I have a lennox HS26 (Elite 13), maybe about 5 years old and the condenser won't turn on when it needs to. If I push the switch (described in this older thread: https://www.askmehelpdesk.com/advice/t-346438.html) it kicks in, so it's getting power and the motor isn't blown. What next?

If you are pushing the contactor in and then the unit runs it is not getting the 24 volt signal source to turn the A/C on outside. Track down the 24 volts and you will find your problem. I figure your inside blower is running since you did not say it was not.

If your inside fan comes on, then you can assume the transformer that gives you the 24 volts you need is working, although there were a few furnaces that had a normally closed fan contactor, but I doubt that is the case here. You need to check and see if you are getting 24v to the coil when it is calling for AC, and if you are, then you need to check the contactor coil and see if it isn't open or blown, which would be an easy fix of replacing the contactor.

If you are not getting 24v outside, then you need to check your unit inside. If it has 24v to the two wires going to your outside unit, then you have a break in the wire someplace between the furnace and outside.

Another test to do would be to diconnect the line voltage to the outside unit, and applying 24v to the contactor wires inside the furnace and usually you can hear the contactor kicking in, if it is working, or have someone go out and listen while you make and break a connection to the contactor to determine if it is the contactor or the wire from the inside unit to the outside unit. I always do this on a new install to check the low voltage side of the wiring.

If your contactor is working, then you have to determine where between the transformer in the furnace and the wires you used to test the contactor are not getting the 24v, which could include the T-stat.

There is always odd stuff to run across like timers to keep the compressor from cycling too fast, but that is why I suggested to supply 24v at the inside of the furnace to the contactor, as it will eliminate a lot of stuff it could be.

Modular/mobile homes with Coleman/Duotherm or any other trailer units, often have a split control system for the heat and AC. Each will have their own transformer and to be quite honest, they can be a real pain to figure out.

One of the bad things about the new stuff is the expensive control board does a lot more now and can be problematic, but it also leaves a big hole in your wiring diagram, which makes it harder to diagnose.

If you have the ability to do the things I mentioned and still don't find the problem, PM me and I will go into further detail, after getting some specific information on your unit.

Just remember, they all can be fixed, you just don't want to spend good (a lot of) money on a old unit, as the new ones will pay for themselves in your power bills.

Dan