WORKED DIAGNOSTIC CASES, EVIDENCE FIRST

SMALL ENGINE / 2026-10-27

Two-stage snowblower, model unknown: Engine runs and discharges snow, but throw distance is short

Short discharge does not prove a worn belt. Compare intake load only under safe operating conditions; blockage and model-specific drive faults remain possible.

SMALL ENGINE / 2026-10-20

8-year-old 24" two-stage snowblower: First-snow morning: cranks briskly, never fires — sat since March with fuel in the tank

Storage makes fuel condition worth checking, but it does not confirm a carburetor fault. Follow the exact manual for fuel checks and approved ignition testing before replacing parts.

HVAC / 2026-10-20

5-year-old 3-ton air-source heat pump: Blows 'cold' air on the first 30°F morning, house 2°F below setpoint by breakfast

Cool-feeling air and a 2°F shortfall confirm no component fault. Record settings and heating behavior; register temperatures and AUX indicators need equipment-specific interpretation.

SMALL ENGINE / 2026-10-19

Toro walk-behind, 6.75 hp Briggs & Stratton: Surges up and down at idle and light load, smooths at full throttle

Storage makes fuel worth checking, but surging confirms no fault. Identify the exact mower and controls before testing fuel, intake, ignition or governor hypotheses.

DIESEL / 2026-10-19

2001 Dodge Ram 2500, 5.9 Cummins 24v: Starts instantly, runs 5–15 seconds, dies — restarts and repeats

A short run does not prove standing-fuel starvation. Record low-pressure supply through the stall; steady pressure still leaves aeration, VP44 and electrical faults open.

HVAC / 2026-10-18

~15-year-old condensing gas furnace: First cold-night lockout — inducer runs, then 'pressure switch stuck open' code

The code is not a failed-switch diagnosis. Inducer operation does not clear every safety, and a pressure reading alone does not justify replacement. Identify the exact manual first.

HVAC / 2026-10-08

80,000 BTU single-stage gas furnace: No heat — 24V at the board, no inducer, no ignition, no status LED

The dark LED says board, the week-old continuous blower says maybe not. The one-wire jumper and two meter probes that prove a board dead before ordering a $300 part.

DIESEL / 2026-09-25

2015 RAM 2500, 6.7 Cummins: Hard start when cold — cranks 4–6 s, fine all day after

New filters, no codes, one battery already replaced. Why the first cold crank with a scan tool splits batteries, rail pressure and cold-start aids before any part comes off.

SMALL ENGINE / 2026-09-24

13 hp GX390 portable generator: Cranks but won't start — "has spark and fuel"

Fuel in the tank and a reported spark do not prove delivery or correct ignition. Start with Honda's controls, oil and fuel checks; no intake priming or safety bypasses.

HVAC / 2026-09-23

3.5-ton R-410A rooftop unit: Running but not cooling — suction 148, head 252, delta-T 15°F

The pressures look close to normal at a glance, which is exactly why this call gets misdiagnosed. What a 15°F delta-T with those readings actually says.

DIESEL / 2026-09-22

2014 Ford F-250, 6.7 Power Stroke: Cranks but won't start — rail pressure 1,200 psi

New filters, no leaks, and a rail that never fills. The return-flow test that splits injector leak-off from a suction-side air leak from a tired CP4.