[its_cold_out_th]

Several days ago my gas furnace quit working unexpectedly during the night without any warning signs. It is a Carrier 58MXA100-16 manufactured in 2005, in service since 2007.

I get status code 34 “Ignition Proving Failure”.

Steps I went through in troubleshooting:
1. Turn heat on at thermostat.
2. Inducer motor runs until pressure is built up.
3. Igniter heats up until orange.
4. Gas valve connection goes from to 27.2-27.5 volts for several seconds and I hear a click.
5. I hear the hissing sound for the gas but see no flames.
6. Gas valve connections voltage drops.
7. Igniter cools down.
8. Steps 3-7 repeat three times and then the system goes into Ignition Lockout (code 14) as expected.

When I remove the burner box cover the gas ignites and the furnace operates properly. When I replace the burner box cover leaving a small gap for air to pass through, air gets sucked in through the gap with a good amount of pressure and the furnace operates. Heck, even having the screws loose on the cover allows the furnace to work.

This leads me to think there could be an obstruction in the inlet for the combustion air. I’m a little suspicious that’s it because (and you’ll love this story)…

When my furnace wouldn’t work when we first tried it for the winter a few months ago a tech came out and determined that the inducer fan was sticking a bit. He wiggled it and that was enough to get it working, though it was making strange noises and seemed stressed. Well, it stopped working again a few weeks later, so I suspected a clog and disassembled the inducer fan and found a dead rotting bird inside blocking the fan blades. Absolutely disgusting. Must have come in through the vent (outgoing combusted air). Needless to say I'm not calling that HVAC company again, and I'd like to try to solve it myself this time.

So now I’m wondering if something has got into the inlet for combustion air and is blocking it just enough that not enough air is being provided for ignition.

Other notes:
• Immediately before entering the furnace it branches to the water heater, which is working fine along with all other appliances in the house.
• I cleared the tube (for the gas valve that connects to the pressure sensor) of any moisture. There was a small amount of moisture, probably less than one drop.
• Checked the voltage of the rollout switch and the pressure switch. Both are fine.
• I verified that the two green/yellow wires are properly grounded with good contact.
• I know it is a common problem but I didn’t check anything related to the flame sensor since there is no flame, so I believe the problem is before that.

According to the manual, other possible issues:
• “Inadequate flame carryover or rough ignition.” – not sure how to check this
• “Low inlet gas pressure.” – doesn’t seem like the culprit here
• “Proper firing rate.” – not sure how to check this
• “Check for air leakage around igniter bracket.” – I don’t detect any leakage

Any ideas?

Any tips from you experts would be much appreciated!

[hvac1000]

Try cleaning the flame sensor. Easy does it if mounted with or near the ignitor since ignitors are sensitive.

[its_cold_out_th]

Try cleaning the flame sensor.

HVAC1000: Thanks for the tip. When I posted I thought you might suggest that as you've done to many others for similar problems. LOL. But my problem is that the flame never even catches when I have the cover on the burner box, let alone make it all the way across to the other side where the flame sensor is.

Nevertheless, I took your advice on cleaning the flame sensor. It looks like the one in the first picture you have. It was a pain to get it out and harder to put back. Guess I don't have the fancy tools. Anyway, I cleaned it even though it didn't seem too dirty.

Nothing has changed -- the gas ignites when the cover is off and doesn't when the cover is on. Any other ideas?

[mygirlsdad77]

Can you take some pics of the inside of your furnace and post them here?

Do you have a rubber tube that goes from the pressure switch to the sealed combustion area. You could just unhook the intake are pipe(may have to cut it) and see if furnace works OK then. Let us know what you find, and we will try to help.

[hvac1000]

Read this old post I was involved in and see what you think.

https://www.askmehelpdesk.com/heatin...er-284552.html

[mygirlsdad77]

I don't think it is the limit in the combustion area,(I think that you would have to have flame present to trip this switch, and you are saying you don't even get any flame with cover on). I would guess that you have a plugged intake air, or possible pressure switch problem(if there is a hose from p.s. to sealed combustion chamber).

HVAC, I may have missed something in your old post. Please let me know what you are thinking. Sometimes my mind gets stuck on one thing, and its far from what the actuall problem is.

[hvac1000]

Actually the question from the other post was never resolved. I posted it so that it would be known that this problem exists. It could be that the cover when in place is dogging down the combustion fan possibly not allowing the pressure switch to operate properly. IE weak draft inducer fan or previous fan damage.

[its_cold_out_th]

can you take some pics of the inside of your furnace and post them here?

Here are two pictures. The first is with the combustion box cover off, the next with it on. If you need something else more specific please let me know. (OOPS: the pictures got corrupted on my camera. I'll try again with a different camera tomorrow.)

do you have a rubber tube that goes from the pressure switch to the sealed combustion area. You could just unhook the intake are pipe(may have to cut it) and see if furnace works OK then. Let us know what you find, and we will try to help.

Yes, the rubber tube goes from the pressure switch to both the gas valve and the sealed combustion area. I've tried the following combinations with that tube: remove from the pressure switch only, remove from the gas valve only, remove from the sealed combustion area only (leaving that end both open and closed/covered with my finger), and covering the place where the rubber hose connects to the gas valve with my finger. None of those got any flame action when the combustion box cover was on.

To unhook the intake air pipe I would have to cut it, so I'm hoping not to have to do that. I did something nearly equivalent which was to remove the plastic cap at the end of the air intake line, just below the sealed combustion area. That should have allowed double the intake air: full flow into the combustion area from both the regular pipe (from outside less any obstructions), plus full flow from indoors through the other side. No flames when I do that with the cover on. (I also looked into the pipe with a mirror and flashlight and saw no obstructions before the vertical bend.)

Read this old post I was involved in and see what you think.

That sounds like a rather similar problem, thought the poster wasn't clear on exactly what the symptoms were. I am getting a different error code, so I'm thinking it would be a different issue. Also, it appears that the igniter is glowing for just as long, and the gas is hissing through for just as long with the cover on or off, so I doubt that a temperature sensor connected to the control board would be causing a problem. But just in case I'll add to my list of things to try to temporarily jumper the temperature sensor (or temporarily remove it from the sealed combustion area).

Thanks for all the great ideas so far. You guys are awesome. I hope we can figure this out.

[hvac1000]

But just in case I'll add to my list of things to try to temporarily jumper the temperature sensor (or temporarily remove it from the sealed combustion area).

Remember never operate the furnace long term in any condition that could jepordise safety! If you jump something out never let the unit operate unattended.

[hvac1000]

FYI your unit. Still looking for other info

Carrier Class Action-Official Court Website

[hvac1000]

Service manual

http://www.docs.hvacpartners.com/idc.../58mxa-2sm.pdf


With variable speed.

http://www.docs.hvacpartners.com/idc...it/58m-2sm.pdf

[its_cold_out_th]

Service manual

http://www.docs.hvacpartners.com/idc.../58mxa-2sm.pdf

That service manual you found is one version newer than the printed one I have! Going through the steps of that troubleshooting guide it narrows it down to two things:

1. Inadequate flame carryover on rough ignition.
2. Low inlet gas pressure

I really doubt it's the first since no flame even catches, so doesn't seem to be a carryover problem. Very well could be the second if my gas pressure dropped just enough that when the valve is open the gas doesn't quite reach the igniter unless the burner box cover is removed. If that is the case I assume this could be solved by either turning the adjustment screw clockwise to increase gas pressure or by slightly increasing the flow from the gas sprayer (not sure of term here) on the manifold that is in front of the igniter.

I'm really thinking now that the reason taking the cover off the burner box helps is not directly related to the combustion air inlet pressure, but related to whether the gas sprayer in front of the igniter can reach the igniter. So I'd thinking adjusting that one sprayer ever so slightly might do it. I'm a bit reluctant to fiddle with the gas settings since I don't want to overdo it and have the heater cycle on and off more or trip the rollout sensor or temperature sensor. But perhaps if I adjust it ever so slightly?

[hvac1000]

I always use a pressure gauge to adjust the gas valve manifold pressure to the factory specs as posted on the furnace label. I guess one half turn clockwise with a maximum of one complete turn might help to see if that corrects the situation BUT that will not tell you what the gas pressure is to start with and if that could be the problem to start with.

You will have no start basis and the regulator could be down at 1 inch water column instead of the usual 3.0-3.5 in wwc. I guess at this point there is nothing to loose but a few facial hairs and you can always turn the regulator screw back to exactly where it came from. Easy does it and be careful.

[its_cold_out_th]

The workaround I'm using right now is to keep the end cap for the inlet pipe open with the main furnace door off. (For some reason that wasn't enough before, but it is working now.) I'm just letting in enough outdoor air to the basement so there is proper combustion. This is the first day in the last 5 days it hasn't been 50 degrees in our house! Yeah!

My camera isn't working, but here is a screenshot from the manual of the inlet piping (with a view from below of the combustion box in the back showing where the attachment is made):



Right now air is flowing into the left from outdoors (probably partially obstructed) and in from the right side from the basement. Any reason this can't be a semi-permanent (through the end of the winter) solution? And yes, I've calculated the BTUs to make sure I'm getting enough air in from outside.

So now I'm thinking again that it is an inlet obstruction. For all I know there is a bird's nest in there. I really hate to cut up the PVC, which runs about 15 feet horizontally before reaching the furnace, so here are my ideas: 1. attach a webcam to a pole with USB extension cables and a flashlight and run it into the pipe so I can see on my laptop if there are any obstructions in that part; 2. get some sort of pipe snake (like what is used for sewers) to push from inside towards outside to see if anything flushes out; 3. use a super-strong magnet and put a steel ball (pinball, probably) in the pipe at the furnace then run it all the way along to outside to see if I can feel where an obstruction might be.

Thoughts?

I'm thinking it could *possibly* be an obstruction in the outgoing vent, or a problem with the inducer motor or fan blades (where there was previously an obstruction from a dead bird).

[hvac1000]

Just have it all hooked up as normal then remove the combustion air inlet pipe from the unit. If the pipe is removed it will just draw air in from the basement. If it runs under those conditions then you have a obstruction for sure. Many units are installed this way to start with so no damage will be done. Then you can play with the toys to see if you can find where the obstruction is in the pipe. NOTE somebody should have installed a very open mesh screen in the pipe end outside as per the instructions. It just keeps out the big chunks of whatever but will not stop wasp nests etc.

[mygirlsdad77]

You can unhook the pvc inlet(or cut it if it is glued). This shouldn't be a problem as long as the furnace isn't in a small sealed room. As hvac stated, a lot of furnaces are installed without the intake air from outside, they just pull the combustion air from inside the house. I would start by disconnection the intake air, if you cut it, you can use an adapter clamp(fernco coupling to reattach pipe, once obstruction is cleared) Try this first and let us know if problem is solved. It it is, you can just leave it this way, or fix the problem when you get time. Please keep us posted.

The fix you have done now should also be okay,

[tophersig]

Hi guys. Was there any resolution to this guy's issue?

Thanks.

[hvac1000]

No

[its_cold_out_th]

At the start of the following winter (Nov 2009), I found another bird in the inducer fan. Perhaps it had been stuck in the combustion inflow all last winter and then advanced to inducer fan when I fired everything up for the first time in this winter.

Since getting that second bird out, the issue has not recurred, the furnace is back to 100%. So my best guess is that the inlet was indeed clogged and unclogging it resolved the issue.

Thanks for the reminder that I should update the post with a resolution. And thanks again to hvac1000 and others for the expert help.