Troubleshooting HVAC Systems Step by Step
Use a repeatable process on any no-cool or no-heat call. Gather facts, check the simple things, measure, then match symptoms to causes.
Table of contents
Good troubleshooting is not about knowing every part. It is about following the same process each time, so you do not jump to a guess. A guess might get lucky, but it usually ends in a callback. Use this process on any HVAC complaint, whether the system is residential or light commercial.
What You Will Be Able to Do
- Gather the facts of a complaint before you touch anything.
- Check the common simple causes in order.
- Separate airflow, electrical and refrigerant problems.
- Match common symptoms to likely causes.
- Confirm your repair before you leave.
Step 1: Gather Information
Ask the person who called:
- What is it doing, or not doing?
- When did it start? Is it constant or on and off?
- Did anything change, such as a power outage, recent work, a new filter or new thermostat?
- Have you noticed any noise, smell or water?
Read any fault codes on the control board before power is cycled. Resetting can erase them.
Step 2: Check the Simple Things
These save a lot of time and embarrassment.
- Is the thermostat set correctly, with batteries or power?
- Are the breakers on, both indoors and outdoors?
- Are the disconnect and the furnace switch on?
- Is the filter clean?
- Are the supply and return registers open?
- Is the outdoor unit clear of debris?
- Is a float switch tripped from a full drain pan?
Step 3: Split the Problem
Most HVAC faults fall into one of four groups. Decide which group first:
| Group | Quick test | Next tool |
|---|---|---|
| Airflow | Filter, blower, temperature split, static pressure | Manometer |
| Electrical or controls | Is there a call? Is there 24 volts? Is line voltage present? | Multimeter |
| Refrigerant | After airflow is good, check pressures and temperatures | Gauges and probes |
| Combustion or gas | Does the sequence stop? What code is showing? | Meter, manometer, analyzer |
Check airflow before refrigerant. Airflow problems can look just like low charge on a gauge.
Step 4: Measure
Take readings and compare them to what is normal. A measurement beats a hunch.
For cooling complaints:
- Check the filter and airflow.
- Measure supply and return air, and find the split. About 15 to 22°F is normal.
- Check that the outdoor unit is running. If not, check for 24 volts at the contactor and line voltage at the disconnect.
- Measure compressor amps.
- Have a certified technician check superheat, subcooling and pressures.
- Inspect the condenser coil.
For heating complaints on a gas furnace:
- Confirm a call for heat at W.
- Watch the sequence and read the board's fault code.
- Note where the sequence stops. The inducer, pressure switch, ignitor, gas valve and flame sensor come in that order.
- Check the filter and temperature rise.
Step 5: Match Symptoms to Causes
| Symptom | Likely causes | Key check |
|---|---|---|
| Outdoor unit will not run | No 24 volts, bad contactor, bad capacitor, tripped breaker | Voltage at the contactor |
| Compressor hums and trips | Bad capacitor, low voltage, locked compressor | Capacitor test, amps at start |
| Fan runs, compressor does not | Bad capacitor, overheated compressor, overload open | Capacitor test, shell temperature |
| Cooling but not enough | Low charge, dirty coil, low airflow | Temperature split, superheat, subcooling |
| Ice on the indoor coil | Low airflow or low charge | Filter, then superheat |
| Short cycling | Dirty filter, thermostat problem, oversized system, a safety tripping | Filter, thermostat setup, sizing |
| Water around the air handler | Plugged drain, frozen coil thawing | Drain and airflow |
| High electric bills | Dirty coils, low charge, duct leaks | Coil, charge, duct leakage |
| Furnace lights then goes out | Dirty flame sensor | Microamp reading |
| Furnace shuts off on high limit | Low airflow | Filter, blower, temperature rise |
| Compressor runs, no cooling, pressures almost equal | Failed compressor valves | Suction and discharge pressure while running |
| Hot air from registers on cooling | Reversing valve fault on a heat pump | O/B wiring, valve solenoid |
| Condenser fan not running | Bad capacitor, bad motor, seized bearing | Capacitor test, then spin the blade by hand with power locked out |
| Pressures swing up and down | TXV hunting from a loose bulb or moisture | Secure and insulate the sensing bulb, check the drier |
| Low suction, cold spot at the drier | Restriction | Temperature drop across the drier, which should be under about 3°F |
When Pressures Are Nearly Equal
If a compressor is running but suction and discharge pressures are almost the same, the compressor is not pumping. Gas is leaking past its internal valves. A failed compressor valve is the most likely cause. On a heat pump, also think about a stuck reversing valve, which can cause a similar pattern.
Warning: Testing live equipment is work for qualified people. Use lockout before you touch parts, and keep clear of fans and terminals when power is on.
Step 6: Fix the Root Cause
A blown fuse has a short behind it. A frozen coil has a filter or leak behind it. A burned contactor may have a weak capacitor or a voltage problem behind it. Ask what caused the failed part to fail, and fix that too.
Step 7: Verify
- Run the system through a full cycle.
- Re-measure the readings that were wrong.
- Check that safeties and any drains work.
- Clean up and explain what happened in plain words.
- Write it down.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Replacing parts until it works | Money wasted, and the customer loses trust |
| Resetting before reading the code | You lose the best clue |
| Hooking up gauges first | Misreading an airflow problem as a charge problem |
| Stopping at the first fault found | The cause of the failure stays |
| Leaving without running a full cycle | A callback the same day |
Check Your Understanding
A customer says the AC stopped after a storm. What should you check first, before gauges? Breakers, the disconnect, the thermostat, and any blown low-voltage fuse.
The indoor coil is frozen. What is the correct first move? Check the filter and airflow, and let the coil thaw before you take refrigerant readings.
The fan and compressor run and pressures are almost equal. What does that point to? The compressor is not pumping, likely from failed valves.
You replaced a blown control fuse, and it blew again right away. What should you do? Stop replacing fuses, and find the short.
Key Takeaways
- Ask questions and read fault codes before you touch anything.
- Check the simple causes first: thermostat, power, filter, registers and drains.
- Decide if a problem is airflow, electrical, refrigerant or combustion.
- Check airflow before refrigerant.
- Fix the cause, and prove the repair with measurements.