Aircon Not Starting Capacitor Fix

Case Details

What We Checked

• Indoor unit was sending the start signal correctly — communication link tested with a multimeter at the outdoor terminal block confirmed the control voltage was present and stable.
• Outdoor fan motor ran normally when powered directly — windings tested with a megger and showed healthy insulation resistance with no short to ground.
• Compressor hummed briefly on each start attempt, drawing a momentary surge current before the thermal overload protector tripped and cut power within two to three seconds.
• Run capacitor measured at 18μF against a rated value of 35μF printed on the casing — barely half the required capacitance, well below the starting threshold for this compressor.
• Compressor winding resistance checked across all three terminals — readings balanced and within the manufacturer's specification, confirming the motor was intact behind the failed capacitor.

The Diagnosis

Run capacitors contain an insulating material that stores and releases electrical energy to provide the phase-shifted current a single-phase compressor motor needs during startup. Over thirteen years of operation in Singapore's heat, the insulating layer had gradually degraded — a natural ageing process accelerated by high ambient temperatures inside the outdoor unit casing. The capacitor's measured value had dropped well below the rated value printed on its casing. Without enough capacitance, the motor could not generate the rotational torque it needed to overcome the compressor's starting load. On each attempt, the motor stalled, drew excess locked-rotor current, and the thermal overload protector cut power within seconds to prevent winding damage. From the outside, the unit appeared completely dead. But the compressor windings were intact, the fan motor ran normally, and the outdoor PCB was sending the correct start signals. The motor was healthy — it simply lacked the electrical push to turn over.

What Fixed It

We fitted a replacement capacitor matching the rated 35μF capacitance and 440V voltage rating printed on the original casing. The compressor started on the first attempt without hesitation — no hum, no stall, no overload trip. We ran a full cooling cycle while monitoring compressor supply current with a clamp meter, reading a steady 5.4A against the rated maximum of 6.5A for this LG model. We checked discharge temperature at the outdoor coil with a contact probe and measured the temperature differential across the indoor coil — both readings confirmed the refrigerant charge was still adequate after the idle period. We also verified the outdoor fan motor current and the contactor contact condition while the panel was open. Everything was within specification and no other components required attention.

One component swap restored full operation. The outdoor unit, compressor, and all existing piping were retained without any further work.

Why This Happens

A silent outdoor unit is not the same as a dead one.

• Run capacitors supply the phase-shifted current a single-phase compressor needs to generate starting torque. When the capacitor degrades, the motor stalls on every startup attempt and the thermal overload protector cuts power. The unit looks completely dead from the outside, which is why it gets mistaken for a compressor failure.
• A capacitor test with a meter takes under two minutes and costs nothing. It confirms or rules out the cheapest fix before any larger diagnosis begins. If your technician does not carry a capacitance meter, that is a concern.
• The classic capacitor failure pattern is distinctive: the compressor hums briefly, then goes silent as the overload trips. A seized compressor draws locked-rotor current and may trip the circuit breaker. A dead PCB produces no hum at all. Each pattern points to a different cause.
• Compressor health, refrigerant integrity, and fan motor condition are what determine whether a system is worth keeping — not the number of years since installation. A thirteen-year-old unit with a healthy compressor and intact piping has years of service left after a capacitor swap.