Just Serviced, Now No Cooling: PM-Induced vs Coincidence Decision Tree

Why this matters

A no-cooling callback within hours or days of a maintenance visit is the single most damaging service outcome for a contractor's reputation. The customer's mental model is simple: the system worked before you touched it, and now it does not. Whether the failure was induced by the PM or simply coincided with it, you carry the burden of proof. Working this callback well means resisting the urge to either reflexively admit fault or reflexively deny it. You isolate the failure to a specific component, then ask the disciplined question: is this a thing a technician's hands could have caused, or a thing that was already on its way out? Getting that attribution right protects margin on warranty exposure and protects trust when you must charge.

Symptom presentation

The unit ran before the tune-up and will not cool after. Variations include: no outdoor unit operation, outdoor fan only, compressor hums then trips, indoor blower runs with no cold air, or normal operation that degrades over the first cooling cycle. Note exactly what changed and when relative to the visit. A failure that appears on the very next call for cooling points toward something disturbed during service. A failure that appears days later, after several normal cycles, is far more likely to be coincidental component end-of-life.

Quick checks

Start at the thermostat and work outward, but front-load the items most likely to have been touched during a PM.

• Confirm the thermostat is in cool, setpoint below room temp, and the C-wire seated. Float switches and condensate safeties get bumped during coil access.
• Verify the disconnect at the condenser is fully seated. A pulled disconnect not fully reinserted is a classic post-service no-start.
• Check the condensate safety switch and any auxiliary float. Coil cleaning fills pans; a tripped float kills the call.
• Inspect filter orientation and registers. A filter reinstalled backward or a closed supply can cause low-airflow lockout on systems with such protection.
• Look for a tripped breaker or blown low-voltage fuse on the board.

Isolation tree

Work the call as two parallel questions: where is the fault, and could service have caused it.

1. No 24V at the contactor coil? Trace the low-voltage circuit. If a condensate float or high-pressure switch is open, that safety tripped for a reason. A float that tripped after a coil cleaning that flooded the pan is PM-adjacent. A float tripped by a pre-existing slow drain clog is coincidental but exposed by the cleaning.

2. 24V present, contactor not pulling in? Suspect the contactor coil or a stuck plunger. Contactors are rarely touched during a basic PM, so failure here leans coincidental unless the tech replaced or cleaned contacts.

3. Contactor pulls in, compressor hums and trips on internal overload? Check the run capacitor with a meter under microfarad reading, not just visual. If a capacitor was replaced during the PM and the new part is the wrong rating or failed early, that is PM-induced. If the original cap is weak and was never touched, coincidental.

4. Compressor and condenser fan both dead with 240V present? Check the capacitor common terminal and wiring. A dual-run cap disturbed during cleaning, or a fan wire knocked off a spade terminal during fin combing, is PM-induced.

5. Outdoor unit runs, no cold air indoors? Check blower operation and refrigerant. A coil cleaning that loosened a fin pack or a recovery/recharge step that left the charge wrong is PM-induced. A slow leak that crossed a threshold is coincidental.

Confirming diagnosis

Do not guess at attribution. Measure.

• Capacitor: read microfarads against the printed rating. Within tolerance rules it out; out of spec on a part you installed is your problem.
• Charge: take superheat and subcooling against the unit's metering type. If you recovered or added refrigerant during the visit and subcool is off target, you own the recharge. If charge is correct and the issue is electrical, decouple the two.
• Safeties: ohm each safety switch in series. An open float plus a wet pan plus a slow drain tells you the cleaning flooded a marginal drain. Document the drain condition.
• Connections: tug-test every spade and screw terminal you could have reached. A loose wire that arcs warm is a smoking gun for a service-induced fault.

Remediation

If the fault is PM-induced, fix it without charge and document the correction in the work order. A wire reseated, a float cleared after a pan re-cleaning, a mis-sized capacitor swapped for the correct rating, or a recharge corrected to target subcool are all your cost. If the fault is a coincidental component failure exposed by the visit, explain the distinction plainly: the part was at end of life, the PM did not cause it, but the timing surfaced it. Provide the measured evidence. Replace the failed component as a normal repair. When attribution is genuinely ambiguous, weigh goodwill against the size of the repair; absorbing a small part often costs less than a damaged review. Always close by verifying full operation through a complete cooling cycle with delta-T across the coil.

References

• ACCA Standard 4 (Maintenance of Residential HVAC Systems), preventive maintenance task scope and verification.
• AHRI Standard 210/240, performance rating context for split-system superheat/subcool expectations.
• ASHRAE Standard 62.1, ventilation and airflow context relevant to filter and register restrictions.
• NFPA 70 (National Electrical Code), Article 440 disconnecting means and motor-circuit conductor requirements for condensing units.