Lennox Furnace Blink Codes — Complete Red Light Guide

How Lennox Blink Codes Work

Lennox furnaces signal errors with a diagnostic LED on the control board. The pattern depends on your model series. Most Lennox furnaces use a green or amber LED that blinks a number of times, pauses, then repeats. A slow steady blink (once every few seconds) means the furnace is standing by with no faults. A counted sequence of 1-10 rapid blinks is an error code.

Some newer Lennox models use a seven-segment display that shows numeric codes instead of blinks. The SLP98 series also displays diagnostic codes on the iComfort thermostat screen.

Identify Your Lennox Series

Lennox makes several furnace series with slightly different code mappings. Find your model number on the rating plate inside the access panel. The model prefix tells you which series you own.

Complete Blink Code Table

Elite, Merit, Signature, and ML193 Series

G60 and G61 Series

The G60 and G61 use a dual-color LED. Amber and green flash in combinations to signal faults.

SLP98 Series (iComfort Thermostat Codes)

The SLP98 communicates error codes through the thermostat. Common codes include:

Most Common Lennox Codes in the Field

6 Blinks (Lost Flame Sense)

This is the most common Lennox error across all series. The furnace lights, runs for a few seconds or minutes, then the flame goes out and the board detects no flame signal. The furnace tries to relight three times then locks out.

The cause is almost always a dirty flame sensor. The flame sensor is a silicon carbide rod mounted in the burner flame. Over time, a coating of combustion byproducts insulates the rod and reduces the microamp signal the control board needs to see (typically 2-6 microamps DC). Cleaning the rod with a fine abrasive restores the signal.

Turn off power to the furnace. Remove the flame sensor (one screw, slide it out). Gently clean the rod with fine emery cloth or a dollar bill. Do not use sandpaper that is too aggressive. Reinstall the sensor and restore power. This fixes roughly 70% of 6-blink calls.

3 Blinks (Pressure Switch)

Three blinks on Lennox means the pressure switch failed to close when the inducer started, or failed to open when the inducer stopped. Start with the condensate drain. High-efficiency Lennox furnaces produce water. If the drain is blocked, water backs up and blocks the pressure switch hose. Check the drain tee for standing water. Clear it with a wet/dry vacuum.

Next, inspect the pressure switch hose. It runs from the switch to the inducer housing. Look for cracks near the barb fittings. Replace cracked hose.

4 Blinks (Open Limit Switch)

Four blinks mean the high limit switch opened. This is an airflow problem 90% of the time. Check the air filter first. Then check all supply registers to make sure they are open. A blower motor running slow because of a bad capacitor or failing bearings can also trigger this code.

414 (Rollout Switch Open, SLP98 and Some Elite Models)

Code 414 means a rollout switch has tripped. This is a safety-critical fault. The rollout switch detects flame coming out of the burner tube instead of staying inside the heat exchanger. Do not reset this without understanding why it tripped.

If you can see the small button on the rollout switch, you can press it back in (manual reset). But if the switch trips again within the same heat cycle, you have a serious problem: blocked heat exchanger, improper gas pressure, or blocked flue. Call a technician.

Diagnostic Workflow

Step 1: Read and Record the Code

Open the blower compartment. Find the diagnostic LED on the control board. Watch the code through three full sequences to confirm the blink count. Write down the number.

Step 2: Power Cycle

Turn the thermostat to OFF. Flip the furnace breaker to OFF. Wait 30 seconds. Restore power and set the thermostat to call for heat. Watch the sequence. If the same code returns, proceed to diagnosis.

Step 3: Follow the Code

Use the code table above to identify the problem area. Start with the cheapest and most common fix for that code. For 1 blink, clean the flame sensor. For 3 blinks, check the condensate drain and pressure switch tubing. For 4 blinks, change the filter.

Step 4: Clear the Code

Lennox control boards have a diagnostic button on some models. Press and hold the button for 5 seconds to clear a soft lockout on Elite and Merit series. If your board does not have a diagnostic button, power cycling at the breaker clears all codes.

Lennox Diagnostic Button Reset

Some Lennox Elite and Merit furnaces have a small black button on the control board labeled DIAGNOSTIC or SERVICE. Press and hold this button for 5 seconds to reset the control board and clear error codes. This bypasses the power cycle for soft lockouts. If the code returns immediately after pressing the button, the fault is still present.

Parts Reference

When To Call a Professional

Call a technician if the code returns after you have checked the obvious causes: filter is clean, condensate drain is clear, pressure switch tubing is intact. Codes that come back immediately usually point to component failures that require multimeter testing.

Code 414 (rollout switch) should always be treated seriously. If it trips more than once, do not reset it again without a professional diagnosis. A cracked heat exchanger or blocked flue can allow carbon monoxide to enter your home.

If you smell gas at any point, leave the building immediately and call your gas utility from outside.

See Also

• Furnace Blinking 3 Times Meaning: Cross-Brand Guide
• Trane Furnace 2 Flashes Pressure Switch Code
• Goodman Furnace Error Codes 1-8 Complete Reference
• Furnace High Limit Switch Keeps Tripping: Complete Diagnosis Guide