In a busy outpatient PFT lab, a portable spirometer reports FVC 18% below the patient's recent lab value, flow-volume loops show jagged expiratory limbs, and the 3-L calibration syringe check fails low on two trials at different flows. No new symptoms are reported. Which troubleshooting action best restores portable spirometer and PFT lab accuracy?

The correct action restores accuracy because the triad of FVC 18% below the recent lab value without clinical change, jagged expiratory limbs, and 3-L syringe checks failing low at different flows indicates sensor contamination or partial obstruction that distorts the flow signal and under-measures volume across flows; removing, inspecting, cleaning, reassembling and recalibrating with the syringe physically removes debris, restores signal quality, and re-establishes volume accuracy to satisfy ATS/ERS verification before further patient testing. Verifying BTPS and ambient corrections then repeating the syringe check fails as temperature-correction troubleshooting because while it would detect software or entry errors, it leaves physical obstruction in the flow path unaddressed so the low failure will simply recur. Installing a factory-calibrated spare sensor and checking it with the syringe fails as premature replacement because although standard when damage is confirmed, it skips the required inspect-and-attempt-cleaning step, adding unnecessary cost and downtime when cleaning is indicated. Coaching harder on blast effort and repeating spirometry fails as effort-optimization because vigorous coaching can smooth variable patient effort but cannot cause a mechanical syringe to read low on two flows, so retesting would reproduce low jagged volumes while instrument error remains uncorrected. For the exam, any failed calibration check prohibits patient testing until resolved regardless of time pressure or apparent patient factors. Practically, repeat the 3-L syringe at slow, medium and fast flows after cleaning, confirming readings within 3% or 0.05 L with smooth real-time flow and documented ambient conditions before resuming clinical measurements.