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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8447
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I noticed my AC unit wouldnt come on last night, nothing at

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I noticed my AC unit wouldn't come on last night, nothing at all inside or outside. I checked the wiring on the thermostat and called it a night. I woke up this morning and the unit turned on right away and was blowing cool air. However after going to get breakfast I came back and everything was off again despite the thermostat calling for it to be on. Any ideas what could be happening?
Welcome to Just Answer.

The symptoms are typical of an electronics problem with the thermostat, or the printed circuit board in the furnace/ blower cabinet.

Next step, tell me which wire colors connect to the thermostat... and which terminal each wire goes to. We can then bypass the thermostat and if the system starts we know its a bad thermostat.

Let me know, we can go from there.

Customer: replied 4 years ago.

Honeywell digital thermostat by the way, White wire to W, Green wire to G, Red wire to R, Yellow wire to Y. FYI, last night I opened the thermostat and saw the G wire out, but I replaced it and made sure it was secure. And again the AC all came on earlier but then stopped.

Customer: replied 4 years ago.
Relist: Incomplete answer.
Hello again, if the unit is not responding to the thermostat at the time, do the following tests

First jumper red to green and tell me if the fan runs.

Next remove red, green and yellow... and twist them together, The cooling should come on.

Next twist the red, and white together, leaving all other wires disconnected...the heat should come on.


Tell me the results of those tests, we can go from there. I am sorry I was late, I am having connection issues on this end.

Customer: replied 4 years ago.

Ok, I jumped the red and green and the fan did not come on. I then put the red, green, and yellow wires together and the cooling did not come on.

Thanks..... this proves the problem is inside the air handler units electronic controls.

The problem is either with the 24 volt transformer going bad intermitently, or loose wire connections in the 24 volt control circuit,,, or the control card (printed circuit board)

Next step requires a multi meter to read volltage at the 24 volt transformer... can also read voltage from the red thermostat wire where it comes from the thermostat and attaches inside the unit... put one test probe on the red wire and the other on the gas pipe... the voltage read should be in the 18 volt range. If you et no read, the transformer is bad, or there is a bad wire connection.

We can go from there.


Customer: replied 4 years ago.

Ok this is where my skills may let me down, but I did test it. I used the multimeter to test the small red wire before it went into the transformer or capacitor. I got very little to no reading from it

Hello again, either you did the test incorrectly, or the transformer is going bad... thats a $40 part. not difficult to replace.

Look at the transformer where the red you tested connects... there should be another terminal right next to it. Test with one probe on each terminal, you should get 24 volts.

Next step.... remove the transformer from its mounting so that the back is exposed and look for any sign of burnt wire connections.


Scroll down that page past the first row of diagrams, you will see many different types of transformers.

We can go from there.

Customer: replied 4 years ago.

I took a picture. It appears that this is the contactor and not the transformer correct? I'm wondering if the transformer is actually in my crawl space and not in my unit?

Hello again, you are correct.. thats the contactor in the outside unit... the transformer is in the air handler/ furnace in the attic.

Customer: replied 4 years ago.

Ok I touched the red wire with the red probe, and the copper gas pipe with the black probe and read 23 or 24 volts

Thanks! that proves the transformer is good... and means the trouble is elsewhere in the control circuit.

Diagnosing that from here gets complex.

Next find the printed circuit board in the unit in the attic.. its about 4 x 6 inches roughly.. look for an automotive type fuse on it, check the fuse.

The fact that the problem is intermittent points to a bad control card... or a loose wire, check all of the 24 volt wire connections in the unit in the attic, if they are good the chances that the control card is bad is in the 80 to 95% range.

Examine both sides of the card for dark or burnt spots, if you see any, the card is bad for sure.

A new control card costs between $100 and $150 wholesale.. and if you get an exact replacement (not a one card fits 35 different furnaces that you have make setting changes on to fit your furnace) you should be able to replace it. yourself.

Do that work with the power off.. handle the card only by its edges.

If you call in a service company and the man they send is very good at trouble shooting he might be able to provide a better diagnoses but it is unlikely.

Give me the brand and model number of the unit. I will locate a parts source for you.

I will put the ratings box up for you now, as long as you rate my ongoing work for you *positively I will hold the question open as long as you like.

Phil and other HVAC Specialists are ready to help you
Customer: replied 4 years ago.

Ok first let me show you pictures to make sure I'm measuring correctly. I'm touching the small red wire and the copper pipe in the pictures, just want to make sure I'm doing that correctly. Second, the fuse looks fine and I don't see any spots on the board.

Hello again, picture number 2 shows you touching the *insulation on the red wire... that should have read 0 volts to the gas pipe not 24 volts.... unless you mean you were touching the screw terminal or bare copper at the end of the red wire.

Notice that the blue wire appears to be broken off at the terminal.. that is normally for a heat pumps 'change over valve', the unit should run even if thats broken but would not switch from heat to cooling as it should.

None the less inspect under that screw terminal for a broken piece of copper you find one, reconnect the blue and tell me how it goes.

Thanks for the positive rating!.
I am leaving for 2 or 3 hours to do a job, I will respond back to you at that time.

Customer: replied 4 years ago.

I think that may have been my fault, I was not touching the copper part of the wire. Once I touched that part of the wire with the copper pipe I was not getting a reading

Hello again, When I use these test instruments myself, I take any read at least 3 or 4 times, being careful to rub the points of test probes hard onto the parts I am testing.... it is quite easy to get a bad read.

There is no way you should get any voltage read at all touching the red plastic insulation on the red wire... in my 50 years in the business I have never seen that.... it makes no sense.

If you get a read it should be between the metal screw the red copper wire is attached to, or the copper part of the red copper wire and the gas pipe.

triple confirm that for me please.


Just so you know what we are into here, the college courses on checking these systems out and working with them safely is 2 years, 20 units a semester, then after that a 4 year apprentice ship.

Its easy for me to start a person checking basic things such as fuses etc... then cross the line and get into the complex electronics with instruments the equipment owner may not be used to operating... and that maybe one owner in a hundred or more is trained to work on....

you need to know that without a lot of experience we can get over your head and waste a lot of your time.


For now we can check a few more things if you want.... tell me about the blue wire that looks like it broke off... look under that screw head for the piece of copper wire... *IF* there is one, reconnect the blue wire since it should obviously be connected.

Double check the read from Red wire to the gas line as discussed above.

Let me know what you find, consider buying a new control card and replacing it.... thats what 90 percent of most HVAC mechanics would do at this point, since diagnosis beyond this point is often not possible... the insides of the control card cannot be tested effectively since the problem is intermittent, and it may be working fine when tested but not later.

Read this note a few times if you like, these are complex issues and it is hard to fathom all of them at once.


Customer: replied 4 years ago.

I wish I were still at home, but I'm with a client for the rest of the day. I appreciate your thoughts on testing the components, it is a little bit more out of my comfort zone although I'd like to learn it.


I pulled out the blue wire and it looked fine so I put it back in.


I did take out the transformer because those connections were easy and I wanted to test it and possibly replace it since it isn't too costly. However looking at the one that the store sells it doesn't have the wires already in it so I'm not sure how I would put the wires in to connect it.


I certainly hoped to avoid calling a technician but if I want AC anytime soon I may have to do it. Your thoughts?

Hello again, you sound a bit like me, I am here in Antigua Guatemala, learning Spanish. When I got here I thought I could learn to get by ok speaking Spanish in 3 weeks, and I allowed 6 weeks more to get fluent. ..

.I was completely wrong. it takes 6 months with a lot of study to barely get by.... and 2 years of constant study and practice to be partially fluent.

You are in a similar situation these systems are **complex and only partly testable.

Going deeper than you already have could cause damage and make the repair more expensive. I recommend that you put that transformer back as it was...if you are 100% sure you can reconnect it correctly... do not guess, you can do a lot of damage that way.

I think the problem is a bad control card but cannot be more than 90% sure since you didst find any burnt spots on the card ... actually its always difficult to be 100% sure on those control cards. It is time to call in a good technician... about 10% of them are good.

If you can tell me which direction you are from the center of the nearest city i can take a look for an outfit with good reviews... and hopefully get you a list of several of them.

These intermittent type problems can baffle even the best technicians,

Customer: replied 4 years ago.

Thanks Phil, I have one that I feel comfortable with, Marshall Heating and Air in Winston Salem. He has been to my house before and was very honest and helpful.


I made sure to take detailed pictures so I can put the transformer back properly so I will do that after work tonight. What a day for my wife's birthday as she's 6 months pregnant!

You are lucky to have a good and honest man to work with.

let me know how it goes, I will appreciate that.


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