Free Auto Repair Advice by Professional Mechanics
Troubleshooting
/ Car Heater Failure
Why Doesn't My Car Heater Work?
Your car's heater
is a life saver in cold weather. Before attempting to repair your car’s heater,
you should understand how your car’s heater system works. First, on this page is
an explanation of how your car’s heater system functions, further down the page
are some pointers on heater system trouble-shooting. To warm the passenger cabin
of your car, your car’s heating system makes use of excess heat from the
engine’s internal combustion process. A car’s engine, in fact, when in operation
produces so much excess heat that if the excess heat is not removed, it would
destroy the engine. It is your car’s cooling system that removes that dangerous
excess heat. Some of an engine’s excess heat is released through the exhaust.
Most of an engine’s excess heat is absorbed by a circulating liquid coolant,
which is a mix of water and antifreeze.
The heated liquid
coolant is carried from the engine through hoses to the radiator, which
transfers the heat from the coolant to the outside air. That heat transfer
lowers the temperature of the liquid coolant, which is then circulated back to
the engine to absorb excess heat again. Whereas the radiator is located at your
car’s front grill, the unit that transfers heat to the passenger cabin is
located inside the dashboard. This unit is something of a mini-radiator and is
referred to as the heater core. Heated liquid coolant circulates through tubes
in the heater core and a heater fan blowing across those tubes, as well as
through little fins encasing the tubes, directs warm air through heating vents
into your car’s passenger cabin. Because your car’s heating system works off of
its cooling system, heating system malfunctions are often caused by problems in
the cooling system.
The heater core in your car
is similar to the
radiator in the front of your car; in fact it looks like a small radiator. The
difference is the heater is mounted inside the car and air is blown through the
fins of the core. The
heater hoses transfer
engine
coolant
from the engine to the heater core, this allows the heat from the engine coolant
to be utilized and warm the passenger compartment. When a heater stops
functioning determine what type of failure has occurred to execute a repair. We
have listed the most common problems below.

Heater Core
Troubleshooting ProcedureA. Check Engine Coolant Level -
Most cars have coolant reservoirs with opaque white plastic tanks and measuring
lines allowing you to see if the coolant is at the recommended level. Coolant
expands when heated, and for that reason, most coolant reservoirs have a
Full-Cold and a Full-Hot line. If there is too little coolant in the reservoir,
you should be alerted to the possibility that there could be a leak somewhere in
your car’s cooling system. Remove the reservoir cap and add coolant to the
Full-Cold line, if a very short time after you refill the coolant reservoir the
coolant level is low, there is a leak in your car’s cooling system. Engine
coolant is used to heat the heater core and if the vehicle is low on coolant the
heater core will stop working. The heater cannot produce heat due to the lack of
heat from the engine coolant. Even if the engine is not over heating the coolant
level can still be low. Inspect the
engine coolant
level in the coolant reservoir tank; coolant level should be between the hot and
cold marks. Always
check the coolant level when the vehicle is cold, preferably over night. Check for
engine coolant
leaks and repair as needed, then refill with coolant and recheck the system.

Radiator Cap and Coolant Reservoir
B. Inspect Heater Control
Valve. The heater control valve is used to shut off hot coolant from
entering the core when the heater is in the off position. A heater control valve
can be actuated by either a
vacuum line or a cable from the
vacuum
heater
control valve to the heater core. This hose should be warm; if not the heater
control valve is stuck and needs to be replaced. After the repair has been made
refill the coolant level and recheck system.

Heater Control Valve
C. Check Vacuum Feed. Most heater
systems are operated by
engine
vacuum. This vacuum is used to actuate servos to move the temperature adjustment
doors inside the heater plenum. If engine vacuum is not present the heater will
not operate properly. To test for this condition remove the vacuum feed line from
one of the vacuum servos. With the vacuum feed line removed insert a
vacuum
test gauge. If no vacuum is present, open the hood and inspect all small
vacuum lines from the engine to the firewall and repair as needed. If a hissing
noise is present under the dash when the engine is running a vacuum leak exists
and must be repaired to restore
heater
function.
D. Trouble Shoot Heater/Air Conditioner Blower Motor. An electrical
motor is used to push air through the
heater
and
air conditioner system. If this electric motor fails it will not force air through
the system. Ground a test light
lead to a good ground source, like a metal brace under the dash, seat mount bolt
or under hood brace. Turn the key to the on position; check the fuse in the heater
or blower motor fuse panel with a test light. Then lightly touch the service port
at the top of the fuse on both sides. If the test light illuminates on both sides
of the fuse, the fuse is ok and working properly. If the fuse lights the test light
on one side it is blown and needs to be replaced.

Power Distribution Center
If you are unsure of the location of the heater fuse consult a
repair manual. If the fuse condition is ok, position the blower speed to the
highest setting. Connect the ground end of the test light to a known ground source.
Then probe both ground and power feed wires of the fan motor. The
test light should illuminate
on only one of the wires (fan motor failed). If the
test light illuminates
on both wires the ground source has shorted. If neither of the wires illuminate
the power source is shorted. To trouble shoot this style of problem a
wiring schematic is needed.
Related Subjects
- If Your Engine is Low on Coolant - Many points in your
car’s cooling system are liable to develop leaks through which coolant can
escape. To inspect for a leak in your car’s cooling system, you should
understand that the main physical components involved in the cooling system
are 1) the engine block; and then, going forwards towards the radiator 2)
the thermostat; 3) the upper and lower hoses leading respectively into and
away from the 4) radiator; 5) the radiator cap; 6) the radiator fan; 7) the
water pump and, going from the engine block towards the passenger cabin; 8)
the heater core and its associated hoses and fan. A leak in the heater core
most often results in escaped coolant on the front passenger-side floor of
the passenger cabin. Most automobile antifreezes are either green or red
and despite a sweet smell they are very toxic. Particular care must be
taken to dispose of old coolant properly, as the sweet smell can attract
children and pets, and disposing of old coolant at random contaminates the
environment. The best repair for a leaking heater core is to replace the
core. Visible signs of leakage are the most obvious indicator of heater
leaks, but you should also physically inspect the heater core for signs of
deterioration. When checking the heater hoses squeeze one hose at a time,
you should feel that the hose is soft and flexible. There should not be any
irregular bulges in the hoses and you should not feel any dry rot, which
manifests as brittleness and/or cracking. You should not feel any weak
spots in the hoses. If you detect any of these conditions, replace the
heater hoses. When replacing heater hoses measure the length and diameter of
your old heating hoses so you will know the size of the new ones. Most cars
use two different sizes of heater hoses; the standard diameters are 1/2”,
5/8”, and 3/4”. When removing the old hoses, take care not to use any
yanking or turning motion that could damage the heater core. Mechanics
often score the old hoses open with a knife and then gradually coax them
loose before removing them.
For exact specific repair procedures for any of the steps above visit our
online repair
manual page.
Troubleshooting
/ Car Heater Failure
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Brake Pads?ASK A CAR REPAIR QUESTION
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