NATE LC Refrigeration common mistakes
NATE LC Refrigeration · Understand the traps and how to avoid them.
Reading suction and discharge pressures without converting them to saturated temperatures before diagnosing, so the candidate reports a 'low charge' when the real problem is a restricted metering device or a dirty condenser.
Candidates memorize pressure numbers (e.g., '35 psig suction is low') instead of using a PT chart or gauge saturation scales, so they compare against a rule of thumb rather than the actual evaporating and condensing temperatures the system is running.
What to do instead
Practice converting every gauge reading to its saturation temperature and computing superheat and subcooling before forming a diagnosis. On the exam, write the saturation temperature next to each pressure reading and base the answer on the temperature relationships, not the raw pressure.
Confusing superheat and subcooling measurement locations, taking superheat at the condenser outlet or subcooling at the evaporator outlet.
The two terms are learned together and the formulas look similar (both are temperature differences), so candidates mix up which side of the system each belongs to under time pressure.
What to do instead
Anchor the definitions physically: superheat is evaporator outlet temperature minus evaporating saturation temperature (measured after the evaporator, before the compressor); subcooling is condensing saturation temperature minus liquid line temperature (measured after the condenser). Draw a quick system sketch on scratch paper and label the measurement points before answering.
Charging a system by adding refrigerant until the sight glass clears or until pressures 'look right' instead of charging to a measured superheat or subcooling target.
Field habits from automotive or older practices carry over, and candidates assume a clear sight glass means a correct charge even on systems with a receiver or a thermostatic expansion valve where the glass is not a charge indicator.
What to do instead
State the correct method explicitly for the system type: fixed-orifice and piston systems are charged by superheat; TXV systems with a receiver are charged by subcooling. Reject answer choices that use sight glass appearance or pressure alone as the charge criterion.
Selecting a line size, trap, or evacuation method without checking the manufacturer's installation instructions or the applicable code, relying only on general rules of thumb.
Candidates study generic refrigeration theory and skip the installation-documentation portion of the exam, assuming the exam only tests theory.
What to do instead
Review the standard installation requirements for line sizing, line routing, oil return, and evacuation (triple evacuation or micron-level decay test). For any installation question, eliminate answers that contradict the manufacturer's instructions or require skipping a documented step such as a nitrogen purge during brazing.
Diagnosing a no-cooling complaint as low refrigerant without first verifying airflow, then confirming with superheat and subcooling readings.
Low charge is the most familiar failure and candidates jump to it, skipping the cheap and common causes like a restricted filter, a failed blower, or a frosted coil.
What to do instead
Use a fixed diagnostic sequence: confirm power and controls, verify airflow and coil condition, then take pressures and temperatures and calculate superheat and subcooling. Match the pattern (high superheat plus low subcooling suggests undercharge; low superheat suggests overcharge or a flooding valve) before recommending a repair.
Misreading a frost pattern or temperature split as proof of a specific component failure without checking the corresponding pressure relationship.
Frost on the suction line and a cold evaporator look like a low charge, but the same symptom can come from a restricted filter-drier, a stuck TXV, or a blower problem, and candidates stop at the first plausible cause.
What to do instead
For each symptom, list at least two possible causes and the measurement that distinguishes them. For example, a restricted drier shows a temperature drop across the drier and low subcooling downstream, while a stuck-open TXV shows high superheat with normal subcooling. Answer only after the distinguishing measurement is identified.
Recovering refrigerant into a system or a non-approved container, or skipping the leak check and evacuation steps during a repair, and treating recovery as an optional step.
Candidates focus on the mechanical repair and treat refrigerant handling as paperwork, and they may confuse recovery, recycling, and reclamation.
What to do instead
Memorize the required sequence for any repair that opens the circuit: recover refrigerant, repair, pressure test with dry nitrogen, evacuate to the specified micron level, then charge by weight or by superheat/subcooling. Know the difference between recovery (removal), recycling (cleaning for reuse in the same equipment), and reclamation (processing to specification).
Replacing a compressor or contactor based on a 'bad compressor' symptom without first checking the electrical supply, capacitor, overload, and start components.
A compressor that hums and trips the overload looks like a failed compressor, and candidates skip the electrical checks that would reveal a failed run capacitor, a stuck potential relay, or a loose connection.
What to do instead
Follow the electrical sequence before condemning the compressor: verify supply voltage and balance, check the contactor and overload, test the capacitor, check start components, then measure winding resistance and insulation to ground. On the exam, choose the diagnostic step that isolates the component rather than the one that replaces it.
Ignoring lockout and safety-control logic, such as assuming a low-pressure switch tripped because of low charge when it may be a control or defrost-timer issue.
Candidates treat safety controls as simple switches and do not trace the control circuit, so they misattribute a lockout to the refrigeration circuit.
What to do instead
Trace the control circuit from the thermostat or controller through each safety and time-delay device to the contactor. Identify which device opened and why, and check defrost termination and fan-delay settings before diagnosing the refrigerant side.
Answering safety questions from general habit rather than from the specific procedure, such as working on energized equipment or skipping PPE and lockout/tagout.
Experienced technicians rely on muscle memory and may not have studied the written safety requirements, and exam questions often test the documented procedure rather than what is convenient in the field.
What to do instead
Study the lockout/tagout sequence, PPE requirements for refrigerant exposure, and the rules for working on energized circuits and for brazing near flammable materials. When a question presents a safety scenario, choose the answer that follows the documented procedure even if a shortcut would work in practice.