Lennox Error Code Green FAST flash / Red ON: Discharge line temperature lockout (Heat Pump)

Updated October 13, 2026 · by Aurora Star Tech Inc.

Lennox error code Green FAST flash / Red ON on a heat pump means: Discharge line temperature lockout. It is also called Lennox diagnostic code Green FAST flash / Red ON.

Medium severityHeat PumpLennox EL16KP1 · Lennox ML14KP1

Homeowner? What Lennox code Green FAST flash / Red ON means for you

Check, then call Your heat pump is reporting: Discharge line temperature lockout. This is often caused by something simple. Try the safe checks below; if the code returns, book a service call before it turns into a no-heat or no-hot-water emergency.

Safe things you can check yourself

Check the thermostat: right mode (Heat or Cool), set a few degrees past room temperature, and fresh batteries if it uses them.
Check both breakers and the outdoor disconnect box beside the outdoor unit; all should be ON.
Look at the air filter. If it is grey and clogged, replace it; a blocked filter causes many shutdowns.
In winter, keep snow and ice off and away from the outdoor unit. A short defrost cycle with steam is normal.
Try one reset: turn the power off for 30 seconds, then back on. If the code comes back, stop resetting and call a technician.

Call (647) 657-2977 Book a service call

Licensed technicians serving Vaughan, Toronto and the GTA.

What does Lennox error code Green FAST flash / Red ON mean?

Discharge temp fault happened 5 times. With the compressor running, the control shuts off its contactor output (and defrost output if on). On these units, suspect the 10K resistor used in place of a discharge sensor.

Applies to: Lennox EL16KP1 (Elite) and ML14KP1 (Merit) R-454B heat pumps, sizes -018 to -060, demand defrost control A108 (CMC1)

Common causes of Lennox code Green FAST flash / Red ON

Shorted 10K resistor on the discharge sensor connection
Wiring short at the discharge sensor terminals

Safety: Gas, electrical and refrigerant work must be done by a licensed technician. Turn off power and gas before servicing, never bypass a safety control, and always follow the manufacturer's manual for your exact model.

Lennox models that use code Green FAST flash / Red ON

EL16KP1-018 EL16KP1-024 EL16KP1-030 EL16KP1-036 EL16KP1-042 EL16KP1-048 EL16KP1-060 ML14KP1-018 ML14KP1-024 ML14KP1-030 ML14KP1-036 ML14KP1-042 ML14KP1-048 ML14KP1-060

Other Lennox Heat Pump codes

Code	Meaning
E600	Load shed event
E601	Low ambient operational lockout
Green & Red alternating FAST flash	Coil (defrost) sensor problem
Green & Red alternating SLOW flash	5-minute anti-short-cycle delay
Green & Red simultaneous FAST flash	Ambient (outdoor air) sensor problem
Green FAST flash / Red OFF	Discharge sensor lockout
Green OFF / Red FAST flash	Discharge sensor fault
Green OFF / Red OFF	No power to the demand defrost control
Green OFF / Red ON	Low pressure lockout
Green OFF / Red SLOW flash	Low pressure fault
Green ON / Red OFF	High pressure lockout
Green ON / Red ON	Demand defrost control failure

All Lennox Heat Pump error codes →

Lennox code Green FAST flash / Red ON: homeowner questions

What does Lennox code Green FAST flash / Red ON mean?

Discharge line temperature lockout. Discharge temp fault happened 5 times. With the compressor running, the control shuts off its contactor output (and defrost output if on). On these units, suspect the 10K resistor used in place of a discharge sensor.

Is it safe to keep using my heat pump with code Green FAST flash / Red ON?

It depends on the cause. This is often caused by something simple. Try the safe checks below; if the code returns, book a service call before it turns into a no-heat or no-hot-water emergency.

Can I reset Lennox code Green FAST flash / Red ON myself?

You can try one reset (power off for 30 seconds, then on). If Lennox code Green FAST flash / Red ON comes back, the cause is still there and a licensed technician should check it.

Who can fix Lennox code Green FAST flash / Red ON near me?

Aurora Star Tech Inc. repairs Lennox heat pumps in Vaughan, Toronto and across the GTA. Call (647) 657-2977 or book a service call online.

Homeowner help: AC Running But Not Cooling: What to Check · Heat Pump Not Heating or Blowing Cold Air

Troubleshooting guides

Heat pump not defrosting / outdoor coil iced - Lennox EL16KP1 / ML14KP1 Lennox · Heat Pump
Low capacity or abnormal pressures - Lennox EL16KP1 / ML14KP1 Lennox · Heat Pump
Outdoor unit will not start - Lennox EL16KP1 / ML14KP1 Lennox · Heat Pump
Two horizontal lines on display (soft disable) - Lennox communicating Lennox · Heat Pump

Source: Lennox EL16KP1 Unit Information 100195 / ML14KP1 Unit Information 100108, Table 4 Demand Defrost Control (A108) Diagnostic LEDs, p.32. Written in our own words as a summary; always check the original manual.

General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.