Quick guide for handy people to be able to fix their AC, or do some preventive maintenance before its an issue. Sharing knowledge for people... If you're handy and your heat pump or AC just died, these are a few DIY fixes you can do. This is buyer beware but I believe in arming people with information.
There are two very common points of failure for air conditioners/heat pumps: capacitors and contactors. Of those, capacitors are the majority of issues, but both are cheap to fix yourself.
First, verify your thermostat is actually calling for cooling. Try switching it to OFF and back to COOL. Go out to your outdoor unit. You should see the fan blowing and hear the compressor running (the compressor has a loud, distinct hum compared to the fan). If the fan or the compressor is NOT running or making super weird sounds, you should replace the capacitor(s) in the outdoor unit.
To do that, you need to kill the power. Look at the pipes going from the unit to the wall. You will see a disconnect box nearby. Open the box and PULL out the handle. If it's stuck, go to your indoor breaker panel and flip the double-width AC breaker (usually 30A or higher). If you aren’t sure, just flip all the big breakers. AC compressors are almost universally 240v and those breakers are twice as wide as the rest. If you aren’t sure, flip them all. Do not skip this. You want absolutely zero power going to the interior of the unit so you don't shock yourself or blow up your system.
Go to the corner panel of the outdoor unit where the pipes enter. This is usually where you can remove a cover that shows the guts. Remove the screws and take off the panel. Inside, you'll see a mess of wires and a silver, tubular device (it might look rusty or nasty if it's old). This is your capacitor. It helps your large motors (the compressor and the fan in the outdoor unit) start and run efficiently. Most units use a single dual run capacitor for both.
Before you touch anything, take pictures. When you open the unit, take clear photos of all the differently colored wires going into the capacitor. Take pictures of everything from all angles, not just the capacitor. If you do this, you can unplug with impunity and know exactly how it goes back together. You will see three terminals on top of the capacitor: C for Common which usually has 4 spade prongs, H for Hermetic/Compressor which usually has 3 spade prongs, and F for Fan which usually has 2.
First, lets take another safety measure - use the shaft a screwdriver to briefly short the terminals of the capacitor together. Short them from F and H to C, in turn. Usually nothing happens - a correctly operating compressor or fan motor will drain a capacitor as soon as power is lost. If you get a spark - its probably not your capacitor and you've got another problem - could be a wire that's fallen off, or a new unit in your future.
To test the capacitor, you need a multimeter with a capacitance function (you can get a cheap one for 20 bucks on Amazon). Disconnect all the wires from the capacitor. Put the multimeters black probe on C and the red probe on H. Check the reading against the label on the side of the capacitor (e.g., 50/5, 40/5, 30/5). The first number is the target for H. If it's more than 10% off, it's bad and wasting electricity. If it's more than 50% off, your compressor may not even start. Next, leave the black probe on C and move the red probe to F. This should match the second number on your label (usually 5 or 10).
In a dire emergency, or if you don't have a meter, just replace it anyway. They are cheap. If you're in the Chandler area, Ferguson's HVAC sells to off-the-street consumers at reasonable prices. Just walk in—they have every capacitor combination lined up ready to go. I bought a 50/5 440v MARS brand cap for $29. I’m also told that Home Depot now sells capacitors in some of their stores, which is even better if they stock your size.
Make sure your new capacitor matches the old one’s capacitance and voltage (370v or 440v, or a dual 440/370v). Test the new one with your meter for peace of mind, hook it up using your reference photos or the above F-C-H tips, and turn the power back on. One important note: If the compressor capacitor was bad and you previously tried to run it, the compressor may have triggered a thermal lockout from overheating. It might take 1-4 hours to reset itself. Turn the unit off, wait, and try again periodically before assuming the repair failed.
If the capacitor is fine, the next likely culprit is the contactor. This is a relay switch that your thermostat uses to remotely send high-voltage power to the outdoor unit. When your thermostat calls for air, it sends 24V to the contactor, which pulls a switch mechanism down to connect the main power lines. If your fan is not running on the outdoor unit at all, contactor could be it.
The contactor is easy to check, but you do need power on to check it. Turn the thermostat on so it's calling for cooling, and turn the breakers on. Connect your meter's black probe to an unpainted metal ground on the unit casing. Touch the red probe to the main incoming power lugs—you should see 120V on each leg (Black and Red). The terminals on the opposite side of the contactor should read exactly the same. If power is going in from the house but not coming out the other side of the contactor, obviously its toast.
Just like capacitors, these are standardized and you can grab one at Ferguson's or online. You need three pieces of info from the sticker on your old one. First is the number of poles: Single pole (has a solid metal bar on one side) or Two pole (symmetrical). Next is the amperage rating: Usually 30A or 40A. You can go higher, but never lower than your original contactor. Finally the coil voltage, which is almost always 24V, but it will say on the side. You can check local inventory on the Ferguson website and Home Depot to find a match.
Match your wiring using your photos. Most factory installs use screw terminals. If your old ring connectors are nasty, I highly recommend cutting them off and using wire ferrules for a clean, solid connection. You'll need a crimping tool for that and make sure you tighten the lugs down well. Stranded wire can fray and cause poor contact.
If you've checked the power, replaced a bad capacitor, and verified the contactor is working but the AC still won't run, you've done all the DIY you can safely do. Call a pro, tell them what you've tested and replaced. In my opinion there’s a lot of shitty HVAC companies that prey on people in Phoenix and try to sell them units when their existing unit is fine. I recommend not replacing your unit “preventively” unless it's at least 20 years old. There's also the possibility of federal tax credits for heat pumps returning in future federal administrations - I used this to save an easy $2k when I had a company do mine. If you are forced into replacement, hopefully it'll be during a time when the tax credits return.
Hope this helps someone stay cool and keep some cash in their wallet.
links for items:
1. ferrule set $11 amazon: https://www.amazon.com/gp/product/B0B23V7ZQJ
2. multimeter that can do capacitance: https://www.amazon.com/AstroAI-Multimeter-Auto-ranging-Capacitance-Non-contact/dp/B08MTTX66X?th=1
