Furnace Won’t Fire Up

[JAYSLOL]

Hello all, our gas furnace decided to stop firing up, I went through the restart procedure listed on the unit (turn power off, turn gas off, wait 5 min, turn gas on, turn power on and turn up thermostat) but no luck.  It is older, I’m not exactly sure how old, and it’s a self igniting model, pilot light does not need to be manually lit.  All I can find online is a billion videos about how to light the pilot light, but nothing on what to do when it should be automatic.  Next steps? 

[techwiz]

The best advice is to contact a professional HVAC technician to fix it.

However, there are some things that a DIY person can check.

Auto-ignite furnaces are designed to automatically light, eliminating the need for a pilot light. Key components of an auto-ignite furnace include a control board, a hot surface igniter, and a gas valve. The functioning of an auto-ignite furnace involves the thermostat signaling the control board to open the gas valve. The hot surface igniter then heats up, igniting the gas and starting the furnace.

Common issues can arise with auto-ignite furnaces such as faulty control boards, dirty burners, or a malfunctioning hot surface igniter.


1. Faulty Ignition Sensor

The ignition sensor ensures that the gas is ignited once released. Over time, the sensor can become dirty or malfunction. Cleaning the sensor with a soft cloth can often resolve the issue. If the problem persists, the sensor might need replacement. A quick search for a video on how to check and replace one for your model of furnace should be easy to find online.

2. Blocked Gas Valve

If the gas valve becomes blocked or doesn’t open correctly, the furnace won’t light. Inspecting the valve and ensuring its proper functioning is crucial. Replacing a malfunctioning gas valve is typically a job for professionals.

3. Inadequate Gas Supply

A disruption in the gas supply line or an empty gas tank can prevent the furnace from igniting. Checking the gas tank and ensuring the supply lines are clear can help. Ensure the gas valve is turned on and the supply is consistent.

4. Electrical Issues

A defective switch or an electrical malfunction can lead to the furnace not igniting. Resetting the electronic ignition furnace or checking circuit breakers can sometimes solve the problem. If the issue remains unresolved, consult an electrician.

5. Malfunctioning Ignition System

Over time, ignition systems can wear out. If the automatic pilot light doesn’t ignite, it might be due to a faulty ignition system. Replacing the ignition system or its components can be a solution.

6. Safety Mechanisms

Some furnaces have built-in safety features that prevent ignition if a problem is detected. If the furnace tries to ignite and then promptly shuts off, this might be a safety feature in action. Check for obstructions or issues with the exhaust vent, and ensure all safety sensors are functional.

[lthenderson]

Hello all, our gas furnace decided to stop firing up, I went through the restart procedure listed on the unit (turn power off, turn gas off, wait 5 min, turn gas on, turn power on and turn up thermostat) but no luck.  It is older, I’m not exactly sure how old, and it’s a self igniting model, pilot light does not need to be manually lit.  All I can find online is a billion videos about how to light the pilot light, but nothing on what to do when it should be automatic.  Next steps?

The first thing I look for is the error code. All newer gas furnaces that I have been around in the last couple decades have a little glass port or a door you can remove to look at the control board where there will be a flashing light. There is usually a sticker on the inside of the door that corresponds to the flashing code to tell you what area is causing the problem.

The three most common things that have happened to me personally over the years in order of commonality:

1. During the winter months, the fresh air intake can freeze over inside the pipe. (You may or may not be able to tell from the outside.) I usually disconnect the pipe inside the house next to the furnace and try starting it again. If it starts right up, that is definitely the problem. I then thaw it out and rehook it up. You don't want to leave it disconnected as it could just speed up a carbon monoxide poisoning if you had issues.

2. Over temp sensor tripping. Usually on the body of the header, there are a few temperature sensors to monitor an overtemp situation. I've had those trip over the years and have had to manually push a button on them to reset them.

3. The ignition sensor gets dirty. As mentioned above, over time, crud can built up on it and cause it to not work properly. I usually take a piece of steel wool (000) and just give it a quick polish.

There are of course many other things that can and do occasionally go wrong which is why figuring out the error code and doing an online search for that model of furnace and that code are my first steps if one of the above three things aren't causing problems.

[GilesMM]

One of our gas furnaces (ca. 2000) can trip when a southerly wind blows in the exhaust flue and causes a "flame-out" fault.  There is a reset button hidden deep inside the unit which I am able to poke with a long chopstick.  The blinking lights on the furnace face indicate this error.

[JAYSLOL]

No blinking lights, readouts or any switches that I can find on the unit, I suspect it’s at least as old as the last time the house was renovated in the early to mid 90s (house was built in the 70s).  I don’t suspect an electrical source issue as the furnace makes noise and fans run etc when I try to use it, and I don’t suspect a gas supply issue as our fireplace, stove and clothes dryer are all gas and are working fine.  No unusual winds lately and while it is cold, it’s not cold enough for things to freeze over here, so I’m guessing it’s the ignition sensor, or igniter or something blocking gas flow in the unit.  I will poke around a bit and see if I can even get replacement parts for this old thing

[sonofsven]

No blinking lights, readouts or any switches that I can find on the unit, I suspect it’s at least as old as the last time the house was renovated in the early to mid 90s (house was built in the 70s).  I don’t suspect an electrical source issue as the furnace makes noise and fans run etc when I try to use it, and I don’t suspect a gas supply issue as our fireplace, stove and clothes dryer are all gas and are working fine.  No unusual winds lately and while it is cold, it’s not cold enough for things to freeze over here, so I’m guessing it’s the ignition sensor, or igniter or something blocking gas flow in the unit.  I will poke around a bit and see if I can even get replacement parts for this old thing

Does it have a condensate pump? If that is blocked or faulty the unit won't fire
Next I would look at the igniter. Remove it and test, if you have an ohm meter. There should be some numbers on it which you can do a google search on (igniter xxxxfir example) and find a replacement. You're basically testing for continuity. https://youtu.be/GYG7kA5uqjk?si=5X9Vra447Ku-0PU_

[SunnyDays]

Maybe the thermocouple needs replacing?  A failed one can cause such an issue I believe.

[Just Joe]

The one gas furnace problem we had was a cracked heat exchanger (combustion chamber). Was allowing the flame to rollout of a vent hole and trip the safety sensor. If you can pop the covers, look for rust and cracks. I thought it was a repairable problem until I realized the heat exchanger was cracked and that led to a whole unit replacement.

if you need to replace the whole thing - don't cheap out. Our house had evidence of 4-5 systems in 25 years. Not sure if they were truly cheap units or HVAC companies taking advantage of previous owners. We put in a quality Trane hybrid system. When we drive through the old neighborhood, its still there in service from many years ago.

[moof]

I'll tell you what fixed mine once.  I turned off the power, opened up the panel to the circuit board, and removed and replaced every connector.  I was guessing surface oxidation was leading to a poor connection, and that reconnecting would break up any surface crud.  Closed things up and it fired up like a champ.  Long shot, but it was easy to try.

[aasdfadsf]

Check the flame sensor. You can most likely unscrew it and clean it off with fine grit sandpaper (or a dollar bill works). This is probably your most common reason why you can't get the unit to fire if the pilot and gas are going fine.

[Beach_Bound]

You can troubleshoot more effectively if you can figure out exactly where in the start up sequence it's failing. This link matches the order of operations for my self igniting gas furnace from the early 2000's.

https://www.technicalhotandcoldparts.com/content/Sequence-of-Operation-on-a-Gas-Furnace.pdf

One time it stopped working because the ignitor failed. I could hear the inducer motor start up (step 3), but the ignitor never came on, and the inducer motor stopped running after a short period of time.

A different time, the furnace stopped working because the flame sensor was dirty. The inducer motor started, the ignitor come on (I could see it and hear it), and the gas valve opened for 5-10 seconds (step 6). I could see the flames from the gas flowing past the ignitor, but the sensor didn't detect the flame, so the gas valve turned off. I paid someone too much money to clean off the flame sensor with some sandpaper or something similar.

[lthenderson]

Hello all, our gas furnace decided to stop firing up

Any progress?

[index]

No blinking lights, readouts or any switches that I can find on the unit, I suspect it’s at least as old as the last time the house was renovated in the early to mid 90s (house was built in the 70s).  I don’t suspect an electrical source issue as the furnace makes noise and fans run etc when I try to use it, and I don’t suspect a gas supply issue as our fireplace, stove and clothes dryer are all gas and are working fine.  No unusual winds lately and while it is cold, it’s not cold enough for things to freeze over here, so I’m guessing it’s the ignition sensor, or igniter or something blocking gas flow in the unit.  I will poke around a bit and see if I can even get replacement parts for this old thing

My money is on the ignition module (makes the click-click-click) or the central control board if it doesn't have a standing pilot light. Its probably the thermocouple if it does have a pilot light. Sometimes steel wool on the thermocouple can get it to fire up and eek the rest of the season out of it. Newer furnaces have a hot surface ignitor (think glow plug in a diesel) that go bad pretty often, so its either that or the thermocouple (turns off the gas if it doesn't get hot enough to prevent an explosion). Open up the furnace and find your model # and google XXXXX will not ignite. 

[Just Joe]

I'll tell you what fixed mine once.  I turned off the power, opened up the panel to the circuit board, and removed and replaced every connector.  I was guessing surface oxidation was leading to a poor connection, and that reconnecting would break up any surface crud.  Closed things up and it fired up like a champ.  Long shot, but it was easy to try.

I did that with an industrial computer last week.

[pecunia]

I'll tell you what fixed mine once.  I turned off the power, opened up the panel to the circuit board, and removed and replaced every connector.  I was guessing surface oxidation was leading to a poor connection, and that reconnecting would break up any surface crud.  Closed things up and it fired up like a champ.  Long shot, but it was easy to try.

I did that with an industrial computer last week.

I've had 3 gas furnaces with the same problem.  All turned out to be the pressure switch that allows the ignition.  I  did the fix myself.  Three flashes on the main board that repeated themselves.  However, the last furnace problem I had required a new main board.  Luckily, it's been running happily all Winter.

I see many are waiting to see what the problem of this original Poster has with his furnace.  It's been over a week.  Well?

[Sibley]

@JAYSLOL  Outcome on your furnace?