Carrier Furnace Troubleshooting: 8 Checks Before You Call

The short answer

Troubleshooting a Carrier furnace in Northern Virginia starts with the code light, not the power switch. Count the flashes first, then work through the checks Carrier’s owner’s manuals give you: the thermostat, the mode, the power, the gas, the gas valve switch, the rollout switch and the vent. If all of those check out and there’s still no heat, that’s when you call.

How a Carrier furnace starts

Knowing the order helps you see where it stops. On a call for heat, the inducer motor starts and pulls air through the furnace. The pressure switch proves that draft. The igniter heats for about 20 seconds. The gas valve opens and the burners light. Then, after up to about a minute, the blower starts and warm air comes out the vents. When it stops partway, the light tells you which step.

Step 1: Read the code before you touch anything

Carrier’s manual says to record the code before you take off the blower door or cut the power. You can see the light through the view port in the blower access panel, the lower door, without opening anything.

Look through the small window in the lower door. Short flashes are the first digit and long flashes are the second, so 31 is 3 short and 1 long, then a pause before it repeats. A steady light is normal, and rapid flashing means the line voltage polarity is reversed. On older boards with a red light, cutting the power erases the code, so count it before you flip the switch. Newer Carrier boards keep the last 7 faults through a power cut and print a code with a third digit, like 31.4, on a small display. Our full list of Carrier furnace codes has every one.

The checks, in order

1. The thermostat is set warmer than the room. Turn it up a few degrees so there’s no doubt.
2. The thermostat is on HEAT. Not Cool, not Off.
3. The furnace has power. Check the breaker and any fuses, and the furnace power switch, which looks like a light switch on or near the furnace.
4. The gas is on. The manual shutoff on the gas pipe to the furnace is open when its handle runs in line with the pipe.
5. The gas valve switch is ON. It’s on the gas valve inside the furnace. Turn the furnace power off before you touch it.
6. The rollout switch hasn’t tripped. It’s a small manual-reset button on the burner box. Carrier’s owner’s manual allows one push to reset it. If it trips again, shut the furnace off and call.
7. The vent is clear. Outside, the vent and intake pipes are free of snow, ice, leaves and nests.
8. Still nothing? Call. Past this point, the parts that fail are inside the furnace.

It runs, but the house won’t warm up

If the furnace runs but can’t keep up, or starts and stops over and over, check airflow first. A clogged filter or blocked return grilles let the furnace overheat, and a safety limit shuts the burners off. Our filter guide and short cycling guide go deeper.

Water around a high-efficiency furnace

A condensing Carrier makes water as it heats, and that water leaves through a drain hose, often to a small pump. A plugged drain or a pump that quit stops the furnace with a pressure switch code. Our Carrier pressure switch guide and condensate drain guide cover it.

The blower won’t stop

If the blower runs all the time, check the fan setting on the thermostat. ON runs it nonstop; AUTO runs it only when the furnace is heating. If it’s on AUTO and the blower still won’t stop, that’s a call.

When to stop and call

• You smell gas. If you smell gas, don’t flip any switches: leave the house and call your gas company or 911 from outside.
• A rollout switch trips a second time.
• The same code comes back after one reset.
• The flame looks yellow and lazy, lifts off the burners, or you see soot.
• Your carbon monoxide alarm goes off.

Never jump or bypass a safety switch to get heat back. Each one is there to stop the furnace from burning gas when something is wrong, and the wrong thing is often exhaust, which carries carbon monoxide.

What it costs to have us look

A repair visit starts with a flat $99 diagnostic, and you get the price of the fix before we touch anything. Our Carrier repair page lists what the common repairs cost. Our heating check is $67 per system, through December 31: furnace, heat pump or boiler.

Related reading

• Every Carrier furnace code and what to do
• Furnace won’t turn on
• How to reset a Carrier furnace
• The Carrier flame sensor
• Carrier code 31

Every check here comes from Carrier’s own documents: condensing gas furnace owner’s manual and 58MCA, 58MTA, 58MVP and 58MXA user’s manual.

Call or book

Call (703) 997-9757 and tell us what you’re seeing, or book online from goswiftpro.com/cookiesoffer and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.

Questions people ask

Why won’t my Carrier furnace turn on?

Start with the code light, then the thermostat setting and mode, the breaker and the furnace switch, the gas shutoff and the gas valve switch, the rollout switch and the vent. If all of those check out, it’s a part inside the furnace, and that’s a call.

Where is the reset button on a Carrier furnace?

There isn’t a main one. You reset the board by turning the furnace switch off for 30 seconds. The rollout switch has its own manual reset; Carrier allows one push, and a second trip is a call.

How do I read the light on my Carrier furnace?

Look through the small window in the lower door. Short flashes are the first digit and long flashes the second, then a pause. Write it down before you cut the power.

Why is my Carrier furnace blowing cold air?

Check that the thermostat is on HEAT and the fan is on AUTO. On ON, the blower runs between heat cycles and the air feels cool. If the burners aren’t lighting at all, the code light tells you why.

How much does it cost to have you look at it?

A repair visit starts with a flat $99 diagnostic, and you get the price of the fix before any work starts. Our heating check is $67 per system, through December 31.