Service Call
Customer says AC does nothing. Thermostat is blank. What are 3 things you check first?
Breaker/power, thermostat, and 24V control power. Also accept transformer, fuse, float switch.
Contactor won't pull in. You have 0V across the coil. What circuit should you troubleshoot?
A: The 24V control circuit.
System isn't cooling. Before connecting gauges, what should you check?
A: Airflow—filter, blower, coil, registers/ductwork.
Evaporator is freezing. Name one airflow problem that could cause it.
A: Dirty filter/coil, failed blower, wrong blower speed, blocked return, etc.
Customer says "AC doesn't work." What's the first information you want from them?
A: Ask questions and verify the complaint.
Indoor blower runs but outdoor unit doesn't. Where do you start troubleshooting?
Check whether outdoor unit has line voltage AND a 24V cooling call.
You have 24V across the contactor coil, but it doesn't pull in. What's your likely problem?
A: Failed/stuck contactor or open coil.
Suction pressure is low and superheat is high. What are two possible causes?
. A: Low refrigerant charge or restriction/starved evaporator.
Filter is extremely dirty. What effects could this have on the system?
A: Low refrigerant charge or restriction/starved evaporator.
You find a failed capacitor. Before replacing it and leaving, what else should you check?
A: Motor/compressor amps, correct µF, voltage, connections and motor condition.
Outdoor fan runs but compressor doesn't. Give me at least 3 possible causes.
A: Capacitor, compressor overload, bad contactor/contact, wiring, low voltage, failed compressor/start components.
You have 240V LINE side of the contactor and 0V LOAD side while it's pulled in. What failed?
A: Failed contactor contacts.
Superheat is high AND subcooling is low. What does that point toward?
A: Undercharge/low refrigerant, assuming airflow is correct.
Blower is running, but airflow from the registers is weak. Give 3 possible causes.
A: Filter, dirty coil/wheel, blower speed, duct restriction, closed damper/register, collapsed duct.
Breaker is tripped. Do you reset it immediately? What should you do first?
A: No. Inspect/test for the reason it tripped before repeatedly resetting it.
System cools normally for 20 minutes, then compressor shuts off while condenser fan continues running. What should you investigate?
A: Compressor overload—check amps, voltage, capacitor, head pressure, condenser airflow, temperature and connections.
Transformer keeps blowing its low-voltage fuse. What type of problem are you looking for?
A: A 24V short—trace R through controls/safeties/wiring to C.
Suction is low, superheat high, but subcooling is high. Does this still look like a simple undercharge? What else might you suspect?
A: Refrigerant restriction/starved evaporator—TXV, filter-drier, liquid-line restriction, etc.
Supply static is +0.42" WC and return static is -0.31" WC. What's TESP, and what does that number tell you?
A: 0.73" WC. Compare it to manufacturer specifications; it may indicate excessive restriction.
You replace a failed blower capacitor. The new capacitor fails two weeks later. What should you investigate instead of replacing another capacitor and leaving?
A: Find the root cause—motor amps, µF rating, voltage, bearings, wheel, heat, wiring and motor condition.
Customer says AC works during the morning but struggles every afternoon. Give me a diagnostic plan instead of guessing what's wrong.
A: Verify complaint → conditions/load → airflow/static → ΔT → electrical → refrigerant → duct/building load → prove diagnosis.
Contactor has 24V sometimes, then loses it while thermostat is still calling. What safety/control devices should you investigate?
A: Trace the control circuit—float switch, pressure switches, board, thermostat wiring, connections and safeties.
System has high head pressure, high subcooling, and normal/high superheat. Give possible causes and tell me what measurement you'd take next.
A: Overcharge, condenser airflow problem, dirty condenser, non-condensables or restriction depending on other readings.
System has correct refrigerant charge but poor cooling capacity. Static pressure is high. Explain how you'd determine where the airflow restriction is located.
A: Take static-pressure measurements across filter, coil, blower and duct system to isolate the excessive pressure drop.
You've replaced the same part twice. It failed again. What's wrong with your troubleshooting approach, and what should you do differently?
A: You're treating the symptom instead of finding the root cause. Diagnose the entire system before replacing it again.
AC runs all day. Return = 74°F, supply = 62°F. What's your next move?
A: ΔT is only 12°F. Investigate airflow, heat load, coil performance and refrigerant operation before diagnosing.
You read 240V at disconnect with system OFF, but voltage collapses when compressor starts. What does this suggest?
A: High-resistance/poor power connection or supply issue. Test voltage under load and isolate where the drop occurs.
Low suction alone means low refrigerant. True or false?
False. Airflow problems, restrictions, load conditions and other faults can also produce low suction.
Filter pressure drop is extremely high. What does that tell you?
A: The filter/filter system is creating excessive airflow resistance.
Another tech already replaced 3 parts and system still doesn't work. What's your first move?
A: Start the diagnosis over. Verify the complaint and take your own measurements instead of trusting previous diagnoses.