The Ultimate HVAC Diagnostic Chart
A measurable, step-by-step HVAC diagnostic chart for licensed MEP technicians and DIY'ers. Every check below is a verifiable reading — voltage, amperage, pressure, temperature, or a coded fault — not speculation. Use it alongside Materva's voice assistant to call out readings hands-free while you work.
How to use this chart
- Confirm power and thermostat call before touching the unit.
- Read any fault code from the control board LED, ECM, or thermostat display.
- Verify the suspected fault with a measurement from the table below.
- Replace or repair only after the reading confirms the diagnosis.
1. First-pass safety and power checks
| Check | Expected Reading | Fault Indication |
|---|---|---|
| Line voltage at disconnect (residential split) | 220–240 VAC | <200 VAC: utility/feeder issue |
| Line voltage (light commercial 3-phase) | 208 / 230 / 460 VAC ±10% | Phase imbalance >2% damages compressor |
| Control transformer secondary | 24–28 VAC | 0 VAC: blown fuse or shorted thermostat wire |
| Capacitor microfarad (run cap) | Within ±6% of rated µF | <90% rating: replace |
| Contactor coil voltage | 24 VAC across coil when calling | 0 VAC: check thermostat / safety chain |
2. Common HVAC fault codes
Codes vary by manufacturer. Always cross-reference the unit's service label. The table below lists patterns seen across Carrier, Trane, Lennox, Goodman, Rheem, York, and Daikin equipment.
| Code / Flash | Meaning | Verify with |
|---|---|---|
| 1 flash / "Status OK" | Normal operation | No action |
| 2 flashes | External lockout (low/high pressure switch open) | Ohm pressure switches; check refrigerant charge |
| 3 flashes | Draft / pressure switch fault (gas furnace) | Inducer amp draw + 1.5–2.5" WC negative pressure |
| 4 flashes | High-limit switch open | Temp rise across heat exchanger vs nameplate |
| 5 flashes | Flame sensed with no call | Flame rod microamps: 1.5–6 µA healthy |
| 7 flashes | Gas valve circuit fault | 24 VAC at MV terminals during ignition |
| E1 / E2 (mini-split) | Indoor/outdoor communication loss | DC bus voltage on signal wire (typ. 0–56 VDC pulse) |
| P4 / F3 (VRF/inverter) | Compressor IPM or discharge temp fault | Discharge line ≤220°F; IPM module resistance |
| LC / "Lockout" | Ignition or safety lockout | Cycle power; reread code on next call |
3. Refrigerant pressure and superheat/subcool ranges
Pressures depend on refrigerant, indoor/outdoor temperature, and metering device. Use these as starting envelopes for a 95°F outdoor / 80°F indoor / 50% RH day.
| Refrigerant | Suction (low) | Liquid (high) | Target Superheat (fixed orifice) | Target Subcool (TXV) |
|---|---|---|---|---|
| R-410A | 118–135 psig | 325–400 psig | 8–15°F | 8–12°F |
| R-32 | 120–140 psig | 340–410 psig | 8–15°F | 8–12°F |
| R-454B | 115–135 psig | 320–395 psig | 8–15°F | 8–12°F |
| R-22 (legacy) | 65–75 psig | 225–275 psig | 8–15°F | 8–12°F |
Rule of thumb: low superheat + low subcool = overcharge or restricted airflow. High superheat + low subcool = undercharge or restricted liquid line.
4. Airflow and temperature split
- Target supply/return Δt in cooling: 16–22°F. <14°F suggests overcharge, low airflow, or compressor wear.
- Target Δt in gas heat: per nameplate (often 35–65°F). Over rise = low CFM; under rise = overfiring or excess CFM.
- Static pressure across air handler: ≤0.5" WC total for residential equipment rated 0.5" ESP.
- Blower amp draw: within FLA on motor nameplate; ECM check via communicating tool.
5. Decision flow (text flowchart)
No cooling call response ├─ 24 VAC at R-Y at contactor? │ ├─ No → thermostat / float switch / control board │ └─ Yes → contactor pulled in? │ ├─ No → coil open / 24V drop under load │ └─ Yes → compressor + condenser fan running? │ ├─ Fan only → check run cap, compressor windings │ ├─ Compressor only→ check fan cap, motor windings │ └─ Both running → check pressures + superheat/subcool
6. Residential vs commercial vs industrial differences
- Residential: single-phase 240 VAC, fixed-orifice or TXV, one stage. Diagnose at the disconnect and air handler.
- Commercial RTU: 208/460 VAC 3-phase, economizer, multi-stage. Add phase rotation, economizer damper position (2–10 VDC), and CO2 setpoint checks.
- Industrial / VRF / chillers: BMS/BACnet alarms, compressor VFD fault codes, oil pressure (chillers), approach temperatures on condenser/evaporator barrels.
7. When to stop and escalate
Refrigerant work requires EPA 608 certification. High-voltage 3-phase work and brazing belong to licensed technicians. DIY'ers should stop at the thermostat, filter, condensate, breaker, and capacitor visual inspection — anything past that calls for a pro.
HVAC diagnostic chart FAQ
Short, voice-friendly answers. Materva reads these aloud on request during a session.
- What is an HVAC diagnostic chart?
- A reference that maps symptoms and fault codes to the exact measurement — voltage, amperage, pressure, or temperature — that confirms the cause before any part is replaced.
- What does 2 flashes mean on an HVAC control board?
- External lockout. A high or low pressure switch opened. Ohm both switches and check refrigerant charge.
- What does 3 flashes mean on a gas furnace?
- Pressure switch or draft fault. Measure inducer amp draw and confirm 1.5 to 2.5 inches water column negative pressure at the switch port.
- What is normal suction pressure for R-410A?
- About 118 to 135 psig on a 95 degree day with an 80 degree return. Pair it with 8 to 12 degrees subcool on a TXV system.
- What is a normal temperature split on an AC?
- 16 to 22 degrees Fahrenheit between return and supply. Below 14 points to overcharge, low airflow, or a weak compressor.
- What voltage should I see at the contactor coil?
- 24 volts AC across the coil when the thermostat is calling. Zero volts means the call is not reaching the unit.
- How do I know if my run capacitor is bad?
- Measure microfarads with the capacitor isolated. Replace if it reads more than 6 percent below the rated value.
- What does E1 mean on a mini-split?
- Indoor and outdoor units lost communication. Check the signal wire for the pulsing DC bus voltage and verify tight terminals on both ends.
- Can a DIY'er use an HVAC diagnostic chart?
- Yes for thermostat, filter, breaker, condensate, and visible capacitor checks. Refrigerant work and 3-phase wiring require a licensed tech.
- Can Materva read HVAC codes out loud?
- Yes. Speak the code or symptom and Materva walks the verification step by step, hands-free.
Use this chart hands-free
Materva Troubleshoot Pro reads codes back to you, accepts spoken meter readings, and walks the flow above without you putting down your tools. Start a free session or review pricing.
Professional MEP diagnostic aid. Not a substitute for licensed inspection. © 2026 Materva Inc.