You cannot tell from symptoms. That is the honest answer, and it is why this decision goes wrong so often.
A system blowing warm air might need a $250 part or a $9,000 replacement. The symptom is identical. What separates the two is six measurements, and every one of them has a passing range you can check yourself.
If you got a recommendation without those numbers, you did not get a diagnosis. You got an opinion.
Here is how to read the report.
Symptoms Do Not Decide This. Measurements Do.
Consider two houses on the same street, both with an eleven-year-old system, both not cooling in July.
House one has a capacitor reading 31 microfarads against a 45 rating. That is a part, an hour, and a few hundred dollars. The system has years left.
House two has a compressor drawing amps well past its nameplate rating and a coil leaking refrigerant it can no longer hold. That is a replacement conversation.
Same age. Same symptom. Nothing else in common.
So when a technician arrives, the useful question is not “what do you think.” It is “what did you measure, and what is that supposed to read.”
The Six Numbers That Decide It
Ask for these in writing. A NATE-certified technician takes all of them as a matter of course, and any of them can be explained to you in a sentence.
1. Temperature Split, Target Roughly 16 to 22 Degrees
This is the difference between the air entering the return and the air leaving a supply register, measured with a thermometer.
A healthy system moves that air down by roughly 16 to 22 degrees. It is the fastest whole-system health check available, and you can take it yourself with a $10 thermometer.
What a failing number means. Under about 16 degrees, something in the list below is wrong. Over about 22 degrees, you likely have an airflow restriction, which is its own problem. A split inside the range with a house that still will not cool points to capacity, ducts, or insulation rather than broken equipment.
2. Capacitor Capacitance Against Its Rated Value and Stamped Tolerance
The run capacitor starts and steadies the fan and compressor motors. It is the single most common failure in a residential AC and one of the cheapest.
Run capacitors carry a tolerance printed on the can itself, typically plus or minus 5 or 6 percent. So a 45 microfarad capacitor stamped at 6 percent passes anywhere from about 42.3 to 47.7. Trade guidance on capacitor testing treats readings outside the stamped tolerance as grounds for replacement.
Ask for three things: the measured value, the rated value, and the tolerance stamped on the part.
Why this matters more than it sounds. A capacitor at 31 against a 45 rating has not failed yet. It will fail in August, and while it is weak the compressor is straining every time it starts. Catching it is the difference between a part and a compressor.
3. Compressor Amp Draw Against Nameplate Rated Load Amps
Every compressor has a nameplate listing its rated load amps. A technician clamps the wire and compares.
A compressor pulling well within its rating in moderate weather is healthy. One pulling at or above its rating, especially on a mild day, is working too hard and telling you something.
What a failing number means. High amp draw can come from a weak capacitor, a dirty condenser, an overcharge, or genuine mechanical wear inside the compressor. That is why this number is read alongside the others rather than alone. A high reading on its own is not a death sentence. A high reading with a good capacitor and a clean coil usually is.
4. Superheat and Subcooling
These two readings tell a technician whether the refrigerant charge is correct, and they are how a real diagnosis distinguishes “low on refrigerant” from “there is a leak.”
What matters to you. Refrigerant is not consumed. A sealed system that is low is leaking. If someone tells you the system needs refrigerant and cannot tell you where it is escaping, you are being sold a recharge that will need repeating.
Ask where the leak is. Coil, line set, service valve, or fitting. Those are four different repairs at four very different prices, and one of them frequently tips the decision toward replacement.
5. Static Pressure Across the Air Handler
This is the one almost nobody measures, and it explains a large share of systems that “never cooled right.”
High static pressure means the blower is fighting the duct system. Undersized returns, crushed flex, closed dampers, and restrictive filters all show up here.
Why it changes the decision. If static pressure is high, a new system installed into the same ductwork will underperform exactly like the old one did. Replacing equipment without fixing airflow is how homeowners spend five figures and stay uncomfortable.
6. The Refrigerant Printed on the Outdoor Nameplate
Not a measurement, but it belongs on the list because it moves the math more than anything else here.
R-22, in systems roughly 2011 and older. Production ended in 2020, so only reclaimed supply remains and it prices accordingly. A leak in an R-22 system generally means replacement.
R-410A, roughly 2011 through 2025. Servicing is legal and refrigerant is available. Costs will climb gradually as the phasedown continues.
R-454B or R-32, 2025 and newer. These are A2L refrigerants with factory leak detection built in. On these systems a sensor fault can lock out cooling with a perfectly healthy refrigerant circuit, which looks identical to a failure. If your system is this new, ask specifically whether a fault code was pulled before anything else is discussed.
Reading the Numbers Together
The combinations matter more than any single reading.
Points to repair: low split, failed capacitor, healthy amp draw, correct charge or a locatable leak in an accessible spot, normal static pressure. That is a part.
Points to repair with caution: low split, good capacitor, high amp draw, correct charge. The compressor is aging. Fix what is fixable and start planning.
Points to replace: low split, good capacitor, high amp draw, low charge with a coil leak, on a system past ten years. Every reading is telling the same story.
Points to something other than the AC: healthy split, good capacitor, normal amp draw, correct charge, high static pressure. Your equipment is fine and your duct system is not.
That last case is the one homeowners most often get sold a replacement for.
Four Findings That Mean Replace, Whatever the Age
Some results end the conversation.
Grounded or shorted compressor windings. A resistance-to-ground test tells a technician the motor has failed internally. Ask for the reading.
A compressor burnout with acid in the system. After a burnout, contamination spreads through the refrigerant circuit. Cleanup on a system of any age is rarely worth it.
A leaking evaporator coil on an R-22 system. The coil cost plus reclaimed refrigerant cost usually exceeds any reasonable share of a replacement.
Multiple simultaneous failures. A bad capacitor is a part. A bad capacitor plus a leaking coil plus a struggling compressor is a system telling you it is finished.
The Diagnosis You Should Not Accept
Four patterns should slow you down.
- “It’s low on refrigerant,” with no leak location identified
- “The compressor is going bad,” with no amp reading against the nameplate
- A replacement quote with no temperature split and no capacitor test recorded anywhere
- Same-day pressure. “One unit left in stock” and “this price is good today” are sales tactics, not engineering. A failed AC in July is uncomfortable. It is not a decision that has to be made in an hour.
None of that means the technician is dishonest. Plenty of good techs are rushed and skip the write-up. But you are the one signing, and you are entitled to the readings.
Get a Second Opinion, and Bring the Right Things
If a replacement was recommended and you are unsure, a second opinion is the cheapest money in this entire process. JonLe provides them at no charge, which exists precisely for this situation.
Bring five things:
- The written diagnosis and quote you already have
- Model and serial numbers from both the outdoor unit and the indoor unit
- Any measurements you were given
- The age of the system and the furnace or air handler it sits on
- Two or three summer electric bills, if you have them handy
And know what a real second opinion looks like. It is an independent set of measurements, not a re-quote of the first company’s conclusion at a different price. If the second technician does not take their own readings, you have two opinions and still no diagnosis.
Sometimes a second opinion confirms the first. That is a useful outcome too. You buy the replacement knowing why.
Where Cost Enters the Picture
Briefly, because the measurements come first.
Once you know which repair you are actually facing, the arithmetic is straightforward. Compare the repair against a real replacement quote, factor the system’s remaining life, and know that federal help changed. The Section 25C tax credit expired for anything placed in service after December 31, 2025, per IRS guidance on the OBBB provisions, and Ohio has no general utility rebate to replace it.
That makes keeping a repairable system alive worth more this year than last, which is another reason to insist on the numbers before agreeing to replace anything.
Frequently Asked Questions
Can I Take Any of These Measurements Myself?
The temperature split, yes, with a cheap thermometer at a supply register and the return. The rest require meters and gauges, and the outdoor electrical compartment holds a charge even with power off. Take the split, then let a technician do the rest.
Is a $79 Diagnostic Fee Worth Paying?
If it produces written measurements, yes, and it is the best value in this process. If it produces a verbal opinion and a replacement quote, you paid for a sales call.
My System Is 14 Years Old. Should I Just Replace It?
Not automatically. Age raises the odds that the numbers point to replacement. It does not substitute for taking them. Plenty of 14-year-old systems need a capacitor and run several more summers.
How Long Should a Proper Diagnosis Take?
Plan on 45 to 90 minutes for a real one. Anything much faster than that did not include static pressure or a full charge check.
Get the Numbers Before You Decide
The repair-or-replace question feels like a judgment call. It is not. It is six readings, and every one of them has a passing range.
JonLe has worked Cincinnati systems since 1959, our technicians are NATE-certified, and we write down what we measure. If your capacitor is the problem, that is what the report will say.
Book AC repair if you need a diagnosis, or schedule a free second opinion if you are holding a replacement quote you are not sure about. If the numbers do point to replacement, we will walk you through AC replacement options and financing.
Call us today to schedule service online.