Overview: Central air conditioner failures and solutions
When a central A/C unit fails
during a heat spell, you may
have to wait days for a technician
to show up, and you'll probably pay at least several hundred for the repair.
But if you're comfortable
working around electricity
and are willing to spend
about $50 on parts, you can
probably repair you air conditioner yourself
in about two hours and
save about $225 on parts
markup and labor.
We talked to local HVAC technicians to get their best
do-it-yourself A/C repair and
maintenance tips. These tips will
help you with the most
common “low cooling” and
“no cooling” problems. You'll
need an inexpensive multimeter,
a voltage sniffer, an
assortment of screwdrivers
and a socket set.
If these fixes don't work, at least you've
covered the most common failures, and
your service guy can concentrate on finding
the more elusive problem. Plus, with
the new parts, you'll likely add years of
breakdown-free air conditioning. Here's how
Make sure the problem isn't the furnace
Set your thermostat to A/C mode and
lower the temperature setting. If the furnace
fan kicks in, the problem isn't in the
furnace. If the fan doesn't run, try resetting
the furnace circuit breaker. If the fan
still won't start, call a pro—the fixes
shown here won't work.
Next, check the outside condensing
unit. The compressor (which sounds like
a refrigerator) and fan should be running.
If not, follow the troubleshooting and
repair procedures shown here.
Turn off the A/C and furnace
breakers in the main electrical panel before
pulling the outdoor disconnect or removing
the condensing unit's access panel. Then
use a voltage tester on the wires coming
into the contactor to make sure the power
is really off.
The contactor (relay) and start/run capacitor(s) (see illustration
below) fail most often and are inexpensive. So it's a safe bet to
buy and install those parts right away, especially if your A/C
unit is older than five years. The condenser fan motor can also
fail, but it runs about $150—hold off buying that unless you're
sure that's the culprit.
To buy replacement parts, find the nameplate on the condensing
unit (not your furnace). Jot down the make, model and
serial number (or take a photo). Get the parts at an appliance
store, furnace dealer or online.
Central air conditioner components
Anatomy of a Central Air Conditioner
Central A/C systems consist of two major components:
a condensing unit that sits outside your house, and the
evaporator coil (often referred to as an A-coil) that sits
in the plenum of your furnace or air handler. The refrigerant
in the A-coil picks up the heat from your home and
moves it to the outdoor condensing unit. The condensing
unit fan blows outside air through the condensing
coil to remove the heat. The condensing unit houses the
three parts replaceable by a DIYer: the contactor, the
start/run capacitor(s) and the condenser fan motor. The
condensing unit also houses the compressor, but only a
pro can replace that. The A-coil has no parts that can be
serviced by a DIYer.
Start with the easy fixes
1 of 2
Photo 1: Shut off the power
Open the electrical box next to the condensing unit and
pull the disconnect block straight out. Check inside the box
with a voltage sniffer to make sure the power is really off.
2 of 2
Photo 2: Clean the condenser coils
Aim your garden nozzle upward into the top of the
condenser coil to remove the crud buildup under the lid.
Work all the way around the coil. Then aim the nozzle down
and flush the debris down the coil fins. Adjust the nozzle to a
gentler stream and shoot water directly into the coils to flush
out any remaining debris.
If you're getting little or no cold air, check these three things
first. Make sure all the registers in the house are wide open.
Then be sure the furnace filter is clean. Then go outside and
clean off the condenser coils (Photo 2). If several registers were
closed or the filter was clogged, the reduced airflow could have
caused the evaporator coil to ice up and stop cooling your
home. If you've changed the filter and opened all the registers
and you're still not getting airflow at the registers, deice the
A-coil. Move the thermostat mode switch from “Cooling” to
“Off” and move the fan switch from “Auto” to “On.” Let the
blower run for at least 30 minutes or until there's good airflow
at the registers. Then turn the A/C back on to test it. If it works for the next 12 hours, you've solved the problem.
If the condenser coils are clogged, the compressor can overheat
and shut down. You'll experience intermittent periods of
minimal cooling, followed by no cooling. Even if you're “sure”
the condenser coils are clean, clean them again. Turn off the
power. Flip the A/C and furnace circuit breakers in your main
electrical panel to the “Off” position. Next, turn off the power
switch right at the furnace or air handler. Then yank the disconnect
block (Photo 1) and clean the condenser coils (Photo 2).
If the A/C still doesn't work properly after you've cleaned the
condenser coils, installed a new filter and opened all the
supply vents, proceed with the following repairs.
Test the fuses
1 of 1
Photo 3: Check the fuses in the disconnnect block
Set your multimeter to the lowest Ohms scale and touch the red and black leads to opposite ends of each fuse. If you get a numerical reading, the fuse is good. But a zero, a minus symbol or an infinity symbol (∞) indicates a blown fuse.
Many disconnect blocks contain two cartridge fuses. Check
them before you proceed with repairs (Photo 3). A blown fuse is a
sign of a failing part inside the condensing unit. So don't just
replace it and think you've solved the problem. Instead, replace
the parts we show here. Then install new fuses and fire up the
unit. If it blows again, call a pro—you've got more serious issues.
Inspect the inside of the access panel
1 of 1
Photo 4: Discharging a dual start/run capacitor
Remove the capacitor from the retaining bracket. Then
touch an insulated screwdriver between the HERM (or “H”)
terminal and the COMMON (or “C”) terminal. Do the same
between the FAN (or “F”) terminal and the “C” terminal. On
single-mode capacitors, just make a short between the two
Follow the electrical conduit from the house—that's where
you'll find the access panel. With the power off, remove and
store the access-panel retaining screws and remove the panel.
Before you replace any parts, check for rodents' nests or evidence
of chewing on wires and electrical connectors.
If you find broken wires or chewed insulation and can safely
handle electrical repairs, discharge the capacitor first (Photo 4).
Then repair the wires and clean out the nest. Otherwise, call a pro.
Replace the start/run capacitor(s)
1 of 1
Photo 5: Install the new capacitor
Slide the new capacitor into the retaining bracket and
tighten the bracket screw. Secure the wires with a zip tie.
All A/C units have at least one capacitor. The capacitor stores
electricity and releases it during compressor and condenser
fan startup to give both motors an extra jolt of power. And it
smooths out voltage fluctuations to protect the compressor
and condenser fan motor from damage.
Capacitors can degrade slowly, providing less startup power
over time. Or they can fail in an instant. Gradual capacitor failure
can go unnoticed for a long time, stressing the compressor
and condenser fan motor windings, resulting in their early failure.
Since capacitors are cheap, it pays to
proactively replace yours about every five years.
Replacing a capacitor is easy. Just take a photo of the wires
before disconnecting anything (you may need a reference
later on). Then discharge the stored energy in the old capacitor
(Photo 4). Use needle-nose pliers to pluck one wire at a
time from the old capacitor and snap it onto the corresponding
tab of the new capacitor. The female crimp connectors
should snap tightly onto the capacitor tabs. Wiggle each connector
to see if it's tight. If it's not, remove the connector and
bend the rounded edges of it so it makes a tighter fit on the
tab. When you've swapped all the wires, secure the new
capacitor (Photo 5).
WARNING: Discharge the capacitor before disconnecting wires or removing it from its bracket.
Replace the contactor
1 of 1
Photo 6: Swap out the contactor
Yank a connector off the old contactor and move it to the same location on the new part. Tighten the connectors where needed. Then secure the new contactor in the condensing unit.
A contactor is a $25 mechanical relay that uses low-voltage
power from the thermostat to switch 220-volt high-amperage
current to the compressor and condenser fan. A/C contactors can wear out and are at the top of the list of common A/C failures.
Even if your contactor is working, it pays to replace it
every five years or so. Unscrew the old contactor before removing
the wires. Then move the wires to the new unit (photo 6).
Test your repairs
1 of 1
Photo 7: Replace the fan motor
Mark the blade to show which side is up. Loosen the
fan blade setscrew and carefully pull it off the motor shaft.
Then swap in the new motor. Route the motor wires through
the old conduit and secure with zip ties where necessary.
Don't skip the zip ties or the blade could cut the wires.
Reinstall the access panel and disconnect block. Turn on the
circuit breaker and furnace switch. Then set the thermostat to
a lower temperature and wait for the A/C to start (see “Be
Patient at Startup,” below). The compressor should run and the
condenser fan should spin. If the compressor starts but the fan
doesn't, the fan motor is most likely shot. Shut off the power
and remove the screws around the condenser cover. Lift the
cover and remove the fan blade and motor (photo 7). Reinstall
the blade and secure the cover. Then repower the unit and see
if the fan starts. If it doesn't, you've given it your best shot—it's time to call a pro.
Be Patient at Startup
A/C units and thermostats have built-in delay features
when they're shut down and then repowered. The delay
can be as long as 10 minutes. And, if you've subscribed to
an energy-saving device from your local power utility, the
unit can take even longer to reset. If you've installed the
parts shown and reinstalled the disconnect block,
repowered the circuit breaker, turned on the switch at the
furnace, moved the thermostat to A/C mode and lowered
the temperature below the indoor temperature, and the
unit doesn't fire up after 30 minutes, it's time to call a pro.