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Quick, low-risk steps

  1. Move vulnerable people to a cooled location if indoor temperatures are dangerous.
  2. Check thermostat: set to COOL, temperature below room temp, and fan on AUTO.
  3. Inspect a visible air filter; if clogged, replace with a standard disposable filter.
  4. Look for a tripped circuit breaker labeled AC or furnace; if tripped, reset once only.
  5. Record the exact model number and any error code displayed on the unit or thermostat.

Start with Safety and Simple Checks

In Phoenix, a non-working AC can quickly become a health risk. Before anything else, ensure that children, older adults, and pets are in a safe, cool environment. If indoor temperatures are extreme, consider relocating to a public cooling center or a neighbor's home.

Once safety is addressed, perform a few basic observations. These steps are safe for any homeowner and can often reveal simple issues like incorrect thermostat settings or a dirty filter. Do not attempt to open the outdoor unit or indoor cabinet.

  • Confirm the thermostat is set to COOL and the temperature is set lower than the current room reading.
  • Check if the fan is set to AUTO; ON may run the fan without cooling.
  • Replace a visibly dirty air filter with a standard disposable filter of the correct size.
  • Look for a tripped circuit breaker; if the AC breaker is in the middle or OFF position, flip it fully OFF then ON once.
  • Ensure the outdoor unit is not blocked by debris, vegetation, or covers.

Gather Evidence for the Technician

Before calling for service, collect information that will help the contractor diagnose the problem efficiently. This can reduce the time spent on-site and may lower the diagnostic charge.

Write down the make and model of your indoor and outdoor units. If your thermostat displays an error code, record it exactly. Note any unusual sounds, smells, or behaviors you observed before the system stopped working.

  • Locate the model number on the outdoor condenser unit (usually on a data plate).
  • Check the indoor air handler or furnace for a separate model number.
  • Photograph any error codes on the thermostat or control board.
  • Describe any recent events: power outage, storm, or previous repair.
  • Note if the system was making unusual noises or short-cycling before failure.

Understand What a Pro Will Check

A licensed HVAC contractor will perform a systematic diagnosis that goes beyond homeowner checks. They will inspect electrical components, refrigerant levels, and safety controls. This work requires specialized tools and training.

Common causes of a no-start condition include failed capacitors, contactor issues, tripped internal safety switches, or control board failures. Refrigerant problems can also prevent the system from starting. Only an EPA-certified technician should handle refrigerant.

  • Technicians check voltage and electrical connections inside the unit.
  • They test capacitors, contactors, and relays for proper function.
  • Refrigerant pressure is measured to rule out leaks or charge issues.
  • Safety switches, such as float switches in the drain pan, are inspected.
  • Thermostat wiring and communication are verified.

Questions to Ask Before You Hire

When you call for service, ask clear questions to understand the process and costs. A reputable contractor will explain their diagnostic fee, what it includes, and whether it applies to the repair if you proceed.

Inquire about licensing, insurance, and experience with your specific brand. Ask for a written estimate before any work begins. Understanding warranty terms on parts and labor can prevent surprises later.

  • What is your diagnostic charge, and does it apply to the repair cost?
  • Are you licensed and insured in Arizona?
  • Do you have experience with [your brand] systems?
  • Will you provide a written estimate before starting repairs?
  • What warranties do you offer on parts and labor?

Take this with you

Evidence and scope checklist

  • Thermostat settings and batteries checked
  • Air filter condition noted
  • Circuit breaker status confirmed
  • Model numbers of indoor and outdoor units recorded
  • Error codes documented
  • Unusual sounds or smells described
  • Recent power events or repairs noted
  • Diagnostic fee and application to repair clarified
  • Contractor license and insurance verified
  • Written estimate and warranty terms requested
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Common questions

What homeowners ask next

Why won't my AC turn on even though the thermostat is on?

Possible reasons include a tripped circuit breaker, a blown fuse, a faulty thermostat, a clogged filter causing a safety shutoff, or a failed component like a capacitor. A professional can diagnose the exact cause.

Can I reset my AC unit myself?

You can safely reset the circuit breaker once if it has tripped. Do not repeatedly reset a breaker that trips again, as this indicates a serious electrical fault. Do not open the unit to press internal reset buttons.

Is it safe to use my AC if it smells like something is burning?

No. Turn off the system at the thermostat and the circuit breaker if you can do so safely. Evacuate if the smell is strong or you see smoke, and call 911. Then contact an HVAC professional.

Evidence

Sources used for this guide

Research checked July 31, 2026

Optional next step

Need an on-site professional?

Use the guide and worksheet first so you can describe the situation and compare the full scope. A referral is optional; you can also contact a licensed local provider directly.

Referral matching is not enabled yet.

We have not enabled a provider route for this site. Use the checklist to contact and verify local providers yourself; urgent hazards should go to the appropriate emergency or public service.