Trane AC Not Cooling — XR15, XR14, XL20i, XV20i, 4TTB Diagnosis Guide

Common Symptoms

• House won't cool — AC runs constantly but indoor temp won't drop
• Compressor hums briefly then shuts off — condenser fan may run but compressor won't start
• AC starts cooling then shuts off after 1–2 minutes, restarts after 5 minutes
• Outdoor unit runs but blows hot air upward instead of feeling warm at the top
• Comfort Link II or Nexia thermostat displays fault code or service alert
• Ice visible on refrigerant lines or indoor coil
• AC runs and cools poorly on hot days but seems fine on cooler days

Most Likely Causes

1. 1

   Failed Run Capacitor (CPT00674 / CPT00651 — Most Common)

   Trane outdoor units use a dual-run capacitor that powers both the compressor start/run and the condenser fan motor. The 2-ton common Trane capacitor is CPT00674 and the 3-ton is CPT00651, but always verify by reading the label on your existing capacitor — the µF and voltage ratings are printed on the can. A failed capacitor shows as a compressor that hums and trips on overload (the thermal overload protector in the compressor will reset after 30–60 minutes of cooling, explaining intermittent no-cooling that 'fixes itself'). The condenser fan may spin slowly or not at all if the FAN section is weak. Test in capacitance mode — replace if more than 10% below rated µF.

2. 2

   Time-Delay Relay (TDR) Preventing Compressor Restart

   Trane installs a time-delay relay (TDR) on the compressor circuit to prevent short-cycling — the compressor cannot restart for approximately 5 minutes after shutting off. This is normal protection. However, a failed TDR can lock out the compressor indefinitely, making it appear the compressor has failed. Symptom: thermostat calls for cool, condenser fan runs, compressor is silent for more than 10 minutes after power is restored. Test by measuring resistance across the TDR coil terminals and checking the normally-closed contacts — an open TDR coil or welded-closed contacts both cause problems. The TDR is typically mounted inside the outdoor unit's electrical compartment.

3. 3

   WeatherGuard™ Top Cap Blocking Condenser Airflow

   Trane's WeatherGuard top cap is a dome-shaped cover that protects the condenser fan motor from rain. If the cap has been removed for service and reinstalled incorrectly — inverted, off-center, or with the discharge louvers facing the wrong direction — it partially blocks the condenser fan discharge airflow. Symptoms: high-side pressure elevated, unit trips on high-pressure lockout, poor cooling on hot days but acceptable on mild days. Check the Trane installation label inside the unit for the correct cap orientation. The cap should allow free upward airflow from the fan.

4. 4

   Dirty Spine Fin™ Coil — Low Pressure Cleaning Only

   Trane Spine Fin aluminum coils use a different fin geometry than standard fin-and-tube designs — the aluminum spines are thinner and more easily bent. Never use a pressure washer on a Trane Spine Fin coil. Use a garden hose at low pressure from inside-out. Despite Trane's CleanCoil™ marketing (a factory coating intended to reduce fouling), Spine Fin coils still accumulate cottonwood, dirt, and biological growth and should be inspected annually. Fouled coil: high-head pressure, elevated outdoor unit temperature, poor cooling capacity on hot days.

5. 5

   Comfort Link II / Nexia Fault Code — Investigate Before Measuring

   Trane Comfort Link II communicating systems and Nexia-connected thermostats log outdoor unit fault codes with timestamps. The thermostat can display high-pressure faults, low-pressure faults, communication faults, and outdoor unit lockout codes. On a ComfortLink II system (typically paired with XV20i or XL20i), navigate to the thermostat's service menu or diagnostics screen before opening the outdoor unit. The fault log often identifies whether the problem is refrigerant-side (high or low pressure trips), electrical (loss of communication, phase fault), or compressor-specific (locked rotor, overload). This saves time compared to blind component testing.

6. 6

   XV20i Variable Speed Compressor — Different Diagnostic Logic

   The Trane XV20i uses a variable speed compressor that stages from approximately 25% to 100% capacity — it does not cycle on and off like a single-stage unit. Diagnosing it with standard single-stage assumptions leads to wrong conclusions. On a mild day, the XV20i may run at low speed continuously — this is normal. Delta-T across the indoor coil (16–22°F target) is more useful than pressure readings for capacity verification. For refrigerant charge, Trane specifies a superheat and subcooling target at specific outdoor temp and indoor load conditions — the XV20i requires more precise charge verification than single-stage units. Communication fault codes from the inverter board are common on XV20i units and usually indicate wiring harness issues before actual board failure.

Quick DIY Checks

1. 1Read Comfort Link II or Nexia fault codes first: on a Comfort Link II thermostat, press the menu button and navigate to Service → Equipment Status or Diagnostics → Fault History. Write down all active faults and recent fault codes with timestamps. On a Nexia-connected tstat, check the app for equipment alerts. Common codes: 79 or 91 = high-pressure trip (fouled coil or refrigerant overcharge), 86 = low-pressure trip (low charge or TXV issue), 26 = loss of communication with outdoor board. On non-communicating Trane units, check the outdoor unit control board LED — the code key is on the inside of the service panel.
2. 2Inspect the WeatherGuard top cap: look at the top of the outdoor unit. The fan should be blowing air upward through the top grille. If there is a dome-style cap, verify it is seated correctly with the discharge louvers aligned with the fan blade rotation — the cap should allow unrestricted upward airflow. If the cap appears cocked, inverted, or if the discharge opening is blocked, remove and correctly reinstall it per the label inside the unit. After reinstalling, run the system for 10 minutes and check that high-side pressure (if gauges are available) drops vs. the previous run.
3. 3Capacitor test: turn off the disconnect box next to the outdoor unit and wait 5 minutes. Remove the access panel. Locate the dual-run capacitor — three terminals labeled HERM, FAN, C (common). Disconnect wires one at a time and test with a multimeter in capacitance mode: HERM-to-C gives the compressor µF, FAN-to-C gives the fan µF. Compare to the label — CPT00674 (2-ton) and CPT00651 (3-ton) are common, but your specific unit may differ. Replace if either section reads more than 10% below rated µF. Match µF ratings and voltage exactly. For units under 5 years old with a failed capacitor, check whether a previous technician installed a mismatched replacement.

1. 4Time-delay relay (TDR) test: on a Trane outdoor unit, the TDR is typically a small relay with two coil terminals and one set of normally-open contacts (NO). With the unit powered off, disconnect the relay from the circuit. Set a multimeter to resistance mode and measure across the coil terminals — should read a few hundred ohms (check your specific relay's datasheet if available). Then check the NC or NO contacts depending on the relay type — contacts should open or close as expected. If the coil is OL (open circuit) or the contacts test the wrong state, replace the relay. Also check the TDR timer function: after restoring power and cycling the system off, the compressor should be locked out for approximately 5 minutes then restart normally.
2. 5Spine Fin coil cleaning: turn off the unit at the disconnect. Remove the top grille if access allows. Using a standard garden hose set to a gentle spray — not a pressure washer, not a garden nozzle at high pressure — spray from inside the coil outward. Work section by section around the coil perimeter. Do not insert the hose nozzle between the fins. Do not use coil cleaners with acidic or caustic chemistry on Spine Fin coils without verifying Trane's recommendation — aggressive chemistry can etch the thin aluminum. After cleaning, restore power and run for 15 minutes. If R-410A high-side pressure was elevated before cleaning (above 400 PSI at 85°F outdoor), confirm it drops to a normal range after cleaning.
3. 6Contactor check: with the disconnect off and capacitor bled, inspect the contactor. Look at the contact faces (visible in the gap when the contactor is open) for pitting or burn marks. Severe pitting means replacement. Restore power and — with the thermostat calling for cooling — use a multimeter to measure 24VAC across the contactor coil terminals (the small pair of low-voltage wires, not the high-voltage line side). 24VAC present and contactor not pulling in = open coil, replace. Contactor pulled in but compressor not running = check capacitor, then verify power to the compressor terminals.
4. 7R-22 vs R-410A identification: Trane residential units manufactured from 2010 onward use R-410A refrigerant. Older XB and XR series units (pre-2010) may use R-22. The refrigerant type is on the unit data plate and also labeled on the service valve ports — R-22 uses 1/4-inch Schrader ports, R-410A uses 5/16-inch ports. Do NOT add R-410A to an R-22 system or vice versa. If you have an older Trane on R-22 that needs refrigerant, the economics of repair vs. replacement change significantly — R-22 is no longer manufactured and is expensive. A licensed tech must handle refrigerant on either system.

Repair vs Replace

Recommended Tools & Parts

• Trane Dual-Run Capacitor (CPT00674 / CPT00651)

  Replacement dual-run capacitor for Trane outdoor condensers. CPT00674 is common for 2-ton units, CPT00651 for 3-ton — always verify µF ratings and voltage from the label on your existing capacitor. Fixes compressor-won't-start and slow condenser fan symptoms. Match µF and voltage rating exactly.

  $20–$60

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• Trane Time-Delay Relay (TDR)

  Replacement time-delay relay for Trane outdoor condenser units. Fixes compressor that won't restart after a short off cycle, or unit that starts then immediately stops. Check relay coil resistance and contact continuity before ordering — confirm failure mode before replacing.

  $15–$40

  Buy on Amazon →

• Trane 2-Pole Contactor

  Replacement contactor for Trane outdoor AC units. Fixes intermittent no-cooling from pitted contacts or open coil. Match coil voltage (24V) and amp rating to your existing unit. Common issue on units over 8–10 years in service — replace if contact pitting is visible.

  $20–$45

  Buy on Amazon →

• Nu-Calgon Coil Cleaner (Coil King or equivalent)

  Mild, non-acidic coil cleaner safe for aluminum Spine Fin coils. Do not use acidic coil cleaners on Trane Spine Fin without manufacturer verification. Rinse thoroughly after application. Used for heavy biological growth that water alone won't shift.

  $15–$30

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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