What it is

There's a huge gap between two things that both get called a "tune-up." One is a tech who shows up, changes the filter, hoses the condenser, slaps a sticker on, and leaves in fifteen minutes. The other is a tech who measures the system — electrical, airflow, refrigerant, and combustion — and catches the part that's about to fail. Only the second one is worth anything to the customer or to the company. This article is about what actually separates them.

How it works

Equipment doesn't usually fail randomly. It fails along predictable curves — a capacitor drifts low, a contact pits, a coil loads up, a heat exchanger fatigues, static pressure creeps as ducts clog. The whole value of preventive maintenance is intercepting those curves before they cross the line into a breakdown. You can't intercept a curve you don't measure. Cleaning helps, but cleaning without measuring is hoping. A meter and a few instruments turn hoping into knowing.

In the field

A real PM is built around measurement. Here's what genuinely predicts failures, regardless of season:

  1. Meter the capacitor(s). A run cap drifting out of spec is the single most common preventable failure. You cannot see this without a meter. This one reading prevents a giant share of emergency calls.
  1. Read amp draws against the nameplate. A compressor or motor pulling high is telling you about a bad bearing, a weak cap, or a coming failure — weeks of warning, free, if you take the reading.
  1. Measure static pressure. High total external static is the hidden killer behind iced coils, overheated heat exchangers, and dead blower motors. A manometer turns "airflow seems fine" into a real number.
  1. Verify the refrigerant side with a method, not a vibe. Superheat or subcooling to the manufacturer's target, plus the temperature split. This catches a slow leak or a developing restriction early.
  1. Run combustion analysis on gas equipment. CO and combustion quality are safety items you measure, not guess. This is non-negotiable on heating.
  1. Inspect the wear items. Contactor contacts, electrical connections for heat, condensate drain and safety switch, belts where present, blower wheel loading.
  1. Document the numbers and trend them. The real magic of PM compounds over time: this year's readings compared to last year's show you what's drifting. A capacitor that read 44 µF last fall and 39 µF this fall is on its way out even if it's "in spec" today.

Normal values & targets

The numbers a real PM captures (residential ballparks; defer to the equipment):

  • Run capacitor: within ±6% of rating; trend it year over year.
  • Amp draws: compressor ≤ RLA, motors ≤ FLA; flag anything climbing.
  • Total external static pressure: design target around 0.5" w.c.; investigate much above ~0.8" w.c.
  • Cooling split: ~16–22°F; furnace temperature rise: within the data-plate range.
  • Superheat/subcooling: to the model's target (commonly ~10–20°F SH or ~8–12°F SC).
  • Flue CO: as low as practical; rising/elevated CO is a stop-and-investigate.

Common faults & what they mean

  • A cap that's "fine" but lower than last year: failure trending — note it, plan for it.
  • Static pressure creeping up year over year: duct/filter/coil restriction building — the future cause of an iced coil or a fried blower.
  • Amps drifting up on a motor: bearings or capacitor degrading.
  • Split or superheat slowly wandering off: a small refrigerant leak or fouling coil developing before it's a no-cool.
  • CO appearing or climbing on a furnace: combustion or heat-exchanger problem — a safety find a wipe-down would have missed entirely.

Tech tips & gotchas

If you only do one thing differently, carry your meters to every PM and actually use them. The wipe-and-sticker visit feels productive because the unit's clean, but it predicts nothing. The customer paid for foresight, and foresight comes from measurement.

Trending is where PM pays off, and almost nobody does it. Write your numbers on the ticket and compare to last visit. A part inside spec but moving in the wrong direction is the highest-value heads-up you can give a customer — it lets them replace a $20 cap on a Tuesday instead of paying emergency rates in a heat wave.

Cleaning still matters — a fouled condenser or blower genuinely hurts performance — but clean and measure. Cleaning treats the symptom; measuring finds the cause and the next failure.

Be honest about what you find. The trust that makes maintenance agreements worth selling evaporates the first time a customer learns a tech invented a problem. Report green, yellow, and red truthfully: what's good, what's borderline and worth watching, what genuinely needs attention.

Don't let "the unit runs" fool you. A system can run today and be one hot afternoon from a failure that a five-minute capacitor and amp check would have caught. Running is not the same as healthy.

Safety / code notes

  • Electrical checks require lockout and capacitor discharge; HVAC circuits fall under NEC Article 440. Prove dead with a verified meter.
  • Combustion and CO checks on gas equipment relate to the fuel-gas code (IFGC; Indiana enforces it) — adequate combustion air and intact venting are safety prerequisites.
  • Condensate safety devices and disposal are covered by the mechanical code (IMC §307).
  • Any safety find — a breached heat exchanger, a CO reading, a missing required safety switch — gets communicated clearly and documented, not glossed over.