Goodman Heat Pump Not Heating: Step-by-Step Fix

Common Symptoms

• Goodman heat pump running but supply air is cold or room temperature in heat mode
• DSZC18 or GSZHC18 outdoor unit running, fan spinning, but house temperature dropping
• Heat pump appears to be cooling the house while set to HEAT
• Outdoor coil fully iced over — entire unit encased in ice
• Auxiliary heat not engaging when outdoor temperature drops below 35°F
• Second stage (Y2) not activating on DSZC18 or GSZHC18 during cold weather demand
• Goodman defrost board LED showing fault or continuously lit defrost indicator

Most Likely Causes

1. 1

   Thermostat Mode and Y2 Terminal Check (Start Here)

   Before any hardware diagnosis, verify thermostat mode and wiring. Set to HEAT and confirm the set point is above current room temperature. On Goodman DSZC18 and GSZHC18 two-stage heat pumps, also verify the Y2 terminal on the thermostat is wired and connected at both the thermostat and the outdoor unit control board. Y2 (second-stage compressor) must be wired for the heat pump to engage its full capacity in cold weather — a missing or loose Y2 wire limits the system to first-stage compressor operation, which is roughly half the heating capacity. This is a common missed wiring point on Goodman two-stage installs and explains why a system heats adequately in mild weather but can't keep up below 40°F. At the outdoor unit control board, Y2 is labeled on the low-voltage terminal strip — verify the wire is present and securely seated. Also verify the O/B terminal is set correctly: Goodman heat pumps use the O terminal (energized in cooling) by default — if a thermostat is set to B mode (energized in heating), the reversing valve will operate in reverse and cause cold air in heat mode.

2. 2

   Defrost Board Failure — Common on Older Goodman Heat Pumps

   Goodman defrost boards on units manufactured between approximately 2005 and 2015 have a higher field failure rate than comparable boards from Carrier or Trane. The defrost board monitors three inputs: outdoor ambient temperature (from an outdoor thermostat sensor), coil temperature (from a thermistor clipped to the outdoor coil), and time (internal timer). When the board fails, it either gets stuck in defrost mode (system runs in reverse/cooling mode continuously, producing cold air) or loses the ability to initiate defrost (outdoor coil ices over completely). On DSZC18 and DSXC18 outdoor units, the defrost board is located inside the electrical compartment and has one or two LED indicator lights — a steadily lit LED or rapidly blinking LED (vs. the normal slow blink for operational status) indicates the board is in fault. On GSZHC18 communicating units, the iComfort or Daikin communicating thermostat logs defrost board fault codes accessible through the diagnostics menu. Goodman defrost board part numbers are stamped on the board — match exactly when ordering. Amana-branded equivalent boards are often the same part number.

3. 3

   Reversing Valve Solenoid Failure — Well-Documented on Goodman

   Reversing valve solenoid coil failures are more commonly documented on Goodman heat pumps than on premium brands — field data shows the OEM Goodman solenoid coils fail open (OL resistance) at a higher rate than Carrier or Trane. The reversing valve is the 4-way brass valve on the refrigerant circuit. Goodman heat pumps use the O terminal convention — the solenoid is energized in cooling mode and spring-returns to heating position when de-energized. When the coil fails open, it can no longer shift the valve to cooling mode, which is actually less problematic in heating season — however, a valve that is mechanically stuck in the cooling position (spool stuck) or a coil that fails with the valve stuck in the cooling position produces exactly the cold-air-in-heat-mode symptom. The Goodman reversing valve solenoid coil reads 20–40 ohms when functional. OL reading on a multimeter confirms coil failure. Coil replacement is a DIY repair (one clip, coil slides off the valve stem) that does not require refrigerant handling. Full valve replacement requires EPA 608 certification.

4. 4

   Low Refrigerant in Heating Mode

   On Goodman heat pumps, low refrigerant manifests differently in heating mode than in cooling mode. In heating mode, the outdoor coil acts as the evaporator — absorbing heat from outdoor air. With low refrigerant, the suction superheat rises dramatically and the system loses heating capacity. Signs of low refrigerant in heating mode on Goodman units: the suction line at the outdoor unit is warm or hot to the touch (should be cold and lightly frosted in normal heating operation), the liquid line at the outdoor unit is ambient temperature or slightly cool (should be noticeably cool in heating), and the outdoor coil frosts heavily and unevenly. Goodman heat pump Schrader valve cores on the service ports are a common refrigerant leak point — the cores can weep refrigerant slowly over years. A Goodman DSZC18 or DSXC18 with a slowly declining refrigerant charge shows worsening heating performance each winter. Refrigerant leak testing and recharging requires EPA 608 certification.

5. 5

   Auxiliary Heat Strips Not Engaging

   Goodman heat pumps are commonly installed with electric heat strips in the GMEC96 or ACNF air handler (or a dedicated electric air handler with strips). If the heat pump is running correctly but the house can't maintain setpoint below 35–40°F, check whether the auxiliary heat strips are engaging. Common causes: the air handler breaker in the main electrical panel has tripped (aux strips run on 240V, often a separate breaker from the blower circuit), the sequencer relay on the heat strip assembly has failed (the sequencer staggers strip engagement to prevent all strips from energizing simultaneously), or the strip elements themselves have burned out. Test strip elements with a multimeter in ohms — a 5kW strip at 240V reads approximately 9.6 ohms, a 10kW strip reads approximately 4.8 ohms. OL reading means the element is open and not producing heat. Also verify the W2 terminal on the thermostat is wired to the air handler auxiliary heat input.

Quick DIY Checks

1. 1Verify thermostat mode, set point, and Y2 wiring before any equipment diagnosis. Set the thermostat to HEAT with the fan on AUTO and set point at least 5°F above room temperature. Wait 5 minutes for the system to respond. For Goodman DSZC18 or GSZHC18 two-stage units, walk to the air handler and look at the low-voltage terminal strip — verify a wire is connected to the Y2 terminal (second-stage compressor output from the thermostat). If the Y2 terminal is empty, the heat pump is limited to first-stage operation. Trace the thermostat wire bundle back to the thermostat — if the Y2 wire is present but not connected at the thermostat, connect it to the Y2 terminal on the thermostat sub-base (some thermostats label this as Y2 or Stage 2 Cooling/Compressor). A missing Y2 connection cuts heating capacity in half on two-stage systems.
2. 2Check the outdoor unit for ice and inspect the defrost board LED. Walk outside and examine the outdoor unit coil. Normal heating operation: light frost on the coil fins is acceptable. Fault condition: coil completely encased in ice with no visible fins. If heavily iced, switch the thermostat to FAN ONLY and let the ice melt naturally (2–4 hours) before restarting. Once ice clears, open the outer electrical compartment access panel on the Goodman outdoor unit. Locate the defrost board — a rectangular circuit board with LED indicator(s). On most Goodman DSZC18 and DSXC18 models, a slow blink (1 flash every 2–3 seconds) means normal operation; a rapid blink or continuously lit LED indicates a fault. Check the wiring diagram label inside the access panel for the LED fault code definitions specific to your board. If the LED shows a defrost fault and the unit was previously iced over, the defrost board or coil temperature sensor has failed.
3. 3Test the reversing valve solenoid coil. Turn off the outdoor unit at the disconnect (pull the fuse block or flip the breaker). Locate the reversing valve inside the outdoor unit — it is the cylindrical brass valve with four refrigerant connections and a small solenoid coil on top. Disconnect the two-wire solenoid connector. Set your multimeter to ohms (Ω). Probe the two solenoid terminals — a functional Goodman reversing valve solenoid reads 20–40 ohms. OL (open circuit) confirms the coil has failed. Coil replacement: note the orientation of the coil on the valve stem (there are usually small alignment tabs), remove the one wire clip or screw holding the coil, and slide the old coil off the stem. Slide the new coil on, secure it, and reconnect the wiring. If the coil tests good (20–40 ohms), restore power and test 24VAC at the solenoid connector terminals with the thermostat set to COOL — should read 24–28VAC. No voltage means the O terminal signal chain has a fault. Verify the thermostat O terminal is not set to B (Goodman uses O, not B).

1. 4Test the defrost coil temperature sensor (thermistor). Turn off outdoor unit power at the disconnect. Locate the coil temperature sensor — a small bullet-shaped thermistor clipped directly onto one of the outdoor coil fins, with a thin wire leading back to the defrost board. Disconnect the two-pin connector at the defrost board. Set your multimeter to ohms and probe the two sensor wire leads. At typical outdoor ambient temperatures (40–70°F), a good Goodman coil thermistor reads approximately 10,000–50,000 ohms (10kΩ–50kΩ) — the resistance drops as temperature rises (NTC thermistor). OL (open circuit) or 0 ohms means the sensor has failed and the defrost board is operating without coil temperature input — this causes incorrect defrost timing and often results in the unit staying iced over. The coil thermistor is an inexpensive part ($10–$30) and straightforward to replace — match the Goodman part number from the defrost board label.
2. 5Check auxiliary heat strip operation. At the main electrical panel, locate the breaker for the air handler (often labeled 'AHU,' 'Air Handler,' or 'Heat Strips') — it is typically a 30A to 60A 240V double-pole breaker. Reset if tripped (OFF then ON). Open the air handler access panel (turn off power first — verify with a non-contact voltage tester before touching any components inside). Locate the electric heat strips — U-shaped wire elements mounted in the supply air plenum. With power off, set your multimeter to ohms and test each strip element across its two terminals: 5kW strip = approximately 9.6 ohms, 7.5kW strip = approximately 6.4 ohms, 10kW strip = approximately 4.8 ohms. OL reading means that element is burned out and not producing heat. Locate the sequencer relays (small rectangular components wired in series with the strips) and test for relay contact continuity with the coil energized — a failed sequencer prevents strip engagement on heat demand even with good elements.

Repair vs Replace

Recommended Tools & Parts

• Goodman / Amana Heat Pump Reversing Valve Solenoid Coil

  Replacement reversing valve solenoid coil for Goodman and Amana heat pump outdoor units (DSZC18, GSZHC18, DSXC18, and compatible models) — fixes heat pump stuck in cooling or heating mode when solenoid tests OL. Does not require opening the refrigerant circuit. Verify voltage (24VAC) and connector style match your existing coil.

  $20–$60

  Buy on Amazon →

• Goodman Heat Pump Defrost Control Board

  Replacement defrost control board for Goodman DSZC18, DSXC18, and related heat pump outdoor units — fixes defrost system stuck on, never initiates, or LED fault indication. Match the part number printed on the existing board exactly. Amana-branded equivalent boards use the same part numbers.

  $40–$100

  Buy on Amazon →

• Goodman Defrost Coil Temperature Sensor (Thermistor)

  Replacement coil temperature thermistor for Goodman and Amana heat pump outdoor units — fixes defrost board operating without coil temperature input, causing unit to ice over or run continuous defrost. Clips directly onto outdoor coil fins. Match resistance specification to your existing sensor.

  $10–$30

  Buy on Amazon →

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

Related Repairs