Before I begin, I would like to request "call a technician" suggestions to stay off. I do know a nice guy who can look it up for me, for a variety or reasons, one of which being my general history of fixing stuff, I would like to take a first stab at solving this problem. Thanks
Background: Both units are middle-of-the-line Lennox, about 2 years old. The thermostat is Nest 1st generation, which also controls the completely separate hydronic heating system used with radiators. The system does only AC (is not a heat pump, in other words).
Problem: I turn the AC cooling on and only the indoor unit (air handler) fan comes on. The outdoor unit (compressor) stays off. I checked power supply at the main panel (both units have a dedicated circuit, 15A for the handler, 30A for the condenser) and using a pen-like current detector.
Wiring Pictures
1. Inside Air Handler
Note that the upper thermostat cable is what goes to the compressor, lower goes to the thermostat. The two white wires are from a small water detector attached to the pan in which the return box sits to switch the system off in case water drips in the pan.
2. Thermostat
Note that the RH red wire and the W1 white wire come from the above mentioned radiator heating system and are completely unrelated to this. RC red goes to the R in the air handler.
3. Compressor
So the C connector from the thermostat is connected to the air handler via the blue wire, which goes to C in the air handler and to which the blue from the compressor cable connects it to the black wire in the compressor. My suspicion is that somewhere along is the problem unless the compressor is broken, for example a problem with its capacitor or something else broken.
Question: How can I diagnose where the problem is using a multimeter? Is it possible to narrow the problem down to the capacitor OR the thermostat using my analog multimeter, i.e. to rule out the wiring, which seems like it is fine? I can turn the system on and I would like to use the multimeter to verify that the thermostat is properly turning the cooling on via the blue wire to the compressor. If so is the case, then the wiring is not the problem.
Best Answer
The first thing to do is check the two wires on the outside unit when the thermostat is calling for cooling. In your picture it looks like the Blue and Yellow thermostat wires are being used. You should have 24v AC across those two wires, which would normally close the 240v relay and start up the compressor and cooling fan.
If you're not getting 24v on the two thermostat wires, there's probably a bad connection somewhere. Since the air handler is working and the thermostat has power, the 24v transformer is working.
If you are getting 24v, then you can move on to diagnosing just the compressor. There are limit switches that can prevent the compressor from turning on if the coolant level is off. There can also be temperature sensors that prevent the compressor from running, so these would need to be checked. The relay should also snap shut when the 24v signal is present. A poor connection might read 24v, but might not have the amperage to actually close the relay. The relay can be corroded or filled with dead ant bodies.
I hope this helps. Clarify anything you can in the comments and I can attempt to edit as needed. I've done a lot to diagnose and solve problems with a WiFi thermostat and heat pump that were not wired correctly.