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My Carrier 58SXA 120 gas furnace keeps locking out after I…

Customer Question
My Carrier 58SXA 120...

My Carrier 58SXA 120 gas furnace keeps locking out after I restart it from my thermostate

Contractor's Assistant: Did you double-check the thermostat? And what about the pilot light?

The unit comes on it starts to heat the house but locks out once it gets to the thermostat temperature

Contractor's Assistant: Do you plan on doing the work yourself?

If I can, but there also seems to be a lose connection to the auto-igniter. I move the connection and the unit will start

Submitted: 7 months ago.Category: HVAC
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Answered in 3 hours by:
12/18/2017
HVAC Technician: Phil, Mechanical Engineer replied 7 months ago
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 9,726
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Verified

Hello,

So far it sounds like a bad printed circuit board. If the furnace is older and you have not been replacing the air filter regularly or if the air filter is dirty it could also be a bad high limit switch... tell me please about how old the furnace is and if you have furry pets in the house.

.

Take close up, exceedingly well focused photo's of the wiring diagram glued inside the furnace/air handler door, and the printed circuit board mounted on the blower, and the connection you moved that allows a restart and I will mark them up with trouble shooting instructions.

Use the 'add files' link next to your send button to get the photo(s) to me.

.

We can go from there with training, coaching and diagnosis.... without a time limit, In depth with as much detail as needed.

.

My goal is to give you the best experience possible! I hope I have earned a 5 star rating! Please remember to rate my service by selecting the 5 stars at the top of the screen before you leave today. If you need more assistance, please use the reply box and let me know.

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HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

Thanks, ***** ***** is to tighten up or repair that loose connection... restore it bright metal.

Next remove power from the furnace and reach inside the blower wheel. if you can dig any dirt from the blower wheel, the cooling coil if you have AC is likely clogged with dirt, that will cause problems such as tripping the high limit switch.

We can go from there without any limits

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Customer reply replied 7 months ago
What does bright metal mean? How do I make it bright metal. I can clean out the blower and ac coil in a few hours . I feel dumb, can you tell me where to look for the ac coil?
HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

Bright metal means no rust, no corrosion, and no burnt wire on the connector.

If the wire is burnt you have to cut the terminal off and fit a new terminal, then you have scrape the female side of the connection with a knife blade until all corrosion, rust, or other stuff is removed and the metal is bright and shinny.

This typing above refers to the LOOSE WIRE CONNECTION ONLY.

The blower if its dirty needs to be removed from the furnace and cleaned with bottle brushes and a vacuum cleaner

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HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

The AC coil fits into a 20" square metal box, often wrapped in insulation, and sitting on top of the furnace in most cases, if its not there its under the furnace.

If the furnace is in the attic it is on one end of the furnace or the other.

It has 2 copper tubes attached, one insulated in black foam rubber, and a white plastic drain line at the bottom.

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HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

Cleaning the coil is a big job....if it needs cleaning we need to discuss it extensively.

.

First step is to see if there is any heavy coating of dirt in the blower. If you are not handy or if you do not like hard dirty work you can have this done by an AC service contractor, prices vary between $300 and $900 I will advise you when the time comes

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Customer reply replied 7 months ago
Excellent thanks . I will work on this when I get home this evening
HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

That will work, I am in until 10 pm Chicago time... and in all day tomorrow

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Customer reply replied 7 months ago
Hello, I did my best to make the connection bright metal. The blower fan was not checked yet. However, the heat worked all night when I had it set at 68. When I try to move the temp up, the thermostat calls for heat but the unit will not turn on. If I turn the heat off at the thermostat, it will start and run until it reaches the set temp. Strange huh? What do you think I should do?
HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

Hello, if I understand what you say correctly, the thermostat is bad.... however that is a very uncommon failure mode, it could be that you do not understand the OFF switch correctly.... play with it for a while and lets see how it goes.

.

Meantime click the 'Finish and rate your expert' link near the top of your page to get me paid.... that way I will be able to hold the question open to you for as long as it takes to get the thermostat figured out.

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Customer reply replied 7 months ago
Thanks. The strange thing about it is that when the house reaches the set temp and I raise the temp at the thermostat, it won’t start and it says heat on. But if I lower the temp so the thermostat stops and raise it about the room temp it will start. But when it gets to room temp it won’t start again . What does that sound like?
HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

If the furnace trips its high temperature safety cut off, it can be reset by turning the thermostat OFF then back ON.... so far I have not seen that work by changing the setting up or down, but its possible.

Make sure you have a clean air filter in the furnace and that none of the vents are blocked... we can go from there

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Customer reply replied 7 months ago
My question is what would cause the above behavior? What could be causing the high temp safety cut off? You said the thermostat could be bad, but not usual. Please see my question today. The unit has a Clean filter. It runs to temp but when it runs to temp, I want to make the house warmer. I raise the temp, the thermostat says heat on, but the unit will not start. I must turn off the heat at the unit or the thermostat before it starts. What does this tell you?
HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

Insufficient air flow due to a dirty air filter, or on older systems a dirty cooling coil cause the furnace to over heat. anything that restricts the air flow will cause the high temperature limit switch to trip..... OR ...a bad limit switch will trip when it should not trip.

A qualified technician would use a multi meter to measure ohms across the limit switch within 10 seconds of the furnace going off, if its 'open' then we know the limit switch tripped.

There are a few dozen other potentials that require tests on the electronics. let me know if you want to dig into those potentials...you will need a few hours and a multi meter

.

Thermostats do not fail in this fashion... the odds that your thermostat is good are nearly 100%

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Customer reply replied 7 months ago
Phil, thanks for putting up with me. Is it correct that unit stops at 68 because it over heats? When I raise the temp at the thermostat about 68, it won’t start, when I remove the ignigtor connection and reconnect it it will start and run until it hits the temp on the thermostat . So should I have a tech change the connection and clean the a/c coil?
HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

It is not absolutely correct that the unit stops at 68 because it over heats.... but that is most likely the case.

Try setting the thermostat o 76F if it still stops at 68F then the problem IS in the furnace, either the high limit switch (80% chance) or a bad printed circuit board etc (20% chance).

If the house warms much above 72F then the problem IS with the thermostat.

Lets do these tests before we discuss the other issues,

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Customer reply replied 7 months ago
Excellent thank you
HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

Let me know how the tests turn out, we can go from there

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Customer reply replied 7 months ago
will do
HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

.

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