Controls, Electrical, and Troubleshooting
What this domain covers
This domain tests low-voltage control circuits, thermostat signals, electrical measurements, safety devices, and a logical troubleshooting sequence. The exam favors questions that ask what to check first or how to interpret a meter reading.
Core rules
- Thermostat terminals. R is 24V power, C is common, W calls heat, Y calls cooling, G calls fan, and O/B energizes the reversing valve. A heat pump uses O or B depending on whether the valve is energized in cooling or heating.
- Sequence. A call at Y sends 24V to the contactor coil; the contactor closes line voltage to the compressor and condenser fan. A call at G starts the indoor blower; W starts heat.
- Electrical formulas. Ohm's law: V = I × R; power: P = V × I. A capacitor stores starting torque; a contactor switches line voltage; a transformer steps 240/208V down to 24V.
- Safeties. High/low pressure switches, float switches, limit switches, and flame sensors can lock the system out. Never bypass a safety to diagnose; prove the cause.
- Troubleshooting order. Verify the call, control voltage, line voltage, airflow, then refrigerant pressures and temperatures.
Worked example
A run capacitor is rated 45 µF ±10%. A meter reads 36 µF. The lower limit is 45 − 4.5 = 40.5 µF, so 36 µF is about 20% low and the capacitor is failed. Replace it with the same microfarad rating and an equal or higher voltage rating. Do not substitute a higher microfarad value; that can overheat the motor.
Traps that cost marks
- Testing a capacitor with power applied instead of discharging it.
- Replacing a capacitor with the wrong µF or lower voltage rating.
- Assuming the thermostat is bad before checking the 24V transformer and fuse.
- Jumpering a pressure or float safety instead of finding the cause.
- Checking refrigerant charge before confirming airflow and control voltage.