You know the call. Same customer, or a new one two doors down, same complaint, on the same string of 95 degree afternoons: the AC is running and the space is not cooling, or the breaker keeps tripping, or the compressor hums for a second and then quits. Every instinct says compressor, capacitor, or a refrigerant charge that walked off since your last visit. Sometimes that is exactly right. During a heat wave in New York City, there is another cause worth ruling in or out before you start swapping parts, and it does not live inside the unit at all.
Con Edison reduces voltage on purpose during extreme heat, by 5 to 8 percent depending on the network, as a way to protect grid equipment and avoid a full outage. It has done this for years, and it happened repeatedly again this past July across parts of Brooklyn, Queens, the Bronx, and Westchester. When it happens in a network you are working in, every piece of motor-driven equipment on that circuit is suddenly running on less than it was designed for, at the exact moment demand on it is highest.
The symptom pattern is specific enough to be a real diagnostic clue, not just a coincidence. Undervoltage shows up as higher than expected amp draw on the compressor, contactor chatter, hard starting, thermal overload trips, or a unit that runs constantly without reaching setpoint. That is a lot of overlap with a genuinely failing compressor. The tell is in the pattern across calls, not just one unit: multiple no-cooling complaints in the same network, in the same window of days, on equipment that was working fine the week before.
Before you replace anything, put a meter on the incoming voltage at the disconnect or panel, under load, during the actual high-demand period, not first thing in the morning when the grid is quiet. Most HVAC equipment is only rated to tolerate roughly a 10 percent swing from nameplate voltage. Compare what you measure against the equipment's rated voltage and you will know within minutes whether you are looking at a supply problem or an equipment problem.
📋 Field habit: Document the actual measured voltage on the ticket, not just "voltage checked, OK" or "voltage low." A specific number protects you if the callback repeats next week during the next heat spike, and it is the difference between a documented diagnosis and a guess that happened to be right once.
If the measured voltage is genuinely running low relative to the equipment's rating, and the pattern lines up with known utility voltage reduction rather than a one-off equipment failure, a buck-boost transformer is the standard fix. It corrects the incoming voltage before it reaches the equipment, sized to the actual load amperage, not guessed at.
This is also where a lot of otherwise capable installs go wrong. Buck-boost wiring configurations change depending on whether you need to step voltage up or down, and additive versus subtractive polarity is not interchangeable. Get it backwards and you can push voltage further from where it needs to be instead of closer, on equipment that was already struggling. If this falls outside your license scope, this is exactly the kind of job worth partnering with a licensed electrician on rather than pushing through it solo. Getting it right the first time matters more than getting it done today.
A measured voltage reading, written down with the actual number, is the same discipline that shows up everywhere else in a well-run service call. It is a Complaint, Cause, Correction record applied to a problem that happens to live outside the unit instead of inside it. It also gives you something concrete to hand the customer that explains why the fix is not a new compressor, which is a much easier conversation to have with a number in hand than with a hunch.
The compressor is not always guilty just because it is the thing that stopped running. Check the supply first. It is the difference between fixing the actual problem once and replacing the wrong part every summer.