose
  • Split rad
  • Split reservoir tank
  • Head gasket failure


Low heat from the heater and hearing coolant splashing around inside the system guarantee the heater core is air locked. Having heat only when you are moving is another major indicator of air locking.

A coolant system test kit is used to pressurize the complete system. If it holds pressure, it’s okay. If, on the other hand, pressure leaks (often you’ll see and hear where it leaks from), you’ll obviously know you found a problem. The kit includes an adaptor to test the Coolant cap (often the failure); replacing the coolant cap is a common cause of coolant loss and air locking and is the easiest repair of all.

The test is simple, as is the kit; I’ve listed it here on the Coolant system tools page.

How To Fix it:

An air-locked system needs to be bled, but bleeding won’t help if there’s an underlying issue, such as a leak.

Bleeding the coolant system:

Coolant hose

Most systems use an expansion tank setup. Begin by parking the car on a slight incline so that the front of the car is uphill. This helps trapped air reach the rad or reservoir where it’s released.

Engine cold before proceeding; hot coolant will spray from rad or reservoir cap if opened when hot. Check engine temperature throughout this procedure; shut off the vehicle if the engine gets too hot. Using a no-spill funnel makes filling and bleeding a ton easier; you can check one out here on the Coolant system tools page.

  1. Open coolant bleed screw if fitted.
  2. Remove the reservoir cap and top up the reservoir tank.
  3. Fit the bleed screw.
  4. Squeezing the hoses helps move trapped air through the system.
  5. Fit the reservoir cap.
  6. Run the engine with the heater set to hot and fan speed at high.
  7. Check and top up the reservoir again, if needed.
  8. Run the engine at 2000 rpm for 5 – 10 minutes to help move trapped air.
  9. Vehicle temperature gauge indicating normal, verify heater is hot.
  10. Check coolant again, allow the engine cool before topping up.
  11. Test drive vehicle and park on level ground
  12. Idle engine until rad fan operates.
  13. Finally check the coolant level after the system has cooled.

3 Stuck Open Thermostat

Car Thermostat

The coolant moves around the system in a circuit, and the thermostat is fitted inside the circuit and its function is to restrict the flow of cold coolant to the radiator.

This is important as a car engine needs to reach operating temperature quickly. It won’t be efficient, and none of the vehicle’s emission systems will function until the engine is hot.

Restricting the flow of coolant causes the coolant around the engine to heat up quickly, and as the heater core is plumbed on the hot side of the thermostat, the cabin heat is available quickly too.

Coolant system info graphic

The thermostat is a clever wax-filled spring-loaded assembly. As the coolant temperature rises, the wax inside the stat melts and expands, causing the valve to open, and as the temperature cools, the valve closes with the help of spring tension. This is a continuous process and is proportional to temperature. Typical stats begin to open at 180° F.

Stats suffer from two problems – 1 Stuck closed, and 2 stuck open. A stuck open stat will cause cabin heating issues as the vehicle will take forever to warm up.


Start and run your engine from cold; after just 10 minutes, shut her down and grab both top and bottom hoses. If the stat operates correctly, one will be warm and the other cold. It’s not important which, some cars fit the stat in the top hose and others the bottom.

If you find both hoses are equally hot, you have confirmed your stat is stuck open. As the engine warms up, it is normal for both hoses to be warm, but in this in-between stage – they shouldn’t be.

How To Fix it:

Removing and replacing the stat isn’t a difficult or expensive job, but be careful to bleed the air from the system; failure to do so can cause a blown head gasket or engine damage.

4 Blocked Heater Core

Heater core

Your heater core is a little rad fitted behind the dashboard. The orifices within it are small and block easily. Debris caused by old coolant is usually the main cause of heater core blockage.

Fresh coolant contains additives that help lube and protects the system from corrosion. The problem with old coolant (over three years) is that it turns acidic, which eats metal components, gaskets, and rubber seals but also promotes rust, which blocks heater cores.


Car heater hoses

With the cabin heat turned on and the engine at operating temperature. Idle for ten minutes before shutting off the engine.

Locate the heater core hoses; they’re fitted on the firewall behind the engine. Grab both hoses; both should be roughly equally warm; if not, the heater core is blocked.

How To Fix it:

Heater core firewall pipes

Back-flush with a specially formulated cleaning agent helps break down the blockage. It is possible to do it DIY style with a garden hose and some buckets. However, a trip to the shop is a wise investment; removing a heater core on most vehicles requires dash removal, and that’s just not fun.

Check out Prestone flush on the Coolant tools page. It’s specially formulated to help clear blockages and clean coolant system internals without damaging gaskets and seals.

5 Faulty Heater Valve

The heater valve’s function is to open and close to allow coolant to flow into the core. The valve remains closed, blocking hot coolant to the cabin when the cabin temperature dial is set to cold. Not all HVAC systems do it this way, so your vehicle may not have a heater valve fitted.

If fitted, it is generally located on the heater hose plumbing right by the firewall; just follow the heater hose back toward the engine. The valve may be operated by old-fashioned cable, vacuum, or electronically.


With the cabin heat turned on and the engine at operating temperature, idle the engine for ten minutes before shutting off the motor. Locate the heater valve, likely fitted on the firewall behind the engine.

Grab the corresponding hose on either side of the valve. Is it equally hot? Try the second pair of hoses. If the hoses are hot on the engine side and cold or just warm on the heater side, it suggests a faulty valve.

How To Fix it:

If your valve is cable operated, check manually that it’s opening all the way and adjust as necessary.

If the valve is vacuum operated, check for split hoses or sticking valves, apply manual vacuum, and check operation.

If electrically operated, back-probe the connection and check the voltage when the heater is in operation (12v normal). Check resistance across the valve solenoid; an “OL” indicates a faulty valve.

Coolant System Maintenance Pro Tips

Your coolant system doesn’t require a ton of maintenance, but if you do these, you’ll reduce the possibility of system failure greatly.

  • Top up with mixed coolant not straight water
  • Check antifreeze before winter
  • Replace water pump every 90k
  • Replace the thermostat every 90k miles
  • Replace drive belt every 90k miles
  • Change the coolant every 3 years
  • Perform coolant back-flush every 6 years
  • Replace rad cap every 6 years
  • Clean debris (bugs + dust) from rad exterior every year

Having a workshop manual for your vehicle is always a good plan; they only cost a few dollars but will save you a packet. A good manual will cover the coolant system in detail, including the electrical wiring diagram, hose routing, system overview, troubleshooting section, fastener torque specs, etc., and all mission-critical info.

Related Questions

Is the car engine not warming up? A stuck open thermostat will cause extended warm-up time and poor cabin heat. It will also affect gas mileage, and long-term effects include cylinder wall washing oil contamination.