HVAC Diagnosis Live Matrix

Safety first, then diagnosis

This guide is not a repair manual. It is a homeowner script for making the service call honest.

Stop using the system and get qualified help if there is gas smell, a carbon monoxide alarm, flame rollout, burning smell, melted wiring, immediate breaker trips, or a technician documents a cracked heat exchanger. Those are not bargaining topics.

For everything else, start with what you actually saw: silent outdoor unit, humming, fan running without cooling, ice, water in the pan, blank thermostat, weak heat, burner lights then shuts off, breaker trips, rooms that never cool, or a recent thermostat/service change. That is enough. The technician's job is to connect that history to measurements and a repair choice.

Cooling: AC seems dead

Do not jump from "outdoor unit is dead" to "replace the system." Ask what the outdoor unit did when the thermostat called.

• Silent: contactor, low-voltage signal, float switch, breaker, disconnect, or board/control issue.
• Hums but fan does not spin: capacitor, fan motor, hard-start, or compressor start problem.
• Fan runs but compressor does not: capacitor, compressor, contactor, wiring, overload, or refrigerant/safety condition.
• Breaker trips immediately: shorted compressor, wiring, motor, or electrical fault. Stop DIY troubleshooting.

The homeowner pushback is simple: "Did you test the capacitor, contactor, voltage, amp draw, and fan motor before quoting replacement?"

Cooling: warm air or weak cooling

Warm air can be low refrigerant, but it can also be airflow, dirty condenser coil, dirty indoor coil, bad blower, blocked filter, iced evaporator, duct leakage, or thermostat/setup issue.

Ask whether the system was thawed before testing, whether static pressure was checked, whether the indoor coil was inspected, and whether the condenser coil was cleaned.

Heater not working: identify the heat type first

"The heater is out" is not enough information for a replacement quote. A gas furnace, electric furnace, heat pump with auxiliary heat, boiler, and dual-fuel system fail in different ways.

Ask the technician to identify the heat type and the failed sequence:

• gas furnace: inducer, pressure switch, igniter, gas valve, flame proof, rollout/limit safeties, and blower delay,
• electric furnace or air handler: sequencer, relay, heat-strip stage, breaker, blower, and thermostat auxiliary-heat call,
• heat pump: outdoor heat output, reversing valve, defrost behavior, auxiliary heat staging, and thermostat setup.

If the quote jumps straight to "replace the heater," ask which symptom and measurement supports that conclusion and what cheaper service-call path was ruled out.

Gas heat: burner lights then shuts off

This is where the flame sensor matters. If the burners light for a few seconds and then drop out, the furnace may not be proving flame. A dirty flame sensor, grounding issue, wiring issue, or board flame-sense circuit can cause that pattern.

That is very different from "replace the furnace." Ask: "What step in the ignition sequence failed? Did you clean and test the flame sensor? Did you verify ground and flame signal?"

Gas heat: inducer runs but no ignition

The sequence matters:

• thermostat calls for heat,
• inducer starts,
• pressure switch proves draft,
• igniter energizes,
• gas valve opens,
• flame is proven,
• blower starts after delay.

If the sequence stops, ask where. Pressure switch, blocked venting, inducer, igniter, gas valve, flame sensor, rollout switch, limit switch, and board can all be involved.

Electric heat: sequencer and heat strips

Electric furnaces and air handlers are high-current systems. A sequencer or relay can fail so the blower runs but heat strips do not energize, or only some stages heat. A thermostat can also fail to call auxiliary heat correctly.

Ask: "Which heat-strip stages energized, what amperage did each stage draw, and did the sequencer or relay fail?"

If the breaker trips when heat starts, treat that as a safety/electrical diagnosis, not a casual reset problem.

Heat pump: separate outdoor heat from auxiliary heat

A heat pump may be fine in cooling but weak in heating, or the outdoor unit may run while auxiliary heat never stages. Thermostat configuration can make this worse by locking out heat pump stages or overusing expensive auxiliary heat.

Ask: "Is the outdoor unit producing heat, is auxiliary heat staging, and is the thermostat configured for this exact equipment type?"

Board replacement: make them prove it

Control boards fail. But "bad board" is also a common lazy diagnosis.

A real board diagnosis should name the failed input or output:

• power enters board but no inducer output,
• thermostat call reaches board but no outdoor call,
• flame signal is present but board drops gas valve,
• relay output fails under load.

Ask what cheaper component was ruled out before the board quote.

Service-call worksheet: questions by symptom

Use this as a private script before the technician arrives. The point is not to diagnose the system yourself. The point is to keep the conversation tied to symptoms, measurements, and cheaper causes before the quote becomes a replacement package.

For cooling that seems dead, write down whether the thermostat is blank, whether the indoor blower runs, whether the outdoor unit is silent, humming, spinning without cooling, or tripping the breaker. Then ask which low-voltage call, safety switch, capacitor, contactor, motor, compressor, and breaker readings support the recommendation.

For weak cooling, write down whether the filter was dirty, whether the coil iced over, whether airflow feels weak, whether one room is the problem, and whether anything changed recently. Then ask whether airflow, static pressure, indoor coil condition, condenser cleanliness, duct leakage, and refrigerant charge were checked in that order.

For gas heat, write down whether the inducer runs, whether the igniter glows, whether flame starts and drops out, whether the blower starts, and whether any safety switch is open. Then ask for the failed sequence step: pressure switch, igniter, gas valve, flame sensor, rollout, limit, board input, or board output.

For electric heat, write down whether the blower runs, whether heat is weak or absent, whether only auxiliary heat works, and whether a breaker trips. Then ask which sequencer, relay, heat-strip stage, thermostat call, and amperage reading was tested.

For heat pumps, write down whether cooling works, whether outdoor heat works, whether auxiliary heat stages, whether defrost happens, and whether the thermostat was recently replaced. Then ask whether the thermostat is configured for the exact equipment type and whether the reversing valve, outdoor unit, and auxiliary heat were tested separately.

If replacement is still recommended, ask for the evidence sentence: "The symptom was X, the measured failed item was Y, cheaper causes A/B/C were ruled out, and replacement is recommended because Z." If the answer is only "it is old," you do not have a diagnosis yet.

Replacement is valid when the diagnosis supports it

Replacement becomes more serious when multiple major parts are failing, the compressor is confirmed shorted or mechanically locked, the heat exchanger is unsafe, the coil is old and leaking, refrigerant economics are ugly, or the system is near end of life and repair cost is high.

But the quote should still connect the homeowner symptom to evidence. "Old" is not a diagnosis.