Bad Compressor or Just the Capacitor? How to Tell

The most expensive mistake in HVAC is condemning a compressor that was fine. The compressor is the priciest part of the system, and telling a client it needs replacing when the problem was a few-dollar capacitor destroys your credibility. This guide helps you diagnose in the right order.

Symptoms that get confused

When the unit powers on but the outdoor unit won't start, or the compressor "hums" and shuts off, the cause could be any of these, cheapest to most expensive:

Rule out the capacitor first

1. Cut power and discharge the capacitor before touching it.
2. Measure it with a multimeter in microfarads (µF) and compare to the printed rating.
3. If it's out of range (or bulging), replace it: in many cases the compressor starts right away.

Confirm the compressor really is bad

Only after ruling out capacitor and contactor:

• Measure resistance between terminals (C, S, R) against spec.
• Check it isn't grounded (continuity to the shell = grounded compressor, failed).
• Check start-up draw: a seized compressor trips the protection.

If everything cheap checks out and the compressor is grounded or seized, then replacement is warranted.

Orderly diagnosis = confident quoting

When you diagnose by elimination and document it, you can quote the repair with confidence: the client understands what was tested, what failed and why that's the cost. A clear estimate, with parts and labor itemized, gets approved faster and protects your margin.

Frequently asked questions

How do I know if it's the capacitor or the compressor? Always measure the capacitor first: it's cheap and fails often. Only if it's fine and the compressor is grounded or seized is it the compressor.

How much does a capacitor cost vs. a compressor? A capacitor is a fraction of the cost. The compressor means an expensive part, refrigerant and labor hours, which is why you rule out the cheap stuff first.

Is it worth repairing an old compressor? It depends on the unit's age. If it's out of warranty and very old, sometimes proposing a full unit replacement makes more sense.