HVAC HVAC Troubleshooting Techniques 2 — Questions and Answers
Question 1: A heat pump runs continuously but fails to reach the set temperature in heating mode. What is the FIRST component to inspect?
- Reversing valve
- Thermostat wiring
- Outdoor coil for ice buildup (Correct answer)
- Blower motor capacitor
Correct answer: Outdoor coil for ice buildup
A frozen outdoor coil severely restricts heat absorption and is the most common cause of a heat pump failing to heat adequately in cold weather.
Question 2: A technician measures superheat of 25°F on a TXV-equipped system. What does this indicate?
- System is overcharged with refrigerant
- TXV bulb may have lost its charge or TXV is underfeeding (Correct answer)
- Condenser fan motor is failing
- Filter-drier is saturated with moisture
Correct answer: TXV bulb may have lost its charge or TXV is underfeeding
High superheat on a TXV system typically points to a restricted or malfunctioning TXV that is underfeeding refrigerant to the evaporator.
Question 3: During a gas furnace call for heat, the inducer motor starts but the ignitor never glows. Which component is most likely at fault?
- Pressure switch (Correct answer)
- Flame sensor
- Gas valve solenoid
- Limit switch
Correct answer: Pressure switch
If the pressure switch fails to close after the inducer starts, the control board will not energize the ignitor because it has not confirmed proper draft.
Question 4: An air conditioner short-cycles off on the high-pressure cutout. Outdoor ambient is 95°F and airflow over the condenser coil is adequate. What should the technician check next?
- Evaporator coil for ice
- System refrigerant charge for overcharge (Correct answer)
- Compressor start capacitor
- Return air filter
Correct answer: System refrigerant charge for overcharge
With adequate condenser airflow and high ambient, an overcharged system is the primary suspect for tripping the high-pressure cutout.
Question 5: A customer reports their split system produces a musty odor whenever it runs. What is the most likely cause?
- Refrigerant leak at the evaporator coil
- Microbial growth on the evaporator coil or drain pan (Correct answer)
- Cracked heat exchanger
- Dirty condenser coil
Correct answer: Microbial growth on the evaporator coil or drain pan
Musty odors from HVAC systems are almost always caused by mold or bacteria growing on the wet evaporator coil surface or standing water in the drain pan.
Question 6: A variable refrigerant flow (VRF) system shows a communication error between the outdoor unit and one indoor unit. What is the FIRST diagnostic step?
- Replace the outdoor unit control board
- Check refrigerant charge in the affected branch circuit
- Inspect the communication wiring for polarity, continuity, and loose connections (Correct answer)
- Recharge the system with refrigerant
Correct answer: Inspect the communication wiring for polarity, continuity, and loose connections
Communication errors in VRF systems are most commonly caused by wiring faults such as reversed polarity, broken conductors, or loose terminals rather than refrigerant or board issues.
Question 7: A furnace control board locks out after three ignition attempts. The flame sensor reads 0.5 µA during a trial-for-ignition test with a visible flame. What is the problem?
- Gas valve is defective
- Flame sensor is contaminated or coated with oxide (Correct answer)
- Inducer motor is undersized
- Ignitor is cracked
Correct answer: Flame sensor is contaminated or coated with oxide
A flame sensor reading below 1–2 µA with a visible flame indicates the sensor rod is coated with oxidation or residue and cannot conduct enough current to prove flame.
HVAC HVAC Troubleshooting Techniques 2
A heat pump runs continuously but fails to reach the set temperature in heating mode.
What is the FIRST component to inspect?