Did your furnace shut down and not start back up?
Furnace lockouts can be a pain to deal with—especially when you don’t know what’s causing them.
In this article, I’ll go over the steps you need to take to reset your furnace. I’ll also cover what furnace lockouts are, and how to diagnose furnace issues using its control board.
How to reset your gas furnace
If your furnace is locked out and not starting up, you’ll need to reset it to get it running again.
But wait! Furnaces don’t just lock out for no reason. There’s a reason why your furnace is locked out in the first place.
That’s why it’s important to fix the issue with your furnace before resetting it.
The first step is to check your furnace’s diagnostic light so you can figure out why your furnace is locked out.
Here are the steps you should take when resetting your furnace:
- Check the diagnostic light
- Turn the furnace power off
- Relight the pilot light (if needed)
- Turn the furnace power back on
Check the diagnostic light
When resetting your furnace, the first thing that you should do is check the control board’s diagnostic light for errors.
The control board diagnostic light will indicate the current status of the furnace.
For example, if your furnace does not receive a flame sensor signal, the diagnostic light will flash in a pattern to indicate that.
Most diagnostic lights blink once every few seconds to indicate normal operation. However, you should check your furnace’s error code chart to see what your furnace’s exact status is.
If your furnace is showing an error, you should take action to correct the error before proceeding further.
There’s usually a good reason why your furnace is locked out—resetting your furnace without taking any corrective action could just repeat the lockout process over again.
One of the most common causes of furnace issues is a dirty air filter, so it’s almost always worth changing the air filter any time your furnace runs into problems.
Turn the furnace power off
After checking the diagnostic light and correcting any issues, the next step is to turn your furnace’s power off.
To turn your furnace’s power off, you need to find the toggle switch or circuit breaker that feeds power to your furnace.
Most furnaces have a small toggle switch (which looks like a light switch) near the furnace. Just flip that switch off to turn your furnace’s power off.
If you have a furnace without a toggle switch, find the circuit breaker that feeds power to your furnace and turn that off instead.
After turning your furnace’s power off, check the diagnostic light again. If the light stays off, then you know that your furnace’s power is off.
Relight the pilot light (if needed)
Older furnaces have a standing pilot light that must be on for the furnace to fire up.
If you have a newer furnace with a hot surface ignitor, it won’t have a pilot light and you can skip this step.
The pilot light is a small flame that stays on—even when your furnace is off.
The purpose of the pilot light is to provide an ignition source for the furnace’s burners.
If your furnace’s pilot light is out, then you will need to relight it before turning your furnace’s power back on.
Check out this video from AMRE Supply that goes over how to relight your furnace’s pilot light:
Turn the furnace power back on
After shutting your furnace’s power off, wait a few seconds before turning it back on again. This gives the furnace’s control board time to completely shut down and clear any errors that may have persisted.
After turning your furnace’s power back on, observe its behavior as it starts up.
Check if your furnace is going through its entire sequence of operation, or if it’s getting stuck somewhere in the process.
If your furnace is getting stuck on one of the steps while starting up, then the furnace still has a problem.
It’s also possible that your furnace starts up completely, but eventually shuts down and locks out after running for a while.
If your furnace keeps shutting down, check out my article below for more help:
How to reset an oil furnace
The furnace reset button on an oil furnace is usually located somewhere in the blower compartment.
Turn off the power to your furnace before resetting it.
If you have an oil furnace, do not press the reset button more than once. Pressing the reset button on an oil furnace multiple times will create a fire hazard.
If your oil furnace keeps needing to be reset, then it has a serious issue. Immediately call an HVAC professional to take a look at your furnace.
What is a furnace lockout?
A furnace lockout is a complete shutdown of the furnace. A furnace lockout is caused by unsafe conditions inside the furnace detected by its sensors.
There are two types of furnace lockouts:
- Soft lockout
- Hard lockout
A soft lockout occurs when your furnace detects an issue from one of its sensors.
During a soft lockout, your furnace will do nothing. It will just sit there until the soft lockout period is over.
For example, if your furnace’s high limit sensor trips, then your furnace will go into soft lockout for around 1-3 hours.
After the soft lockout period is over, then your furnace will automatically attempt to start up again.
A hard lockout occurs when your furnace exceeds its “maximum allowable attempts” parameter. This usually happens after several repeated soft lockouts.
For example, if your furnace’s high limit sensor trips and goes into soft lockout 3 times, then your furnace will go into hard lockout.
During a hard lockout, your furnace will do nothing. It won’t try to restart until it is reset.
A hard lockout indicates that there is a serious issue with your furnace. Check what your furnace’s error code is and address the issue before resetting your furnace.