How to make sense of your furnace diagnostic codes.

All high efficient and most mid efficient furnace manufactured after 1995 are equipped with a circuit board that will provide some diagnostic information by way of using one or more flashing LED lights. Most newer models have a small viewing window on the lower front door panel. Each manufacturer will have its own code language that is not always the same from year to year but is specific to all models of the same production year. Most often there will be a fault code chart right on the inside of furnace compartment service panel.

First of all, if you know that the furnace has a led light and it is not on, you need to confirm that the circuit from your electric panel is on along with the service switch if you have one.
(looks like a regular lite switch) Otherwise the fuse on the circuit board could be blown and the control circuit would then be off along with the led light. You can reset your furnace by cutting the power to the unit via the service switch or the breaker leaving it off for about 30 seconds and then turn it back on. This will clear the board and let the unit try to restart so you can watch to see what is happening. Removing the furnace service door panel will also kill the power to the furnace.

If the diagnostic lite is on steady and not flashing, that is indicating a normal steady or intermediate state. Once the power is introduced to the furnace, there is a startup sequence that will either have the blower fan or draft inducer fan run for a purge or pre-ignition proving period. At this point error codes may begin to show up.

Be sure to get the code the furnace is showing you before you reset the power. This code will be lost once the unit is powered down. Sometimes, if you’re lucky, the unit will run after you’ve restarted it. Not that it’s fixed but if luck is on your side, you might have heat again, at least for the time being.

The failures that the codes represent are relative to cause and effect. That is they will not always point to a specific failed component or part. Failures however can be broken down into 4 main areas as follows:

1.      An Ignition problem. Failure to ignite after a specific time can be caused by a bad flame sensor, Igniter, gas valve, wire connection or circuit board.

2.      A gas problem. Failure to ignite can be caused by pilot failure, bad gas valve, or a gas supply problem (local utility)

3.      An air flow problem. Furnace will eventually lock out system due to high limit tripping open. Caused by many things that can reduce or impede airflow. Dirty air filter, too many vents blocked or closed, dirty evaporator coil, plugged secondary heat exchanger, dirty blower wheel, blower fan speed set too low or failing blower motor.

4.      A pressure switch problem. Furnace may not even initiate a trial for ignition. Anything that can restrict the amount of pressure that the draft inducer is intended to deliver will shut the furnace down. There are many places within the system that can cause this failure. Snow or ice blocking the vent terminations outside, a blocked condensate drain, pinched or blocked hoses, a bad pressure switch, a weak or failed draft inducer, a blocked secondary heat exchanger, improper installation of the venting system.

A code for “lockout” means just what it sounds like; everything is locked out for use.

Lockout happens when a furnace has attempted to fire multiple times (usually three) and failed. It may run the blower for a few seconds as a default and then shut down completely.

Fixing your own furnace can be costly and dangerous.

I hope this information is helpful to you but remember, working on a gas furnace can be dangerous and if you aren’t sure about what you’re doing, not only can you hurt yourself and possibly others but you can cause further damage to the furnace.

It’s never a good thing when you end up calling a professional HVAC technician and have to explain how you tried to fix your furnace and now it’s in pieces. Even if it’s not in pieces, it’s usually pretty easy for a technician to see when someone has been messing with things. Always consider your safety first.

Enjoy fixing the things that you can but know when to call it a day and let someone else handle it.

Check here for fault code charts for your furnace: http://tradesmandrcool.com/winnipeg-heating-company/furnace-fault-codes/

All of us here at Tradesman are always happy to help: alwaysopen.ca