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my compressor outside is not kicking in. Here's the sequence of events:

  1. I came home and the thermostat - a Honeywell LCD - was no more numbers and the battery indicator was flashing.
  2. replaced the batteries and no change in that
  3. bought a new basic thermostat and hooked up the wires correctly - note there is a Green, White, Red and Yellow wire and the hookup locations are very obvious
  4. installed the thermostat face, and all controls behave as expected, and the cooling shows as being on - but the compressor outside is not kicking in.

It's possible I crossed some wires? but I don't think so. I know years ago someone replaced a capacitor on my compressor but I've had no problems since then. How would I troubleshoot this?

  • Do you have a multimeter, and know how to use it? Are you comfortable poking around inside HVAC equipment? – Tester101 Jul 18 '15 at 2:56
  • Sometimes just matching colors to letters, is not the proper way to install a thermostat. You should have labeled the wires as you removed them from the old thermostat, to be sure where each one belonged. Now to be sure of the wiring, you'll have to investigate in the furnace/air handler. – Tester101 Jul 18 '15 at 11:59
  • Do the other functions work (heat, fan)? – Tester101 Jul 18 '15 at 11:59
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Test the thermostat

There's a couple ways to do this, the "professional" way, or the "caveman" way.

  • Label and remove the wire connected to the Y terminal on the thermostat.
  • Set your multimeter to test continuity, or resistance.
  • Set the thermostat to cooling, and reduce the set temperature until the unit starts to call for cooling (may hear a click, and the unit should indicate that it's calling for cooling).
  • Touch one of the meter probes to the R terminal on the thermostat.
  • Touch the other probe to the Y terminal on the thermostat.

The meter should show continuity, and/or a low resistance. If not, the thermostat is bad, or not configured properly.

If the thermostat checked out, reinstall it and move to the next test.

Alternatively, a caveman would simply touch the "yellow" wire (the wire connected to the Y terminal) to the "red" wire (the wire connected to the R terminal).

This should turn on the A/C. If not, continue to the next test.

Test the wiring

WARNING: This test requires you to open up the furnace/air handler, and work on live equipment. If you do not feel comfortable, contact a local licensed HVAC technician. Next, you'll want to make sure the signal is getting to the furnace/air handler.

  • Carefully open the furnace/air handler, to expose the control wiring.
  • Set your meter to test AC volts.
  • Touch one probe to the R terminal.
  • Touch the other probe to the Y terminal.
  • Have a partner turn the thermostat to cooling mode, and reduce the set temperature until the unit starts to call for cooling.

Initially; when the thermostat is not calling for cooling, you should read about 24 volts. When the thermostat calls for cooling, that should drop to 0 (or very near 0, depending on how sensitive the meter is).

If this checks okay, move to the next test.

Test the contactor

WARNING: This test requires you to open up the condensing unit, and work on live equipment. If you do not feel comfortable, contact a local licensed HVAC technician.

  • Open the condensing unit to expose the contactor.
    • Using a nonconductive implement (broom handle, etc.), push the contactor closed.

If the unit does not come on, it's likely time to contact a local licensed HVAC technician.

If it does come on, move to the next step.

Test the contactor control wiring

For these tests, you can pull the disconnect for the condensing unit.

  • Label and remove the control wires from the contactor.
  • Set the meter to measure AC volts.
  • Touch one probe to each of the wires you just disconnected.
  • Have a partner turn the thermostat to cooling mode, and reduce the set temperature until the unit starts to call for cooling.

Initially you should read 0 volts. When the thermostat calls for cool, that should jump to about 24 volts.

If everything to this point tested okay, it's time to contact a local licensed HVAC technician.

  • I'm still working through this, and want this to be a resource for everyone, which if one reads carefully will get a lot of gotchas. Your answer is incorrect under "Test the Thermostat" at least from a standpoint of saying that if the Y wire works then the AC works - in fact one needs also the G I found out, i.e. the fan/blower to run. – Oliver Williams Jul 18 '15 at 19:58
  • @OliverWilliams The problem you're having, is that the condensing unit is not coming on, right? I agree that the blower also must come on, for the A/C system to operate properly. However, if you're trying to figure out why the condensing unit is not coming on, testing the blower is out of scope. This is a short answer format site. If you want a comprehensive troubleshooting guide, go buy a book or take a few courses at the local community college. – Tester101 Jul 19 '15 at 3:15
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The best I can do on this is give a narrow set of guidelines:

  1. If a system was working and then there is a failure, usually there is only one critical point of failure that prevents the system from operating and not multiple simultaneous failures. (Of course, there may be multiple underlying problems that are not system critical, but that is not the issue here.)

  2. Since you do not mention a black wire, I conclude that you had a battery powered thermostat that required no external power. Given the blank display and battery replacement not correcting the problem, it sounds like your old thermostat failed. The old thermostat was therefore likely the critical point of failure.

  3. Since the system still does not work correctly after installing a new thermostat, the new thermostat or perhaps the wiring to the thermostat is currently the critical point of failure.

  4. You mention that everything works as expected except the compressor, meaning the blower works. To determine if the inoperable compressor is a thermostat problem or a wiring problem, carefully inspect the wiring at the thermostat. Look for damage on the yellow wire. If you cannot find any damage, then check for a thermostat problem by connecting both the green and yellow wires together at the "G" terminal on the thermostat. In this configuration, the compressor should come on whenever the blower is activated. Put the thermostat back on the wall and turn on the blower. Does the compressor come on with the blower? (If yes, you have a thermostat problem. If no, then you have some other problem beyond the scope of what I can tell you here.)

  5. If you verify that you have a thermostat problem, then check that the new thermostat is configured correctly for your system by reading the setup section of the installation manual very carefully. If there are no setup issues then you have a thermostat that is DOA. Replace it again.

Good luck.

  • Simply touch the yellow wire to the red wire at the thermostat, that should turn the A/C on. If it does not, the problem is not the thermostat. Alternatively, you could use a multimeter in continuity mode, to check if the switch to call for cooling is closing. – Tester101 Jul 18 '15 at 11:55
  • Just to clarify, the test Tester101 mentions in the comment above will likely turn on the compressor without turning on the blower which is only okay to do for a relatively short period of time. Personally, I am all for the "caveman" method if it is simple, effective, and safe, but to each his own. – user39367 Jul 18 '15 at 16:41
  • Guys, appreciate the suggestions. One gotcha I'm experiencing is that yes, the first time I jumped Yellow to Red, I saw the compressor go on outside. But then doing it again, it doesn't; and then again later, it does. Is there some type of built-in delay mechanism that is causing this? – Oliver Williams Jul 18 '15 at 20:02
  • Frequently there is a 5 minute delay to prevent restarts from being too close together. Many systems rely on the thermostat for this delay. Others have the delay built in to the condenser outside. If you do not have a delay in the unit outside, then you may have inadvertently caused the compressor contacts to chatter when you "touched" the red and yellow wires together, possibly tripping the breaker. If the two pole compressor breaker in your electric panel has been tripped, reset it. – user39367 Jul 19 '15 at 0:13
  • I see I was a little off target on my last comment. You probably do have a delay on your outdoor unit based on what you said. – user39367 Jul 19 '15 at 3:50

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