Last updated July 30, 2026
AC Repair & Installation Troubleshooting: Common Problems and How to Fix Them
The single most expensive word in HVAC service is “compressor.” It’s also the diagnosis technicians reach fastest when they’re guessing, because a compressor replacement is billable whether or not it was actually the problem. In 13 years running Redwood Heating & Air in San Diego, Gregory Hahn has seen homeowners quoted $3,000+ for compressor replacements when the real issue was a $45 capacitor or a clogged return grille. This guide—along with our The Complete AC Repair & Installation Guide for San Diego Homeowners—teaches you the diagnostic logic behind AC symptoms, not just a checklist that ends with “call a pro.” You’ll learn how to distinguish three problems with nearly identical symptoms but wildly different repair costs, how to read your system’s electrical sequence, and the five observable clues that tell a real diagnostic story. San Diego’s coastal-to-inland climate swings, from Pacific Beach’s salt air to Santee’s 110°F summer peaks, create unique failure patterns that generic national guides miss entirely.
Quick Answer
Most AC problems fall into three categories: airflow restrictions (dirty filters, blocked vents, failing blower motors), refrigerant circuit issues (leaks, undercharge, TXV failure), or electrical/control failures (capacitors, contactors, circuit boards). Homeowners can safely check filters, thermostat settings, and visible ice buildup, but refrigerant work and electrical diagnostics require EPA certification and proper tools, which is why our AC Repair & Installation services include thorough professional diagnostics. The key is knowing which symptoms point to which category so you describe the problem accurately and avoid unnecessary repairs.
Table of Contents
- How Your AC Works: The Electrical Sequence That Reveals Failures
- “It’s Running But Not Cooling”: The One Field Test That Separates Causes
- Refrigerant Undercharge vs. Failing TXV vs. Dirty Evaporator
- The Five Homeowner-Observable Clues That Tell a Real Story
- What Modern Error Codes Actually Mean (And What Technicians Sometimes Claim)
- Common Mistakes to Avoid
- When to Call a Professional
- Frequently Asked Questions
How Your AC Works: The Electrical Sequence That Reveals Failures
Your air conditioner is an electrical machine that happens to move refrigerant. Understanding the sequence of operation, what we call the “control chain,” lets you pinpoint where a failure occurs instead of describing vague symptoms like “it’s not working right.”
Here’s the sequence in every standard split system, from thermostat call to cooling output:
- Thermostat sends 24V signal to the control board, requesting cooling
- Contactor pulls in, connecting high voltage to the outdoor unit
- Capacitor delivers start boost to both compressor and condenser fan motor
- Compressor starts, pumping refrigerant through the system
- Condenser fan runs, rejecting heat to outside air
- Indoor blower circulates conditioned air through ductwork
Each step depends on the previous one. When you know this chain, you can describe failures with precision that saves diagnostic time and prevents misdiagnosis.
Contactor failure: You’ll hear the thermostat click, but nothing happens outside. No hum, no fan movement. The contactor’s coil may have failed, or its contacts are pitted from San Diego’s occasional voltage fluctuations during Santa Ana wind events.
Capacitor failure: The compressor tries to start, you’ll hear a hum or buzz, then it may trip the breaker or overheat and shut off on internal protector. Capacitors degrade faster in coastal San Diego communities like La Jolla and Coronado where salt air corrodes electrical connections.
Compressor failure (actual): Capacitor and contactor test good, electrical connections are clean, but the compressor draws locked-rotor amps or won’t pump. This is the real, expensive failure, and it’s rarer than the number of compressor replacements sold.
When you call for service, describe exactly where in this sequence your system stops. “The outdoor unit hums but the fan doesn’t spin” points to capacitor. “Nothing happens outside when the thermostat clicks” points to contactor or low-voltage issue. This specificity signals to any technician, including our team at AC Repair & Installation in San Diego, that you’ve done real observation, not guesswork.
“It’s Running But Not Cooling”: The One Field Test That Separates Causes
This is the most common AC complaint in San Diego, where systems run long hours through September and October heat waves. The thermostat shows cooling mode, air is blowing from vents, but the house isn’t getting comfortable. Three completely different problems cause this identical symptom, and the wrong diagnosis costs hundreds or thousands.
The field test: Measure supply air temperature at the vent closest to the indoor unit, and return air temperature at the return grille. Use any kitchen thermometer, digital preferred. Run the system 15 minutes, then check.
- Healthy system: 15-20°F difference (supply air 55-60°F, return air 75-80°F)
- Airflow problem: Less than 15°F difference, but supply air still feels cool. Weak volume from vents, dirty filter, blocked return, or failing blower motor
- Refrigerant problem: Less than 15°F difference, supply air feels only slightly cool or room-temperature. Low charge, restriction, or compressor not pumping effectively
- Controls problem: Normal temperature difference but house won’t cool. Thermostat in wrong location, system undersized, or duct leakage pulling hot attic air
In San Diego’s older neighborhoods like North Park and South Park, we’ve found that 30% of “not cooling” calls in homes built before 1970 are actually severe duct leakage in unconditioned attics, not equipment failure at all. The temperature difference test is normal, but cooled air never reaches the rooms.
Here’s how to interpret your results without a pressure gauge set:
- Check the filter first. A filter clogged with Pacific Beach sand or Mission Valley dust reduces airflow before it reduces temperature
- Feel the large insulated refrigerant line (suction line) at the outdoor unit. Should be cold and sweating in cooling mode. Warm or ambient temperature suggests refrigerant issue or compressor problem
- Check for ice anywhere. Ice location tells the story, covered in the next section
- Listen to compressor pitch. A healthy compressor has a steady, medium-low hum. A high-pitched whine or clicking suggests mechanical distress
This four-step observation gives any technician, including Gregory Hahn’s team, a head start on diagnosis and demonstrates that you won’t accept a guess.
Refrigerant Undercharge vs. Failing TXV vs. Dirty Evaporator
Three problems with nearly identical symptoms: frozen indoor coil, reduced cooling, long run times, and eventually compressor damage if ignored. Yet their repair costs range from $150 (cleaning) to $400 (TXV replacement) to $1,200+ (refrigerant leak detection and repair). Knowing how to distinguish them protects you from the technician who sees ice and immediately recommends “a full refrigerant recovery and recharge.”
Refrigerant undercharge (leak): Ice forms first on the large suction line at the outdoor unit, then progresses to the indoor evaporator. The suction line feels cool but not cold near the compressor, and may be oil-stained at leak points. System runs constantly, never reaches setpoint. In San Diego’s coastal climate, formicary corrosion from salt air accelerates pinhole leaks in copper linesets, especially in beach communities where we’ve replaced dozens of linesets in 8-10 year old systems that should have lasted 20 years.
Failing TXV (thermostatic expansion valve): Ice forms unevenly across the evaporator coil, often concentrated at the valve outlet. Superheat readings swing wildly if measured. The TXV may hunt, opening and closing, causing short cycling or erratic pressures. Common in higher-end systems from Carrier and Trane with electronic TXVs, particularly after power surges during San Diego’s fall Santa Ana windstorms—our Choosing the Right HVAC Brand: A Buyer’s Guide for San Diego explains how brand selection affects long-term reliability.
Dirty evaporator: Ice forms uniformly across the entire coil face, starting from the air-entering side. Airflow is visibly restricted at vents. The coil itself may be visible through a maintenance panel and show gray, fuzzy buildup. This is the cheapest fix, but some technicians won’t mention it because cleaning pays less than refrigerant work.
| Symptom | Undercharge | Failing TXV | Dirty Evaporator |
|---|---|---|---|
| Ice location | Starts at suction line outdoors | Uneven, near TXV outlet | Uniform across coil face |
| Airflow at vents | Normal volume, warm air | Normal or fluctuating | Noticeably reduced |
| Suction line temp | Cool, not cold; possible oil stain | Erratic, swinging | Cold, but low airflow |
| System age when seen | Any age, often 5-12 years | 7-15 years, electronic TXV | 3+ years without maintenance |
| Typical San Diego repair cost | $400-1,200+ (find and fix leak) | $350-600 (TXV replacement) | $150-250 (professional cleaning) |
The critical distinction: a dirty evaporator can be confirmed visually and fixed without touching refrigerant. A TXV failure requires superheat and subcooling measurements with proper gauges. A true undercharge means there’s a leak, and recharging without finding it violates EPA regulations and wastes your money.
At Redwood Heating & Air, Gregory Hahn’s approach is to prove the diagnosis before quoting the repair. We’ve seen competitors in San Diego charge for “2 pounds of R-410A” on systems that were simply dirty, or replace TXVs that were actually responding correctly to low airflow from a collapsed duct.
The Five Homeowner-Observable Clues That Tell a Real Story
Professional technicians gather dozens of data points, but homeowners can provide five critical observations that dramatically improve diagnostic accuracy. Write these down before calling any service company.
1. Ice formation location
As detailed above, where ice first appears predicts the failure category. Note whether it’s on the outdoor suction line, indoor coil, or both. Photograph it if safe to do so.
2. Refrigerant line temperature at the outdoor unit
The large insulated line (suction line) should be cold and sweating during normal operation. The smaller uninsulated line (liquid line) should be warm but not hot. In San Diego’s direct summer sun, a liquid line that’s too hot suggests condenser airflow problems or overcharge. A suction line that’s ambient temperature with the compressor running strongly indicates refrigerant issue or compressor valve failure.
3. Condenser fan behavior
Does the outdoor fan start with the compressor, or delay? Does it ever stop while the compressor keeps running? On variable-speed systems from Daikin and some Carrier models, fan speed modulation is normal. On standard single-stage systems, fan stopping while compressor runs indicates control board or fan motor issues, not compressor failure.
4. Indoor humidity levels
San Diego’s coastal humidity, especially in neighborhoods within a mile of the bay or ocean, makes this clue particularly valuable. A system that cools but leaves humidity at 65%+ likely has oversized equipment, short cycling, or low airflow across the evaporator. The coil doesn’t get cold enough, long enough, to dehumidify. This is common in older San Diego condos where original 2-ton systems were replaced with 2.5-ton “to be safe,” destroying comfort.
5. Short cycle timing
Time the cooling cycles. A system that runs 3-5 minutes, shuts off 2-3 minutes, then restarts is short cycling. Causes include:
- Oversized equipment (most common in San Diego retrofits)
- Low refrigerant charge triggering low-pressure cutout
- Dirty condenser causing high-pressure cutout
- Thermostat in poor location (direct sun, near kitchen)
- Restricted airflow causing coil freeze and pressure switch trip
Short cycling destroys compressors through excessive start-stop wear and never dehumidifies properly. The timing pattern helps distinguish causes: immediate restart suggests electrical/control, several-minute delay suggests pressure switch protection, random intervals suggest hunting TXV or failing capacitor.
These five observations, communicated clearly, transform a service call from “come look at my AC” to “my suction line is ambient temperature, the condenser fan stops after 10 minutes while the compressor keeps running, and it’s short cycling on a 5-minute pattern.” That technician arrives with test points in mind, not a sales pitch prepared.
What Modern Error Codes Actually Mean (And What Technicians Sometimes Claim)
Communicating HVAC systems from Carrier Infinity, Trane ComfortLink, Bryant Evolution, and high-end Daikin units display error codes that homeowners can read on thermostats or control boards. These codes are specific, but their interpretation requires understanding what the system is actually reporting versus what a technician might claim.
Carrier/Bryant Infinity and Evolution systems: Code 13 indicates limit circuit lockout, but this can mean high limit, roll-out limit, or auxiliary limit depending on which stage failed. A technician who says “your limit switch is bad” without specifying which one and why it tripped is guessing. The real question: what caused the limit to open? Restricted airflow, failing inducer, or cracked heat exchanger in heating mode.
Code 31 on these systems indicates pressure switch fault. In San Diego’s coastal environment, we’ve found that pressure switch tubing gets waterlogged from condensate backup or corrodes at the barbed fittings, causing false readings while the actual refrigerant pressures are normal. Replacing a pressure switch without checking the tubing is a $200 mistake.
Trane ComfortLink systems: Code 126 indicates “low stage pressure switch not closed.” This often gets diagnosed as “low refrigerant,” but on systems with two-stage compressors, it can mean the compressor failed to transition to low stage, the pressure switch itself is stuck, or the control board isn’t sending the stage signal. Each has a different repair cost.
Daikin VRV/VRF systems: These display U-series codes (U0, U1, U2, etc.) that refer to communication errors between indoor and outdoor units. In San Diego commercial installations, we’ve traced U0 codes to corroded communication terminals in rooftop units exposed to marine layer moisture, not to failed boards. The fix was terminal cleaning and dielectric grease, not $800 in replacement parts.
The pattern: error codes report what the control system detected, not necessarily what failed. A pressure switch code means the switch didn’t close or open as expected. The switch might be bad. The pressure might actually be wrong. The wiring to the switch might be damaged. The control board might not be reading correctly.
When a technician quotes based solely on the code, ask: “What specifically tests did you run to confirm the component failed rather than the sensor reading incorrectly?” This question separates technicians who understand systems from parts-changers. At Redwood Heating & Air, Gregory Hahn’s diagnostic process always verifies the component, not just the code.
For homeowners with communicating systems, photograph the code and note whether it occurs consistently or intermittently. Intermittent codes that clear on reset often indicate connection or environmental issues, not hard component failures.
Common Mistakes to Avoid
- Replacing the thermostat first. Thermostat failure is rare; misdiagnosis is common. A $200 thermostat replacement rarely fixes a low refrigerant charge or airflow problem. Test the control chain before blaming the thermostat.
- Adding refrigerant without leak detection. EPA Section 608 regulations require repair of substantial leaks. More importantly, refrigerant doesn’t “wear out” or get consumed. If it’s low, it’s leaking. Any technician who recharges without finding the leak is violating regulations and guaranteeing a repeat failure.
- Ignoring condenser clearance. San Diego’s dense neighborhoods mean outdoor units often get enclosed by fences, landscaping, or additions. A condenser needs 24 inches clear on all sides for proper airflow. We’ve seen systems “fail” that simply couldn’t reject heat because a fence was installed 8 inches from the coil.
- DIY capacitor replacement without discharge procedure. Capacitors store lethal voltage even with power disconnected. This is genuine high-voltage electrical work. The safety caveat here is absolute: do not attempt capacitor replacement without proper training and discharge tools. Call a professional.
- Assuming “bigger is better” for replacement sizing. An oversized system short cycles, dehumidifies poorly, and fails prematurely. San Diego’s mild climate means proper load calculation matters more than raw tonnage. We’ve replaced 3.5-ton systems with properly sized 2.5-ton units that delivered better comfort and lower bills.
- Neglecting maintenance in “mild” climates. San Diego’s moderate temperatures lull homeowners into skipping annual service. But salt air, dust from Santa Ana winds, and long run seasons create accelerated wear. Systems here often run 2,000+ hours annually, comparable to much hotter climates where maintenance expectations are higher.
When to Call a Professional
Some AC issues genuinely require professional diagnosis and repair. Call a licensed HVAC contractor when you encounter: refrigerant-related symptoms (ice on lines, hissing sounds, oil stains), electrical failures beyond a tripped breaker, any burning smell or visible electrical damage, compressor issues confirmed by amp draw testing, or error codes that persist after power cycle. San Diego’s municipal code requires permits for equipment replacement and major refrigerant line modifications, and unpermitted work can complicate home sales.
Redwood Heating & Air San Diego offers free estimates throughout San Diego, from coastal communities to inland valleys, and you can explore more guides & resources on our blog before scheduling. Gregory Hahn brings 13 years of field experience and Master Technician credentials to every diagnostic call, across nine major equipment brands. When you’re facing an AC problem that doesn’t match simple fixes, call (844) 875-9451 for an accurate diagnosis before any repair commitment.
Frequently Asked Questions
Common AC repairs in San Diego range from $150 for capacitor or contactor replacement to $400-800 for TXV or blower motor replacement, and $1,200-3,500 for compressor replacement or major refrigerant leak repair. Coastal corrosion and inland heat exposure create different failure patterns that affect pricing. Call (844) 875-9451 for a free estimate with upfront pricing before any work begins.
Constant running with poor results indicates either insufficient cooling capacity (undersized equipment, severe duct leakage, or refrigerant undercharge) or excessive heat gain (poor insulation, single-pane windows, or significant air infiltration). In San Diego’s older housing stock, particularly in neighborhoods like Kensington or Talmadge, we’ve found that 25% of “undersized” diagnoses are actually 30%+ duct leakage in unconditioned crawl spaces. A proper load calculation and duct pressure test reveals the real cause.
No. EPA Section 608 certification is legally required to handle refrigerants, and DIY kits cannot accurately measure charge level or detect leaks. More importantly, adding refrigerant to a leaking system wastes money, violates environmental regulations, and often overcharges the system, causing compressor damage. The proper repair is leak detection, repair, and precise evacuation and recharge with proper measurement.
Standard 1-inch pleated filters need replacement every 30-60 days during heavy use (May through October in San Diego), and every 60-90 days during lighter seasons. Homes near construction, with pets, or in dusty inland areas like Santee or El Cajon need more frequent changes. Media filters (4-5 inch) typically last 6-12 months. A clogged filter is the single most common cause of service calls we see, and it’s completely preventable.
A heat pump reverses the refrigeration cycle to provide both cooling and heating from the same equipment, while a standard AC only cools and requires a separate furnace for heat. In San Diego’s climate, heat pumps operate efficiently year-round and eliminate combustion heating in many homes. Modern cold-climate heat pumps from Daikin and Rheem handle San Diego’s occasional 40°F nights without backup heat. For homes considering conversion, our Heat Pump & Mini Split in San Diego page covers options in detail.
Musty odors indicate biological growth on the evaporator coil, in the condensate pan, or on blower components. San Diego’s coastal humidity, especially in homes without whole-home dehumidification, creates conditions where mold and mildew colonize wet surfaces. The smell typically fades after a few minutes as condensation rinses the coil surface, but the underlying growth persists. Professional coil cleaning, pan treatment, and UV light installation address the source; air fresheners mask it temporarily while potentially spreading spores.
Replace when repair costs exceed 30-40% of replacement cost on a system over 12 years old, when the system uses phased-out R-22 refrigerant (repairs become increasingly expensive), or when cumulative repair costs over two years approach replacement cost. In San Diego, where electricity rates are among the nation’s highest, efficiency improvements from modern 16+ SEER equipment often justify replacement on operating cost alone. A 13-year-old 10 SEER system replaced with a 16 SEER unit saves approximately 38% on cooling costs, which in San Diego’s rate structure can mean $400-700 annually.
The Bottom Line
Effective AC troubleshooting isn’t about memorizing symptoms, it’s about understanding the diagnostic logic that connects symptoms to causes. The electrical sequence, the temperature difference test, ice location, and the five observable clues give you a framework to describe problems precisely and evaluate technician recommendations critically. San Diego’s unique coastal-to-inland climate creates failure patterns that generic advice misses, from salt-air corrosion to humidity challenges in marine-influenced neighborhoods. When diagnosis exceeds safe homeowner observation, Redwood Heating & Air San Diego provides the technical depth that comes from 13 years of founder-led service, 428+ verified reviews, and genuine multi-brand expertise across Carrier, Trane, Lennox, Rheem, York, Daikin, Bryant, Goodman, and American Standard equipment.
Ready for an accurate diagnosis? Call Redwood Heating & Air San Diego at (844) 875-9451 for a free estimate. Gregory Hahn personally oversees technical standards, and we service every major brand with the accountability that comes from owner-level involvement in every job.
Written by Gregory Hahn, Founder & Master Technician at Redwood Heating & Air San Diego, serving San Diego since 2013.