Service, Troubleshooting, and Efficiency
The official CSLB outline calls this content area Troubleshooting, Repair, and Maintenance and puts it at roughly 20 percent of the examination. Troubleshooting questions are written as short scenarios: a set of symptoms and readings, and four candidate causes. The way to answer them reliably is not to memorize scenarios but to know what each measurement can and cannot distinguish. This chapter organizes the material that way, starting with the electrical side, then the gas furnace sequence of operation, then the airflow-versus-charge problem that underlies most no-cooling calls, and finally the efficiency ratings and the customer conversations that go with them.
Capacitors, Contactors, and Transformers
A run capacitor provides a phase shift to a single-phase motor's start winding so the motor runs smoothly and efficiently; it stays in the circuit the whole time the motor runs. A start capacitor provides a much larger momentary boost of starting torque and then drops out of the circuit, usually through a potential relay. The classic field symptom that points straight at a capacitor is a condenser that hums but does not start, where both the fan and the compressor begin running after you spin the fan blade by hand: the motor has no starting torque because the capacitor is not providing the phase shift. Test the capacitor against its rated microfarad value. A dual run capacitor marked 45 and 5 microfarads serves the compressor with the larger value and the condenser fan motor with the smaller. A 45 microfarad capacitor measuring 31 microfarads is far outside tolerance and gets replaced, not watched. Always discharge a capacitor safely across a resistor with power removed before handling it. A contactor switches line voltage to the compressor and the condenser fan on a call from the 24-volt control circuit. Visibly pitted contacts with a compressor that chatters at startup are a real hazard, because voltage drop and arcing across the contacts damages the compressor windings over time. A quick diagnostic: 240 volts measured across the open line terminals with the unit off, and 0 volts across the contacts once the contactor pulls in, is a contactor operating normally, because a closed switch should have no voltage drop across it. A 24-volt control transformer that burns out twice is not a bad transformer; look for a shorted control conductor or a failed contactor coil drawing excessive current.
The Gas Furnace Sequence of Operation and Where It Breaks
Knowing the sequence turns furnace troubleshooting into a matter of finding the step that did not happen. On a modern induced-draft furnace, the thermostat closes the heat circuit, the inducer starts and pulls a negative pressure that closes the pressure switch, the ignition sequence begins with a hot surface igniter or spark, the gas valve opens, the flame is proved by the flame sensor, and after a delay the main blower starts. If the inducer runs but the pressure switch never closes, the switch is usually telling the truth: check for a blocked vent or termination, a plugged condensate drain on a condensing furnace, a disconnected or cracked sensing hose, or a failing inducer. A flame sensor proves flame by rectification, passing a small direct current through the flame from the sensor rod to ground, so a coating of oxide on the rod interrupts a very small current. That produces the most recognizable furnace symptom there is: the burners light, run about five or six seconds, and shut off, then the board retries. The cause is almost always a dirty or misaligned flame sensor. A rollout switch that has tripped is never to be reset and forgotten; it means flame escaped the burner compartment, and you must find and correct the cause, usually a blocked heat exchanger, a blocked vent, or a failed inducer. A furnace that repeatedly trips its high-limit switch is being starved of air, from a dirty filter, closed registers, a collapsed flex duct, or a failing blower. A yellow, lazy flame indicates insufficient primary air or a dirty burner. A combustion analysis showing elevated carbon monoxide air-free in the flue is not a reading to record and leave; it is a combustion problem to investigate and correct.
Airflow Diagnostics
Airflow is the most commonly neglected half of a cooling system, and it produces symptoms that look exactly like a refrigerant problem. Low airflow across an evaporator during cooling gives a cold or frozen coil and low suction pressure, because less heat is reaching the refrigerant. Total external static pressure is the measurement that identifies a restrictive duct system: it is the supply plenum pressure plus the return pressure at the equipment, and it is compared with the external static the blower was rated for. A system measuring 0.95 inches of water column against a blower table built around 0.50 is spending nearly twice the pressure it was designed for, and the productive next step is to measure the pressure drop across the filter, the coil, and each section of duct to find where it is going. A flow hood placed over a register gives the most direct measurement of what that register is delivering. Where no flow hood is available, the practical field method on a furnace is to measure temperature rise and calculate CFM from the furnace output: CFM equals output BTU per hour divided by 1.08 times the temperature rise. Several field causes of low airflow are worth naming because they appear as scenarios. A blower wheel caked with dust no longer grips air and moves less of it. A high-MERV one-inch filter substituted for a standard pleated filter adds pressure drop and cuts airflow, which is why a homeowner who upgraded a filter and then lost cooling capacity is describing a cause and effect. Closing registers in unused rooms raises static pressure and reduces total system airflow rather than saving energy. A constant-airflow ECM blower will hold commanded CFM against high static by increasing speed and torque, which masks a duct problem from the homeowner while costing them electricity.
Sorting Charge Problems From Airflow Problems
Both a low charge and low airflow produce low suction pressure, which is why the first reading alone never settles it. The distinguishing measurements are superheat, subcooling, and the temperature split across the coil. Low airflow leaves the refrigerant plenty of surface but not enough heat, so the air that does pass over the coil is chilled hard, producing a large temperature split, while superheat stays low or normal and subcooling stays normal or rises. Low charge starves the coil, so superheat is high and subcooling is low, and the temperature split is smaller than expected because there is not enough refrigerant to absorb heat. That single contrast answers a large family of exam questions. Round out the pattern table with the high side. High head pressure with a normal charge is most often poor condenser airflow, from a dirty coil, a failed condenser fan, or recirculated discharge air. High head pressure with high subcooling on a clean condenser on a mild day is an overcharge. Head pressure higher than the condensing temperature would predict points to non-condensable gas. High suction pressure together with low head pressure indicates the compressor is not compressing, from worn valves or an internal bypass, and pumping ability is verified in the field by closing the suction service valve and observing how low the compressor can pull the low side. A compressor drawing amps far above its rated load amps points to high head pressure, low supply voltage, or failing windings, and a common-to-ground reading near zero ohms means the windings are shorted to the shell and the compressor has failed. Low voltage is its own hazard: 197 volts at a unit rated 208/230 while the compressor tries to start means high current draw and overheating windings. Finally, low suction pressure with high superheat and a frost line just past the filter drier is a restriction at the drier, not a charge problem.
Heat Pump Complaints and Seasonal Patterns
Heat pump service calls have a small set of recurring shapes. Cold air on a call for heat points to the reversing valve or its solenoid. A heat pump that runs continuously on a 35 degree morning with auxiliary heat energized the entire time should first be checked for a frosted outdoor coil and a defrost control that is not initiating or not terminating properly; a coil buried in ice cannot absorb heat, so the compressor runs and the strips carry the house. A customer who reports that the heat pump blows lukewarm air compared with their old furnace is not describing a fault. A heat pump normally delivers a lower supply air temperature over much longer run times, and that is a conversation to have at the sale rather than at the service call. Seasonal patterns are diagnostic in themselves. A system that cools well at night but struggles every afternoon is telling you the high side is marginal at peak ambient, so the check that addresses the pattern directly is to measure condensing temperature against outdoor ambient at peak load. A commercial rooftop unit that trips its high-pressure switch every afternoon points to the same place: condenser airflow, a dirty coil, or discharge air being recirculated. A rooftop economizer damper stuck fully open on a 100 degree day will prevent the building from holding setpoint, because the unit is conditioning outdoor air it should be excluding. An air conditioner that runs four minutes and stops for three all afternoon is significantly oversized for the load. A condensate float switch that keeps interrupting the cooling call is not a switch to bypass; clear the blockage, verify the trap and the slope of the drain, and the switch will stop tripping.
Efficiency Ratings and What They Actually Measure
Four acronyms cover residential efficiency. SEER, and its successor SEER2, is a seasonal cooling efficiency figure that averages performance across a range of operating conditions rather than reporting a single test point; EER, by contrast, is a single-point ratio of cooling output to power input at a stated condition. AFUE is annual fuel utilization efficiency and applies to gas and oil furnaces. HSPF, and its successor HSPF2, rates a heat pump's seasonal efficiency in heating mode. Coefficient of performance, or COP, is the ratio of heat output to energy input expressed in consistent units, so it is a dimensionless number and it is how heat pump heating performance is compared with electric resistance heat, which has a COP of 1. The reason SEER2 and HSPF2 replaced the earlier ratings is worth knowing because it is asked: the test procedure now uses a higher external static pressure that is closer to a real installed duct system, so the newer numbers reflect field conditions better and are not directly comparable to the older ones. Two honest customer answers follow from this. Whether a higher AFUE furnace lowers a bill in a mild coastal climate depends on how many heating hours that house actually runs, because efficiency multiplies consumption and a house that barely heats has little consumption to multiply. And an ECM blower's advantage over a permanent split capacitor motor is variable speed and substantially better electrical efficiency, particularly at part load, not extra capacity.
Maintenance and Documenting a Diagnosis
Routine maintenance is not filler on an invoice; it protects the two things that determine capacity, airflow and heat rejection. Changing filters, cleaning the evaporator coil, and clearing the condensate drain preserve airflow. Cleaning the outdoor coil and clearing the area around it is the single task that most directly protects capacity on hot days, because that is when a fouled condenser costs the most. When you find an outdoor coil packed with cottonwood, disconnect power first and rinse from the inside out with low-pressure water, so the debris is pushed back out the way it came rather than driven deeper into the fins. Two matters of professional conduct close this chapter, and both appear on the exam. When a diagnosis calls for equipment replacement rather than repair, document it in writing with the measured readings that support the conclusion; a written record protects the customer's ability to get a second opinion and protects you if the recommendation is later questioned. And when you find a visibly cracked heat exchanger, the appropriate immediate action is to shut the appliance down, tag it out of service, and inform the customer in writing, because a cracked heat exchanger can let combustion gases mix with the supply air. Being asked to sign off on work installed by someone else that you have not inspected is a request to decline; you verify work before you certify it.
Last updated: September 2026