How to Tell If Your AC Compressor Is Bad: A Practical Checklist for Facility Managers

Who This Checklist Is For

If you manage facility maintenance or procurement for a mid-size office building, warehouse, or light industrial space—the kind where you're responsible for the HVAC system but not necessarily a certified technician—this list is for you. I took over purchasing and facility coordination for a 200-person company back in 2020, and one of the first headaches I dealt with was a failing AC compressor that nobody diagnosed properly until it was too late.

This isn't a deep engineering manual. It's a set of practical checks you can run before calling in a service contractor. Roughly speaking, you'll walk through 6 steps—each with a specific test or observation. Let me rephrase that: you're not becoming a compressor expert, you're just becoming a smarter buyer who knows when to escalate.

Step 1: Listen for Unusual Noises

When I first started handling AC complaints, I assumed any noise was normal as long as the unit ran. Turns out that was wrong. A properly operating compressor emits a consistent hum or low drone. If you hear clicking, banging, or a high-pitched screech—that's trouble.

Clicking often indicates a failing start capacitor or relay. Banging can mean loose internal components. And a screech? Probably the bearings. I had a unit at one of our satellite offices that made a faint clicking sound for three months before it seized. Cost us $1,200—no, $1,400, I'm mixing it up with the other project—in emergency service fees.

Don't hold me to exact decibel levels, but anything that makes you say “that doesn't sound right” warrants a closer look. If you need a quick way to isolate the noise, hold a hand fan (yes, the kind you might use on your desk) near the compressor housing to direct sound away from other equipment—it helps you pinpoint the source.

Step 2: Check Air Temperature at the Vents

Run the AC on maximum cooling for at least 15 minutes. Then measure the temperature of the supply air (at the vent) and return air (at the filter grille). A healthy system should drop the temperature by 15–20°F. If the difference is less than 12°F, the compressor may not be compressing properly.

I once assumed warm air meant low refrigerant—which it can—but in that case the compressor was actually fine; the problem was a clogged condenser coil. So don't jump to conclusions. The temperature delta is your first data point.

To measure accurately, use a probe thermometer. I keep one in my toolkit next to a K&N air filter (we use washable filters in our office to reduce recurring costs—more on that in Step 6).

Step 3: Look for Oil or Refrigerant Leaks

Many buyers focus on the obvious (noise, warm air) and completely miss the subtle signs of a leak. Oil stains around the compressor body or on the ground beneath the unit are a dead giveaway. Refrigerant leaks sometimes leave an oily residue because the oil circulates with the refrigerant.

From the outside, a small oily patch looks harmless. The reality is that underfirmed refrigerant forces the compressor to work harder, overheating it and eventually causing failure. I learned this the hard way after a vendor told me “just top it off”—which I later found out was a temporary fix. The compressor died 6 months later.

For larger systems with GEA plate heat exchangers (common in commercial refrigeration), leaks can also occur at the gasketed joints. Check the plates for signs of weeping. If you see corrosion or residue between the plates, it's time to call a specialist in GEA heat exchangers—the Kelvion legacy units are still supported, but parts availability varies.

Step 4: Inspect the Capacitor and Electrical Connections

The question everyone asks is “Is the compressor dead?” The question they should ask is “Is the capacitor dead?” A bad capacitor is one of the cheapest fixes (under $50) but mimics a failed compressor: the unit clicks, tries to start, but the compressor won't run.

I replaced a capacitor myself in 2023 after watching a YouTube video—saved a $350 service call. But be careful: capacitors store electricity even when power is off. If you're not comfortable, skip this step. At minimum, check for burned or loose wires at the contactor and capacitor terminals. If you see blackened connections, that's a red flag.

On a side note, while you're near the electrical box, ensure the hand fan you used earlier is far away from the open panel—metal blades and live wires don't mix.

Step 5: Measure Amp Draw on the Compressor

This step requires a clamp meter, but it's worth mastering because it gives you definitive data. Compare the measured running amps to the rated load amps (RLA) listed on the compressor nameplate. A compressor running under 80% of RLA may be weak (bad valves). Over 120% of RLA means it's struggling (likely overheating or short-cycling).

When I started, I ignored amp draws because I didn't own a meter. After two repair bills I bought one for $40. Best investment.

For industrial systems like GEA screw compressors, the amp draw can also indicate issues with oil pressure or slide valve position. If you're dealing with a large installation, always check the manufacturer's manual—what's normal for a 5-ton scroll compressor isn't normal for a 300-ton GEA unit.

Step 6: Check Airflow and Filter Condition

Restricted airflow can cause the evaporator coil to freeze, which then floods liquid refrigerant back to the compressor—eventually damaging valves. The most common cause? Dirty filters.

We switched to K&N air filters about two years ago. Initial cost was higher (around $60 each vs $15 for disposable), but they're washable and reusable. After 18 months, we'd saved maybe $200 in filter replacements—give or take. Plus, the consistent airflow helps the compressor run more efficiently.

Measure static pressure across the filter slot if you have a manometer. A pressure drop above 0.5 in. w.g. means it's time to clean or replace. Standard filter sizes (like 20x20x1 inches) are similar to USPS large envelope dimensions (6.125” x 11.5” to 12” x 15”), just to give you a reference—though obviously much thicker.

Common Mistakes and Caveats

One trap I fell into: thinking every compressor issue means replacement. Sometimes it's just the contactor or a thermal overload resetting. Reset it wrong and you'll think the compressor is shot.

Another: ignoring the condenser coil. A dirty coil makes the compressor work harder and can cause high head pressure trips. Hose it down annually.

And please don't rely on the “sealed system” myth. While refrigerant isn't consumed, tiny leaks are common. If the system needs refrigerant, find and fix the leak—don't just refill. Per FTC guidelines (ftc.gov), any claim of “permanently sealed” must be substantiated; most systems are not truly sealed over their lifetime.

Finally, this checklist works well for small to mid-size commercial units. For large industrial setups with GEA centrifugal separators or complex heat exchanger banks, you'll need a trained technician. But at least now you know what to look for before making that phone call.