Why Does My Furnace Go Into Furnace Lock Out Mode?

Bypassing Furnace Lock Out Mode is Not Advised.**

What is furnace lock out mode? How to bypass furnace lock out mode?

So you have been told (or read the diagnostic manual) that your home heating system has been disabled and is in lock out mode. This is a technical term meaning that the circuit board is preventing the furnace from igniting the flames because a problem was detected in the ignition sequence. Typically this means that the furnace has tried to ignite the flames on the burners 3 times in a row and has failed all three times to recognize the flame. This doesn't mean that your burners don't ignite; it just means that the circuit board fails to acknowledge that flames are present. The reason for the lockout is safety. The circuit board thinks that gas is being pumped into the combustion chamber and is not being burnt. It wants you to call out a service technician to diagnose what problem(s) are present.

So What Causes the Circuit Board to Fail to Read that Flames are Present?

A poor furnace ground. All furnaces must be properly grounded.
Power on the incoming ground or neutral. This typically occurs when someone taps into the dedicated home heater circuit to install lighting or motor driven appliances. Fluorescent lights are the worst. Imagine your furnace works perfectly fine until you switch on that bathroom fluorescent fixture. Once the light is off the furnace can resume working if it has gone in lock out mode.
Poor burner ground. This wire can become corroded or disconnected.
Improperly mounted circuit board. Most circuit boards are grounded through one of the mounting screws. If this is tight and clean a poor ground and intermittent burner firing can occur.
Dirty burners
Dirty flame sensing rod. The flame rod needs to be cleaned from time to time. Corrosion inhibits the flow of electricity to the and from the burners.
A defective, cracked or high ohm hot surface igniter. This is the item that glow whit hot prior to the gas valve releasing gas for the ignition sequence. Yes, it can light the burners and still be defective. These can and do wear out and do not always show any visible sign of damage. It must be tested with an electrical meter and different furnaces and hot surface igniters ohm out at different levels.
Cracked firebox blowing into the furnace burners. This can cause flame movement and it only takes an instant for the circuit to sense a lack of DC current to shut down.
Too much gas pressure. The flames pass up the flame rod or igniter.
Too little gas pressure. The flames lazily wick around the igniter and/or flame rod.
Dirty burners.
Dirty pilot assembly.
Defective induced draft motor.
Blocked flue vent.
Induced draft motor failure.
Vacuum-pressure switch for the flue assembly failure
Defective circuit board

I know, it sounds like an awful lot of things can cause a furnace lockout. You are correct. The furnace lockout is used as a catch all safety in the event that something is going improperly in your furnace ignition sequence. A furnace lockout is a symptom, not a problem.

Give the experts a call and we can help you through this and usually your furnace can be repaired on the same initial visit. Most furnace repairs are little more than a minimal service fee.

** To temporary reset furnace locku0ot mode simply disconnect power to the furnace for few seconds and then restore it. If the furnace was having furnace lockout problem it will come back. Lock out mode cannot be permanently bypassed.