Trane Heat Pump Not Heating: Diagnosis and Fixes

Common Symptoms

• Trane heat pump running in heat mode but supply air feels cool or only slightly warm
• Outdoor unit running normally — fan spinning, compressor operating — but house temperature dropping
• Thermostat set to HEAT but system behavior mimics cooling mode
• Outdoor coil completely encased in ice that does not clear after 45–60 minutes of normal operation
• XV20i or XL18i ComfortLink II thermostat displaying a fault code or alert in the service menu
• Aux heat or Emergency Heat indicator on thermostat stays lit, but house still not reaching setpoint
• Heat pump runs continuously in cold weather but cannot maintain indoor temperature above 65°F
• Audible hissing from the reversing valve area of the outdoor unit refrigerant circuit

Most Likely Causes

1. 1

   Reversing Valve Stuck in Cooling Mode — Most Frequently Misdiagnosed on Trane Heat Pumps

   The reversing valve is a solenoid-operated 4-way valve that redirects refrigerant flow between heating and cooling cycles. On Trane heat pumps using the O terminal wiring convention (standard Trane configuration), 24VAC energizes the reversing valve solenoid during cooling — in heating mode, the solenoid is de-energized and the valve spring-returns to the heating position. When the solenoid coil fails open (OL on a multimeter), the solenoid never energizes and the valve defaults to whichever position it was last in. If it was last in the cooling position, the heat pump runs in cooling mode even when the thermostat calls for heat — actively refrigerating the house in winter. A heat pump stuck in cooling mode with the compressor running produces cold supply air in heat mode and a rising house temperature. The reversing valve solenoid coil on Trane XR and XL series outdoor units reads 20–40Ω when functional — OL confirms coil failure. Solenoid replacement is a DIY-accessible repair that does not require opening the refrigerant circuit. If the coil tests within spec but the valve spool is mechanically stuck, refrigerant circuit work by a certified technician is required to replace the full valve.

2. 2

   Defrost Board Failure — Stuck in Defrost or Never Defrosting

   Trane residential heat pump defrost boards — particularly on XR15 and XR17 units manufactured between 2005 and 2018 — have a documented above-average failure rate in the field. The defrost board monitors the outdoor coil temperature thermistor and a time clock to initiate defrost cycles. When the board fails, it commonly fails in one of two modes: (1) stuck in defrost — the system is permanently in reverse/cooling mode with the outdoor coil heating and the reversing valve energized; supply air is cold, and the homeowner sees steam rising from the outdoor unit continuously; (2) defrost never initiates — the outdoor coil progressively ices over until it is a solid block of ice, completely blocking airflow and halting heat absorption. Both failure modes produce no useful heat. On Trane XR15 and XR17, the defrost board is located in the outdoor unit electrical compartment and has LED status indicators — a continuously lit defrost LED with no visible ice clearing confirms the board is stuck in defrost. Trane defrost boards are brand-specific (not interchangeable with Carrier/Goodman equivalent) — order by model number from the wiring label inside the access panel.

3. 3

   Low Refrigerant in Heating Mode

   Low refrigerant affects a heat pump differently in heating mode versus cooling mode. In heating mode, the outdoor coil is the evaporator — it absorbs heat from outdoor air. Low refrigerant charge reduces the system's heat absorption capacity: the suction line at the outdoor unit feels unusually warm or even hot (instead of cold and sweating normally), because low refrigerant causes suction superheat to spike. The liquid line may feel barely cool or ambient temperature. On Trane XV20i and XL18i communicating units, a low-pressure fault code will appear at the thermostat. On non-communicating XR15 and XR17 units, the symptom is simply inadequate heating with no stored codes. Low refrigerant charge on a Trane heat pump is almost always due to a leak — common sites are the Schrader valve core service ports, the flare connections at the service valves, and the brazed evaporator coil connections. Refrigerant recovery and recharge requires EPA 608 certification.

4. 4

   Outdoor Coil Heavily Iced — Defrost Thermistor Failure

   Even when the defrost board itself is functional, a failed outdoor coil temperature thermistor prevents the board from sensing when the coil is cold enough to require defrost. Without accurate coil temperature data, the board either never initiates defrost or runs on timer logic alone. A Trane XR15 or XR17 defrost thermistor that has drifted out of specification will show readings outside the normal 10–50kΩ range at ambient temperature — either open circuit (OL) or shorted near 0Ω. The thermistor is a small NTC sensor that clips onto the outdoor coil fins and connects to the defrost board via a two-wire harness. Replacing a failed thermistor ($20–$40 part) often resolves defrost board symptoms without requiring a full board replacement — always test the thermistor before condemning the board.

5. 5

   Auxiliary Electric Heat Strips Not Engaging

   Most Trane heat pumps are installed with electric auxiliary heat strips in the air handler that supplement the heat pump below the balance point (typically 35–40°F). If the heat pump is operating correctly but the house cannot maintain setpoint in cold weather, the auxiliary strips may not be engaging. Causes: a tripped breaker on the air handler circuit (aux strips run on 240V and can trip their own dedicated breaker independently), a failed sequencer relay, a burned-out strip element (a 5kW element reads ~9.6Ω, a 10kW element reads ~4.8Ω — OL means burned out), or an outdoor thermostat lockout temperature set too low. On Trane XV20i and XL18i ComfortLink II thermostats, the aux heat lockout temperature is configurable — verify it is set above the current outdoor air temperature or the strips will not engage.

6. 6

   Thermostat Mode Error or Control Wiring Fault

   Before opening the outdoor unit on any Trane heat pump no-heating call, spend 30 seconds confirming thermostat configuration. Trane XV20i and XL18i thermostats use ComfortLink II communication — navigate to the fault history menu and check for stored codes before going outside. On standard non-communicating thermostats, confirm mode is HEAT (not COOL or Emergency Heat), fan is AUTO, and the set point is at least 3–5°F above current indoor temperature. A Trane heat pump set to Emergency Heat bypasses the compressor entirely and runs electric strips only — if the air handler breaker has tripped, Emergency Heat mode produces little or no heat. Also verify the O terminal wiring: Trane heat pumps use O (orange wire) for reversing valve, not B — if a thermostat was replaced by a technician who wired it incorrectly to B, the heat pump operates in cooling mode whenever the thermostat calls for heat.

Quick DIY Checks

1. 1Step 1 — Thermostat mode and configuration check: confirm the thermostat is set to HEAT, fan to AUTO, and the set point is at least 3–5°F above current indoor temperature. On Trane XV20i or XL18i with ComfortLink II thermostat, navigate to the menu and check the fault history — note all stored codes before going outside. Common Trane heat pump fault codes: code 24 = low-pressure fault in heating (low refrigerant charge or iced evaporator), code 41 = defrost fault, code 175 or 178 = outdoor unit communication or reversing valve circuit fault. Confirm the thermostat wiring has O terminal active (not B) — Trane heat pumps use O for reversing valve. If the thermostat was recently replaced, verify the O/B selection matches Trane's convention.
2. 2Step 2 — Check the outdoor unit for ice accumulation: walk outside and visually inspect the outdoor coil. In cold, humid weather, some frost on the outdoor coil is normal and will clear during a normal defrost cycle. A completely ice-encased outdoor coil — where no coil fins are visible and the cabinet looks like a solid ice block — means the defrost system has failed. Do NOT pour hot water on the unit or chip the ice. Set the thermostat to fan-only mode or turn the system off. Allow 2–4 hours for the ice to melt naturally. After the ice clears, restart in heating mode and observe whether the coil begins re-icing within 30–60 minutes. If it does, the defrost board or thermistor has failed. On Trane XR15 and XR17, open the outer electrical compartment and look for the defrost board LED status indicators — a continuously lit defrost LED confirms the board is stuck in defrost mode.
3. 3Step 3 — Test the reversing valve solenoid coil: turn off the outdoor unit at the disconnect box. Locate the reversing valve on the refrigerant circuit — it is the cylindrical brass 4-port valve (typically mounted horizontally near the compressor) with a small solenoid coil body on top. Disconnect the two-wire connector from the solenoid coil — one wire traces to the outdoor unit control board terminal labeled O or RVS. Set your multimeter to ohms (Ω) and probe the two solenoid coil terminals. A functional Trane reversing valve solenoid reads 20–40Ω. OL (open circuit) means the coil has failed. Solenoid coil replacement is a DIY-accessible repair — slide the coil off the valve stem after removing its mounting clip, install the new coil, and reconnect the wires. This does not require opening the refrigerant circuit. If the coil resistance is within spec, restore power and test 24VAC at the solenoid connector during a cooling call (thermostat set to COOL) — should read 24–28VAC. No voltage in cooling mode confirms a wiring or thermostat O terminal fault.

1. 4Step 4 — Test the defrost board thermistor: with power off, locate the defrost thermistor — a small NTC temperature sensor clipped to the outdoor coil fins, connected to the defrost board via a two-wire harness (typically white wires). Disconnect the thermistor connector from the defrost board. Set your multimeter to ohms and measure resistance across the thermistor leads. At ambient outdoor temperature (40–70°F), a functional Trane defrost thermistor reads 10–50kΩ; OL indicates open circuit failure; near 0Ω indicates short circuit failure. A failed thermistor prevents the defrost board from sensing coil temperature — replace it before condemning the full board. Trane defrost thermistors are model-specific; order the replacement using the part number from the outdoor unit wiring diagram label inside the access panel. After replacing the thermistor, restore power, run in heating mode, and verify the defrost cycle initiates and clears the coil within 90 minutes.
2. 5Step 5 — Check auxiliary heat strips: locate the air handler in the attic, closet, or basement. Check the main electrical panel for the air handler breaker — aux heat strips often have a separate dedicated 30A–60A breaker labeled 'air handler' or 'AHU' that can trip independently. Reset any tripped breakers. With power off at the breaker, open the air handler access panel and locate the electric heat strips (U-shaped wire elements in the air stream). Using a multimeter set to ohms, test each strip element across its terminals — a 5kW element reads ~9.6Ω, a 10kW element reads ~4.8Ω; OL indicates a burned-out element. On Trane XV20i and XL18i, also verify the ComfortLink II thermostat aux heat lockout temperature setting — if it is set below the current outdoor temperature, the strips will not engage. The TrueComfort variable-speed system on XV20i should modulate compressor output before engaging aux strips — if aux heat is running constantly, the compressor capacity or refrigerant charge may be compromised.
3. 6Step 6 — Assess refrigerant charge indicators: with the heat pump running in heat mode, touch the large suction line at the outdoor unit (this is the evaporator in heating mode). It should feel cold and slightly moist — distinctly below ambient temperature. An unusually warm or hot suction line with the compressor running in heating mode indicates low refrigerant charge causing high suction superheat. Touch the liquid line (smaller diameter copper tubing) — in heating mode it should feel slightly warm to warm, carrying hot liquid from the outdoor condenser function. Ambient-temperature or cold liquid line in heating mode suggests low charge or a failed expansion device. Check the Schrader valve service port caps for oil staining — oil residue confirms a refrigerant leak at the valve core. If refrigerant indicators are present, stop DIY and call an EPA 608-certified HVAC technician. Trane XR15 and XR17 systems use R-410A; older units may use R-22.

Repair vs Replace

Recommended Tools & Parts

• Trane / American Standard Reversing Valve Solenoid Coil (24VAC)

  Replacement reversing valve solenoid coil for Trane XR15, XR17, XL18i, XV20i, and American Standard equivalent outdoor units. Fixes heat pump stuck in one mode when coil tests open (OL on multimeter). Slides on and off the valve stem — does not require opening the refrigerant circuit. Match the voltage (24VAC) and connector type to your existing coil. Order using the part number from the wiring label inside the outdoor unit access panel.

  $30–$80

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• Trane Heat Pump Defrost Control Board

  Replacement defrost control board for Trane XR15 and XR17 heat pump outdoor units — fixes defrost system stuck in defrost mode, defrost that never initiates, or failed status LED. Match the part number from the existing board or the outdoor unit wiring label inside the access panel.

  $80–$180

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• Trane Defrost Thermistor (Outdoor Coil Temperature Sensor)

  Replacement outdoor coil temperature thermistor for Trane XR and XL series heat pump outdoor units — fixes defrost board that cannot sense coil temperature, causing the system to either never defrost or continuously defrost. Clips onto the outdoor coil fins. Test resistance before ordering: should read 10–50kΩ at ambient temperature; OL or 0Ω indicates failure.

  $20–$40

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• Manifold Gauge Set for R-410A

  Professional 4-valve manifold gauge set for R-410A systems — required to verify refrigerant charge on Trane XR15, XR17, XL18i, and XV20i heat pumps. For use by EPA 608-certified technicians only. Includes color-coded hoses and gauges rated for R-410A operating pressures up to 800 PSI high-side.

  $45–$120

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

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