Compressor Won't Start: A Checklist
You pulled up to a “no cooling” call, the outdoor fan may or may not spin, and the compressor sits there humming or dead silent. Work this checklist in order — power, capacitor, contactor, windings — and you'll land on the fault fast without condemning a good compressor.
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In This Guide
Safety First: Lockout and Capacitor Discharge
A no-start compressor call puts your hands near line voltage, a charged capacitor, and a system that can spin up the instant power returns. Before anything else, verify the thermostat is calling for cooling, then pull the outdoor disconnect and lock it out. Confirm zero volts at the contactor load side with your meter — trust the meter, not the switch.
Discharge the capacitor every time
A run capacitor can hold a lethal charge for minutes after power is removed. Bleed it across its terminals with an insulated 15k–20k ohm resistor (or a proper discharge tool) before you touch a single wire. Never short it with a screwdriver — that welds contacts, damages the cap, and throws sparks in a tight cabinet.
Keep your EPA 608 obligations in mind too: this is an electrical diagnosis, so there is no reason to break into the refrigerant circuit. If your diagnosis ends at a failed compressor, recover the charge properly before any brazing.
Step 1: Confirm Power and Control Voltage
Half of “dead compressor” calls are really dead power. Before you condemn anything expensive, prove that voltage is actually reaching the compressor terminals.
- Line side: Confirm ~240V (or the nameplate voltage) between L1 and L2 at the line side of the contactor with the disconnect closed.
- Control voltage: On a cooling call, you should read 24V across the contactor coil. No 24V means the problem is upstream — transformer, thermostat, float switch, high/low pressure lockout, or a tripped breaker on the air handler.
- Load side: With the contactor pulled in, verify line voltage is passing through to the load side and out to the compressor.
Common trap: A tripped condensate float switch or an open low-pressure switch will kill the 24V signal and leave the compressor silent. Don't chase the compressor until you've confirmed the coil is actually being energized.
If you have line voltage at the compressor terminals and it still won't run, move on — the fault is the capacitor, the windings, or an internal overload.
Step 2: Test the Run Capacitor
A failed run capacitor is one of the most common reasons a single-phase compressor hums but won't start. The capacitor provides the phase shift that gets the start winding doing its job; without it, the motor can't develop starting torque and sits there drawing locked-rotor current until the internal overload opens.
With the cap discharged and disconnected, set your meter to capacitance and read across the terminals. Compare to the printed rating on the can.
The 6% Rule
Replace the capacitor if the measured MFD is more than 6% off the rated value, or if it reads open or shorted. A 45 MFD cap reading 39 MFD is out of tolerance and will cause hard starting or a no-start. A dual run cap with a good HERM leg but a dead FAN leg (or vice versa) still gets replaced as a unit.
Replacement rules
- Match the MFD rating to the nameplate exactly — both the HERM (compressor) and FAN values on a dual cap.
- Voltage rating can be equal or higher, never lower. A 440V cap can replace a 370V cap; the reverse will fail early.
- Bulging tops, oil leakage, or rust are visual condemnations — replace on sight even if it still reads close.
Common dual-run capacitor sizes by unit tonnage:
| Unit Size | Typical Dual Run Cap (HERM/FAN) | Notes |
|---|---|---|
| 1.5 Ton | 30/5 MFD | Verify nameplate |
| 2 Ton | 35/5 MFD | Verify nameplate |
| 2.5 Ton | 40/5 MFD | Verify nameplate |
| 3 Ton | 40/7.5 or 45/5 MFD | Varies by OEM |
| 3.5 Ton | 45/5 or 50/5 MFD | Varies by OEM |
| 4 Ton | 50/7.5 or 55/5 MFD | Varies by OEM |
| 5 Ton | 60/5 or 70/5 MFD | Verify nameplate |
These are typical starting points, not substitutes for the data plate. Always match what the manufacturer specifies.
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Step 3: Inspect the Contactor
The contactor is the workhorse relay that carries line voltage to the compressor and outdoor fan whenever the 24V coil is energized. It arcs every cycle, so pitting and welding are extremely common failure modes.
- Pitted or burned contacts: Heavy resistance across worn contacts drops the voltage the compressor actually sees, causing hard starting or a no-start. Pull the disconnect and inspect the contact faces.
- Welded contacts: If the contacts fuse together, the compressor may run continuously and ignore the thermostat — the flip side of a no-start, but the same worn part.
- Coil test: On a call, you should read 24V across the coil. If you have 24V but the contactor doesn't pull in, the coil is open — replace it.
- Chatter: A buzzing or chattering contactor usually means low control voltage or a weak transformer, not necessarily a bad contactor.
Quick voltage-drop check
With the contactor pulled in and the unit trying to run, measure voltage across the closed contacts (line side to load side of the same pole). You should read close to 0V. A reading of several volts across a closed contact means the contacts are burned and starving the compressor.
Residential contactors are typically 30A or 40A with a 24VAC coil. When you replace one, match the amp rating and pole count, and confirm the coil voltage.
Step 4: Ohm the Compressor Windings
If power reaches the terminals, the capacitor is good, and the contactor passes voltage cleanly, it's time to test the compressor itself. Pull the terminal cover and identify the three terminals: C (Common), R (Run), and S (Start). With power off and leads removed, measure resistance between each pair.
Worked example: reading a single-phase compressor
You're on a 3-ton R-410A condenser that hums for a couple of seconds and trips out. Capacitor and contactor checked good. You ohm the terminals.
Step 1: Measure C to R (the run winding)
C–R = 1.2 Ω (lowest resistance path)
Step 2: Measure C to S (the start winding)
C–S = 3.4 Ω (higher than the run winding)
Step 3: Measure S to R (the total)
S–R = 4.6 Ω
Step 4: Verify the winding math
C–R (1.2) + C–S (3.4) = S–R (4.6) ✓ Windings are intact
The rule to memorize
On a healthy single-phase compressor: C–R + C–S = S–R. The run winding (C–R) always reads the lowest resistance, the start winding (C–S) reads higher, and the largest reading is between S and R.
If any pair reads infinite / OL, that winding is open. If a reading is near zero between two windings, they're shorted together. Either result condemns the compressor.
Then check to ground
Put one lead on each terminal in turn and the other on clean copper (the suction line or the compressor shell). You should read OL / infinite resistance to ground. Any continuity — even a few hundred thousand ohms — means a grounded (shorted-to-shell) winding. That compressor is dead, and on a burnout you'll need to clean the system and install a suction line drier.
Locked Rotor vs. Open Overload
Sometimes the windings ohm out fine but the compressor still won't turn. Two scenarios explain most of these, and a clamp meter on the common leg tells them apart.
Locked Rotor
The compressor hums and immediately draws locked-rotor amps (LRA) — roughly 5 to 6 times running amps — then trips the internal overload. Windings good, voltage good, capacitor good, but the pump is mechanically seized. This is a mechanical failure; the compressor is done.
Open Internal Overload
A hot compressor may open its internal overload and read OL from Common until it cools. Let it sit 30–60 minutes, then re-ohm. If continuity returns and it runs, look for the root cause of the overheating — overcharge, dirty condenser, high head pressure, or a weak capacitor.
Clamp the common leg on start-up and compare to the nameplate. Reference the data plate values:
- RLA (Rated Load Amps): The compressor should settle below this once running. Drawing above RLA points to high head pressure, overcharge, or a mechanical bind.
- LRA (Locked Rotor Amps): The inrush spike at start. If the unit sits at LRA and never drops into running range, the rotor is locked.
Field note: A weak run capacitor can mimic a locked rotor — the motor can't make torque, so it draws LRA and trips. This is exactly why you test the capacitor (Step 2) before condemning the compressor. Never call a compressor bad on a hum-and-trip alone.
When a Hard Start Kit Helps
If a compressor is slow to start, groans on start-up, or occasionally trips but otherwise ohms out healthy, a hard start kit (a start capacitor plus a potential relay, or a solid-state PTC device) can boost starting torque. It's a legitimate fix for an aging compressor with worn bearings or a system with a long line set and low starting voltage.
Use it as a real fix, not a bandage
A hard start kit is appropriate for genuinely hard-starting units and TXV systems that don't equalize between cycles. It is not a way to nurse a compressor with open, shorted, or grounded windings — those are dead regardless. Confirm the windings and capacitor are good first, then add the kit to solve a torque problem.
On single-phase scroll compressors, low-voltage starting is a common culprit. Check voltage at the compressor during the start attempt — if it sags well below nameplate, chase the voltage-drop source (loose lugs, burned contactor, undersized or long conductors) before reaching for a start kit.
Field Tips and Common Misdiagnoses
- Fan spins, compressor doesn't: On a dual cap, the FAN terminal is often fine while the HERM terminal is dead. Prove the compressor leg of the capacitor independently.
- Both dead: If neither the fan nor compressor runs but you have 24V at the coil, suspect the contactor or a completely failed capacitor before the compressor.
- Intermittent no-start on hot afternoons: Classic thermal overload trip from high head pressure — check the condenser coil, charge, and capacitor before condemning the pump.
- Reads good cold, dead hot: Internal overload opening on temperature. The compressor isn't necessarily bad; find why it's running hot.
- Trust the winding math: C–R + C–S = S–R keeps you honest. If the numbers don't add up or any leg reads OL or ground, you have your answer.
Worked in order, this checklist catches the overwhelming majority of no-start calls at the capacitor or contactor — the two cheapest, most common failures — and reserves the compressor verdict for the times the windings or a locked rotor actually prove it.
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