Lennox Heat Pump Flashing Light: Meaning, Causes & Fixes
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Lennox heat pump flashing light: what it means
A flashing light on a Lennox heat pump usually comes from the diagnostic LEDs on the defrost control board inside the outdoor unit. Many boards have two LEDs, often labeled DS1 and DS2. The board uses their on, off and flashing patterns to report its status.
The pattern matters more than the flashing itself:
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Normal operation: On many Lennox boards, both LEDs flash slowly and together. This is a heartbeat showing the board is powered and running. It does not mean anything is wrong.
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Fault codes: Other patterns report specific problems. Depending on the board, these include:
- alternating flashes
- fast flashes
- one LED on and the other off
- both LEDs solid on or solid off
Lennox boards commonly report these conditions:
- a pressure switch has opened (high-pressure or loss-of-charge/low-pressure)
- a pressure-switch lockout after repeated trips within one thermostat call
- a defrost or coil temperature sensor fault
- an ambient temperature sensor fault
- a board malfunction
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Communicating systems: iComfort and S30 heat pumps report the same kinds of faults as alarm codes on the thermostat screen.
Lockout and reset behavior varies by board. On many Lennox defrost boards:
- A single pressure-switch trip causes a short timed shutdown, and the unit restarts on its own.
- Several trips during one thermostat call put the board into lockout. A lockout typically holds until 24-volt power is cycled, either by turning the thermostat off and on or by turning the power off and on at the breaker or disconnect.
The exact trip count, delay times and LED patterns differ between board generations. The code chart printed inside the outdoor unit’s access panel is the authority for your unit.
This page covers the LED itself. For specific symptoms, see the related pages listed at the end.
Lennox heat pump flashing light: common causes, ranked by probability
Causes are listed in the order Lennox defrost-board charts typically present them: pressure switches first, then sensors, then the board.
- The normal heartbeat was mistaken for a fault. Both LEDs flashing slowly and in sync is standard on many Lennox boards. This is the most common reason homeowners worry about a flashing light.
- The high-pressure switch opened. Common triggers:
- a dirty or blocked outdoor coil in cooling mode
- a failed outdoor fan motor or capacitor
- restricted indoor airflow in heating mode, such as a clogged filter, closed registers or an indoor blower problem
- refrigerant overcharge or a restriction in the lines
- The low-pressure (loss-of-charge) switch opened. Usually caused by a refrigerant leak or low charge. Can also be caused by:
- a restriction in the refrigerant system
- a stuck metering device
- very low airflow across a frozen indoor coil
- Pressure-switch lockout. Repeated high- or low-pressure trips within one thermostat call. The cause is still one of the two problems above. The lockout is the board protecting the compressor.
- Coil (defrost) sensor fault. The sensor is open, shorted or out of range, or its wiring or connector has failed. This may also cause defrost problems.
- Ambient (outdoor air) sensor fault. This sensor is only present on some boards. It may be open, shorted, unplugged or damaged.
- Defrost control board failure. The board reports an internal fault, or its LEDs show a pattern the chart lists as a board malfunction. Possible causes include lightning or surge damage, moisture, or age.
- Compressor protection or wiring issues reported by the board. Some newer boards also monitor compressor operation and 24-volt signals. Whether your board does this, and how it reports it, depends on the model.
How to fix Lennox heat pump flashing light: safe checks first
Keep the outdoor unit’s panels closed. Do not touch the board, wiring or switches.
- Read the pattern from outside the cabinet, if you can. Some units have a viewing window or show the code on the thermostat. If the LEDs can only be seen by removing a panel, leave that to the technician. Instead, write down what the thermostat shows, when the problem started, and whether the unit runs at all.
- Check the thermostat.
- Confirm the mode (heat or cool) and the set temperature.
- Replace the batteries if your thermostat uses them.
- On iComfort or S30 systems, record the alert code and description.
- Replace or check the air filter. A clogged filter is a known contributor to high-pressure trips in heating mode.
- Open the supply registers and clear the return grilles. Make sure furniture, rugs or closed dampers are not blocking airflow.
- Clear the area around the outdoor unit. Gently remove leaves, grass clippings or snow from around the unit and keep about 2 feet of clearance. Do not open the unit, and do not spray its interior.
- Check the condensate drain at the indoor unit. Look for overflow, a tripped float switch alarm, or a full pan. A blocked drain can shut the system down.
- Reset once.
- Turn the thermostat to off.
- Switch off the heat pump’s breaker or outdoor disconnect for 5 minutes.
- Restore power and call for heat or cooling.
- If the fault pattern or lockout returns, do not keep resetting. Call a technician.
- Confirm all exterior access panels are seated and secured.
How a technician will diagnose it
Knowing these steps helps you judge whether a quote is reasonable.
- Reads the LED pattern and matches it to the chart for your board revision. Many boards also store a fault history that can be read or cleared with a test-pin procedure.
- Checks power and control voltage:
- line voltage at the disconnect
- 24-volt control voltage at the board
- all connections at the board
- For pressure-switch faults:
- Checks continuity across the high- and low-pressure switches.
- Inspects the outdoor coil, outdoor fan motor and fan capacitor.
- Verifies indoor airflow.
- Connects gauges or digital probes to measure operating pressures, superheat and subcooling. A low charge means leak-searching, not just topping off.
- For sensor faults:
- Unplugs the coil or ambient sensor and measures its resistance against Lennox’s temperature/resistance chart.
- Inspects the wiring and connector for rodent damage or corrosion.
- For a board fault, rules out the inputs first: power, sensors and switches. Then:
- may run a forced defrost or test-mode sequence
- replaces the board only if it fails to respond correctly with good inputs
- Clears the fault history and verifies operation through a full cycle, including a defrost cycle where practical.
A quote that replaces the board without checking the switches, sensors and refrigerant pressures first should prompt questions.
Symptom, cause and what to do
| Symptom | Likely cause | DIY action | Technician job |
|---|---|---|---|
| Two LEDs flash slowly together; system heats and cools normally | Normal heartbeat | None needed | None |
| Unit stops, then restarts on its own after a delay | Single high- or low-pressure switch trip | Replace filter, open registers, clear debris around outdoor unit | Check coil, fan, airflow and refrigerant pressures |
| Unit stays off; the LED pattern indicates lockout | Repeated pressure-switch trips | Reset power once; call if it recurs | Find the root cause: refrigerant leak, fan failure, airflow or restriction |
| Outdoor fan not turning while compressor hums or runs | Failed fan motor or capacitor causing high pressure | Turn unit off; do not reset repeatedly | Replace capacitor or fan motor |
| Poor heating; ice build-up; sensor fault pattern | Coil (defrost) sensor open or shorted | Turn thermostat to emergency heat if needed, then call | Test sensor resistance; replace sensor or repair wiring |
| Odd defrost timing; ambient sensor fault pattern | Ambient sensor fault (some boards only) | Note the pattern and call | Test and replace the sensor |
| LEDs off, solid on, or showing the board-fault pattern | Defrost control board failure or no 24-volt power | Check breaker and thermostat | Verify voltage, then replace the board if confirmed |
| Alert code on iComfort or S30 thermostat | Communicating system fault | Record the code; reset once | Diagnose using the alarm code and the unit’s service data |
Lennox heat pump flashing light repair cost
Typical US prices, including parts and labor. Prices vary by region, warranty status and model.
| Repair | Typical cost |
|---|---|
| Filter, thermostat batteries, clearing debris (DIY) | $0–$40 |
| Diagnostic service call | $90–$200 (often credited toward the repair) |
| Outdoor fan capacitor | $150–$350 |
| Outdoor fan motor | $350–$800 |
| High- or low-pressure switch replacement | $200–$500 (requires refrigerant recovery or valve work on some units) |
| Coil (defrost) or ambient sensor | $150–$350 |
| Defrost control board | $350–$800 (Lennox OEM boards cost more; parts may be covered under warranty) |
| Refrigerant leak search and repair, plus recharge | $400–$1,500+, depending on leak location and refrigerant type |
| Outdoor coil cleaning | $100–$250 |
Check your Lennox registration before paying for parts. Many units carry a 5- or 10-year parts warranty if they were registered.
Related codes
- Lennox Heat Pump Frozen and Not Defrosting: Fixes & Cost
- Lennox Heat Pump Stuck in Defrost Mode: Causes & Fixes
- Lennox Heat Pump Compressor Not Running: Causes & Fixes
- Lennox Heat Pump Outdoor Fan Not Spinning: Causes & Fixes
- Lennox Heat Pump Short Cycling: Causes, Fixes & Costs
- Lennox Heat Pump Won’t Turn On: Causes, Fixes & Costs
- Lennox Heat Pump Not Heating: Causes, Fixes & Costs
- Lennox Heat Pump Not Cooling: Causes, Fixes & Costs
- Lennox Heat Pump Ice on Outdoor Unit: Causes & Fixes
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Frequently asked questions
Is a flashing light on my Lennox heat pump always a problem?
No. On many Lennox defrost boards, the two diagnostic LEDs flash slowly and in step with each other during normal operation. That pattern is a heartbeat, not a fault. A fault shows as a different pattern, such as alternating flashes, fast flashing, one LED steady while the other flashes, or both LEDs on solid. The meaning depends on your board, so compare what you see with the chart on your unit's access panel.
Can I reset a Lennox heat pump that is showing a lockout?
You can reset it once. Turn the thermostat to off, switch off the outdoor unit's breaker or disconnect for about five minutes, then restore power. Some boards also clear a soft lockout on their own after a timed delay, while a hard lockout holds until power is cycled. If the fault comes back, stop and call a technician. Repeated resets can damage the compressor.
Why does my Lennox thermostat show an alert instead of a flashing light?
Communicating Lennox systems, such as iComfort and S30 models, send fault codes to the thermostat display, often with a text description. A flashing LED on the outdoor board is mainly a technician tool on conventional 24-volt systems. If you have a smart Lennox thermostat, write down the alert code and give it to your contractor.
Does a dirty filter cause a flashing light on a Lennox heat pump?
It can contribute. A badly clogged filter reduces airflow across the indoor coil. In heating mode, that can push refrigerant pressure high enough to trip the high-pressure switch, which many Lennox defrost boards report as a pressure fault and eventually a lockout. Replacing the filter is a sensible first check.