Furnace Ignition Lockout: Causes & Solutions

Posted on: October 5, 2020

What does ignition lockout fault mean? Is your furnace continually shutting down? Has it stopped responding entirely after several attempts to turn it on? If so, your furnace may be in an ignition lockout and must be reset.

Modern furnaces are equipped with a number of safety sensors used to keep the proper functioning of the unit in check. If any sensors detect an unsafe condition, the furnace shuts down to prevent safety hazards or damage to the unit. Unfortunately, the furnace will remain in this state until you or your local Murrieta heating repair technician reset it manually.

What does ignition lockout fault mean?

When a sensor detects an unsafe condition, the furnace controller will shut off the power and fuel to prevent hazards. This is known as an ignition lockout, a condition that can be caused by a number of different sensors found in your furnace. If your furnace is locked out, it will most likely remain in that state until it’s reset. Many homeowners send their furnace into a locked out state when they attempt to get it running several times in a row without success, unaware that the sensor has detected an unsafe condition.

What causes furnace ignition lockout?

Most of the time, furnaces go into ignition lockout because of issues with flame sensors, igniters, or limit switches. Here are the most common lockout culprits explained.

Defective flame sensor

Defective flame sensors are one of the most common causes of lockouts. When your furnace begins to start, the sensor checks for the presence of fire. If the sensor is faulty or dirty, it won’t be able to detect the flame and the controller will stop the ignition sequence. Most modern furnaces will let you start the furnace two more times before it goes into a lockout.

Faulty igniter

The igniter is another frequently encountered lockout culprit. If your furnace igniter does not activate within a couple of seconds, the flame sensor won’t detect the flame, and the controller will stop the furnace from starting. After the third attempt, your furnace will most likely go into a locked-out state. If your gas furnace has a pilot light, the lockout may also be caused by its failure to light the main burner.

Limit switch response

Your furnace also has a limit switch that monitors heat exchanger temperatures and fuel pressure. If the temperature is too high or the pressure too low, the limit switch will close, instructing the furnace to shut down and enter lockout mode. A dirty filter may also cause the limit switch to trip by reducing the air flow. If you have a modern furnace, you may be able to tell what caused the lockout via the status code lights.

What is the difference between a hard and soft lockout?

There are two main types of furnace lockouts you should know about – a hard and a soft lockout. As we’ve already explained, your furnace controller will stop the ignition sequence and prevent unsafe operation if the sensor doesn’t detect the flame within a set time period. This semi-shutdown is known as a soft lockout, and you will be able to attempt to start the furnace again, usually two more times.

A hard lockout happens after several repeated soft lockouts. For most furnaces, you will have three attempts to relight the unit before it goes into hard lockout. If this happens, you will have to reset the unit manually. The best way to do this is by calling your go-to HVAC technicians to perform the reset sequence.

How do you fix a furnace ignition lockout?

The exact procedure for getting your furnace out of lockout will depend on the culprit. Here’s what you can do before resetting your furnace:

  • Clean or replace the flame sensor: Inspect the furnace flame sensor to look for cracks and signs of damage. If you notice only soot or slight corrosion, follow our guide on cleaning flame sensors. If that doesn’t fix the problem, replace the sensor and try again after you restore the furnace from lockout.
  • Clean or replace the air filter: A clogged air filter can prevent adequate air supply to the system, tripping the limit switch. Simply replace the filter and see if the issue has been resolved. It is generally advisable to replace or clean furnace air filters regularly to ensure the safety of your family.
  • Inspect and clean the vent pipes: Look for debris and signs of obstruction inside your vent pipes and clear any blockage you find. If you are unable to clear the blockage, the best course of action would be to call HVAC experts to replace the clogged pipe section.

Resetting the furnace

Regardless of the culprit, you will have to perform a reset procedure to get your furnace out of lockout. Most modern gas furnaces can be reset by turning off the power, waiting around 20 seconds, then switching the power back on. Other furnace models may exit lockout after 1-2 hours and attempt to power on again. If your furnace has a pilot light, you may need to relight it following the manufacturer’s instructions.

If none of the above-mentioned works for you, the best course of action is to reach out to knowledgeable HVAC technicians and have them inspect your unit and perform the necessary tests and repairs to get your furnace out of lockout.

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What causes furnace ignition lockout? After being named the favorite HVAC company in San Diego in the 2020 Union-Tribune Poll, we can proudly say that you won’t find better heating and cooling professionals in the area. Action Air Conditioning, Heating & Solar is a long-standing HVAC company offering top-quality service to homeowners across Riverside County and San Diego County. Whether you need help with your furnace or air conditioner, don’t hesitate to give us a call. Get in touch today!