Common Diagnostic Test Procedures Reference

Why this matters

The technician without a diagnostic method guesses + swaps parts; the one with a method diagnoses + fixes the actual problem. Customer's "it doesn't work" needs a systematic test sequence: power → control → component → load. Every trade has a workflow; this card captures the universal ones. This is the field card.

Universal diagnostic framework

Step 1 - Verify the complaint: customer's words ≠ actual fault. Reproduce the failure yourself.

Step 2 - Power test: 24V/120V/240V present at expected location? Use multimeter, not voltage pen alone.

Step 3 - Control test: thermostat / control board calling? Voltage at output terminal of control?

Step 4 - Component test: motor / element / valve receiving voltage? Drawing current?

Step 5 - Load test: load resistance OR amp draw within spec? Capacitor capacitance?

Step 6 - Mechanical test: blower spinning freely? Coil clean? Drain clear?

Step 7 - Logical conclusion: which test failed → which component → repair.

Electrical continuity test

Set multimeter to ohms (Ω) OR continuity beep.

Disconnect power from circuit.

Touch leads to two ends of conductor / device.

• 0 ohms = continuous (closed circuit)
• ∞ ohms = open (broken circuit)
• Some resistance (50 - 5,000 Ω) = component-specific (motor windings, heating element)

Use cases: broken wire, blown fuse, open thermal cutoff, switch tested, heater element integrity.

Voltage test

Set multimeter to AC OR DC (matching circuit).

Probe positive lead to live, negative to neutral OR ground.

Read voltage:

• 120V AC: nominal range 108 - 132V (±10%)
• 240V AC: nominal 216 - 264V
• 24V AC (HVAC control): nominal 19.2 - 28V
• 12V DC: nominal 11 - 14V

Low voltage under load may indicate undersized supply OR poor connection.

Amperage / current draw test

Clamp meter around single conductor.

Read amperage during operation.

Compare to nameplate FLA (full load amps):

• Within spec: normal
• High: locked rotor (motor starting issue), restriction (compressor with high head pressure), short
• Low: open winding (motor losing windings), no load (belt slipping)

LRA (Locked Rotor Amps) = 5 - 8x FLA at startup; normal for 2 - 5 seconds.

Resistance / motor winding test

Disconnect motor leads + power.

Set multimeter to ohms.

Measure between motor terminals + to ground:

• Common motor: 10 - 50 Ω winding-to-winding
• 0 Ω OR very low: shorted winding
• ∞ Ω: open winding
• Winding to motor frame: should be ∞ Ω (insulation good)

Use megohmmeter (megger) at 500V for true insulation test.

Capacitor test

Discharge first: short the leads with insulated screwdriver (some capacitors hold lethal charge).

Set multimeter to capacitance (μF) OR use dedicated capacitor tester.

Read μF: compare to nameplate rating.

• Within ±6%: serviceable
• ±10% lower: degraded; replace
• Below 6 μF on 35 μF capacitor: dead

Common run capacitors: 5 - 75 μF; start capacitors 50 - 300 μF.

Refrigerant pressure + temperature test (HVAC)

Manifold gauge on schrader ports.

Read low + high side pressure during operation.

Match P/T chart to refrigerant (R-410A, R-32, R-22, R-454B):

• Compare evaporator pressure to expected saturation temp
• Compare condenser pressure to expected saturation temp
• Calculate superheat (low side) + subcool (high side)

Target superheat typical 8 - 12°F; subcool 8 - 14°F. Out of range = charge OR airflow problem.

Combustion test (gas furnace)

Combustion analyzer in flue.

Read during steady-state operation (5+ min after startup):

• O2: 6 - 10% for atmospheric; 4 - 8% condensing
• CO: < 100 ppm air-free for atmospheric; < 50 ppm condensing
• Stack temp: 350 - 500°F atmospheric; 100 - 150°F condensing
• Draft: -0.02" WC atmospheric

High CO + low O2 = incomplete combustion; needs adjustment.

Static pressure test (HVAC ducts)

Manometer on supply + return.

TESP (Total External Static Pressure) = supply + return.

Target: < 0.5" WC for residential equipment (per nameplate).

Higher = restriction (dirty filter, undersized duct, closed registers). Lower = leaks.

Airflow / CFM measurement

Anemometer at registers.

Sum CFM at all registers; should match equipment nameplate (within 10%).

Low CFM = duct leak, blower issue, restriction.

Drain / drainage test

Bucket drain pan + watch flow.

Pour water in AC condensate line + verify flow at exterior termination.

Time it: < 5 sec drain rate = clear; longer = partial clog.

Vacuum the drain line from outside to clear OR snake.

Vacuum / leak test (refrigeration)

Pull system to deep vacuum (500 microns OR lower).

Hold vacuum 15 minutes with vacuum pump valved off.

Reading climbs:

• < 1000 microns rise: dry + tight
• 1000 - 2000: minor moisture OR slow leak
• 2000+: significant leak; find OR repair

Pressure test (plumbing / gas)

Cap all openings + pressurize with air OR water.

Hold pressure for time per code:

• Plumbing water: 1.5x working pressure 15 min
• Plumbing air: 50 PSI 15 min
• Gas: 3 PSI 15 min (residential)
• Refrigerant lines: 250 PSI nitrogen 15 min

Drop in pressure = leak; find + repair.

Smoke test (drainage)

Smoke generator pushed into drain stack vent.

Smoke escaping anywhere except roof vent = leak in plumbing system. Common: dry traps, broken seal under toilet.

Functional test (after repair)

Cycle the system through full operation:

• Heating: stat to call → ignite → reach setpoint → cycle off
• Cooling: stat to call → compressor on → reach setpoint → cycle off
• Plumbing: turn water on → check fixtures + drainage
• Electrical: switch on → load operates → switch off

Verify in front of customer before invoicing.

Common diagnostic mistakes

Swapping parts before diagnosing (wastes parts + customer money). Skipping voltage test before assuming bad component. Assuming voltage pen accurate (always verify with multimeter). Not disconnecting power before resistance test (damages meter; risk to tech). Forgetting capacitor discharge before testing (lethal jolt). Skipping pressure test after repair (callback within days). Not documenting readings (customer asks; can't prove). Skipping functional test (callback for "didn't fix it" feeling).

References

• ASHRAE Fundamentals Handbook
• NEC + relevant trade codes
• Manufacturer technical service bulletins
• Refrigerant P/T charts (R-410A, R-32, R-22)
• Manuall internal: Common Gauges + Meters, Electrical Conversion Math