AC Tune-Up Checklist: What Should Be Included (2026)

The tech places a calibrated thermometer near the thermostat and compares readings. A thermostat that reads 3°F high means the system under-cools — your house sits at 75°F when you think it's 72°F. Calibration drift is common on older mechanical thermostats and can be adjusted. Digital thermostats rarely drift but should still be verified.

The tech calls for cooling at the thermostat and verifies: the indoor blower starts, the outdoor compressor engages within 1–3 minutes, and both shut off when the setpoint is reached. Delayed starts, failure to engage, or failure to shut off indicate relay, contactor, or control board issues that should be caught now — not during a 100°F afternoon.

The system has multiple safety devices: high-pressure cutout, low-pressure cutout, condensate overflow switch, and anti-short-cycle timer. The tech should verify each one is present, connected, and functional. A bypassed or disconnected safety switch puts the compressor at risk of catastrophic failure.

The tech should pull the filter, assess its condition, and replace it if needed. A clean filter is the single highest-impact maintenance item. If you've been changing filters regularly, the tech should note the current filter size, MERV rating, and condition. Some tune-up contracts include a filter; others charge $10–$30 separately.

Static pressure is the resistance to airflow in the duct system — measured in inches of water column (iwc). The tech places probes before and after the indoor coil/filter. Normal total external static pressure is 0.5–0.8 iwc. Above 0.8 iwc indicates restricted airflow — dirty coil, undersized ducts, or filter too restrictive. This is the most underperformed tune-up step and the most diagnostic. Ask for the reading.

The tech should visually inspect the evaporator coil surface. A layer of dirt on the coil reduces heat transfer and efficiency 10–20%. Mold growth on the wet coil surface is common and affects indoor air quality. If the coil needs cleaning, it's typically a separate charge ($100–$300) — but the inspection and recommendation should be part of every tune-up.

A dirty blower wheel is like a clogged filter — it reduces airflow even when the filter is clean. The tech should spin the wheel to check for bearing roughness, wobble, or excessive play. A squealing or grinding blower motor is headed for failure. Catching it at tune-up means a scheduled replacement ($300–$700) instead of an emergency call ($500–$1,000+).

The tech connects gauges to the service ports and measures superheat (at the evaporator) and subcooling (at the condenser) to determine if the refrigerant charge is correct. Proper charge is not just a pressure reading — it requires temperature measurements at specific points. Overcharged systems are as problematic as undercharged ones. The tech should report the readings and whether they're within manufacturer specifications.

The tech should trace the refrigerant lines from indoor to outdoor unit, checking for: damaged insulation (reduces efficiency), rubbing against metal (eventual leak), and oil stains at joints (indicates slow leak). Oil stains are the most important finding — refrigerant carries compressor oil, so an oil stain means refrigerant is escaping at that point.

The tech measures air temperature at a supply register and at the return grille. Normal split is 15–20°F. Less than 14°F with clean filter and adequate airflow suggests low refrigerant, dirty coil, or compressor issue. This is a quick cross-check on the refrigerant measurements — the numbers should tell the same story.

The tech applies a commercial coil cleaner, allows it to dwell, then rinses with a garden hose from the inside out. This removes embedded dirt, pollen, grass clippings, and cottonwood fuzz that reduce heat rejection. A dirty condenser coil raises head pressure, increases compressor load, and reduces cooling capacity 10–30%. This is one of the most impactful tune-up tasks.

The contactor is an electrical relay that sends power to the compressor and outdoor fan. Over time, the contact points pit and arc, increasing resistance and eventually failing. The tech should pull the contactor, inspect the contact surfaces, and test for proper operation. Pitted contacts waste electricity and can weld shut — leaving the compressor running continuously. Replacement costs $150–$300 and takes 20 minutes.

The run capacitor stores electrical charge to keep the compressor and fan motors running efficiently. A weak capacitor causes hard starts, increased electrical draw, and eventual motor failure. The tech measures microfarad rating with a capacitor tester — it should be within 5% of the rated value printed on the capacitor. Weak capacitors are the #1 cause of AC compressor failure. Replacement costs $150–$300.

The tech checks the condenser fan for wobble, bent blades, and motor bearing noise. A wobbling fan creates vibration that loosens electrical connections and refrigerant joints over time. A motor with rough bearings is approaching failure. Catching these at tune-up allows scheduled replacement before the fan fails during peak cooling demand.

Thermal cycling loosens wire connections over time. Loose connections create resistance, heat, and eventually arcing. The tech should tighten terminal screws on the contactor, disconnect, control board, and compressor terminals. Voltage should be within 10% of rated (typically 208–240V). Compressor amperage should be at or below the rated load amps on the nameplate — high amps indicate a struggling compressor.

UV exposure degrades outdoor wiring insulation. Indoor units in attics and crawl spaces are vulnerable to rodent damage. Discolored wires indicate overheating from loose connections or overloaded circuits. The tech should flag and repair any compromised wiring — this is a safety issue, not an upsell.

The tech should flush the drain line with water or compressed air to verify it flows freely. Algae and slime build up inside the drain line during cooling season — especially in humid climates. A clogged drain backs water into the drain pan, triggers the safety float switch (shutting down the system), and can overflow onto ceilings and floors if the switch fails. The tech should treat the line with algaecide tablets or a vinegar flush.

The primary and secondary drain pans collect condensate from the evaporator coil. Metal pans corrode over time; plastic pans can crack. Standing water in the pan with the drain clear means the pan is not sloped properly. The tech should check both pans and the float switch. A cracked or corroded pan that goes unnoticed leads to water damage between tune-ups.