Burner Flame Lost After Ignition
🔎What the code means
This error means the burner initially ignites, but the flame goes out while the unit is running.
🔧 Error Code 121 – Causes & Troubleshooting Steps
Error 121 almost always means one of these issues:
Unstable gas supply (pressure drop, insufficient BTU supply, regulator issues)
Poor combustion (venting blockage, recirculation, dirty burner)
Dirty or weak flame rod (flame sensor cannot detect flame)
Low water flow (causing flame to modulate too low)
Incorrect setup (gas type, altitude DIP switches, vent length)
Below is a step-by-step diagnostic procedure.
🚫 Safety & Compliance Reminders
Use correct tools and gauge sets for gas pressure and vent static pressure measurement.
Follow all local codes and manufacturer’s installation instructions — mis-installation may void warranty.
Call a Professional
- If you smell gas, see scorch marks, or find water in the burner compartment.
- Inlet gas pressures are out of spec, pressure drop >1.5" W.C., or the unit keeps throwing 111 alongside 101/991 or 391.
Quick Links
🧰 STEP 1 — Confirm Gas Type, Pressure & Supply
A drop in gas pressure while running is the #1 cause of Error 121.
✔ For LP models:
Ensure tank is not near empty
Regulator should be 10–14" WC (dynamic under load)
Undersized/old regulators often cause 121
✔ For NG models:
Supply should be 4–10" WC dynamic
Confirm gas line is sized correctly for full BTU load (HS120Plus requires high flow)
What to check:
Verify correct dip switch for LP/NG is set
Check supply pressure while the unit is firing
Inspect regulator (replace if old or freezing in cold climates)
A pressure drop even for a moment = flame drops = Error 121.
🧰 STEP 2 — Clean & Inspect the Flame Rod
A dirty flame sensor is the most common internal cause.
How to clean:
Disconnect power
Remove front cover
Locate the flame rod (thin metal probe near burner)
-
Lightly clean using:
Scotch-Brite pad
Fine sandpaper (600+ grit)
DO NOT use heavy abrasive
Reinstall and tighten connections
Also inspect:
Flame rod wire for damage
Loose connectors on PCB
Cracked ceramic insulator
A weak signal causes false flame dropout.
🧰 STEP 3 — Check Venting & Combustion Air
Poor air supply or exhaust obstruction = unstable flame.
Look for:
Blocked intake/exhaust
Vent icing
Bird/squirrel nests
Too long vent runs
Incorrect vent diameter
Exhaust recirculation (air intake sucking exhaust)
Actions:
Remove terminal screens & inspect
Check that both intake/exhaust pipes are fully seated and sealed
-
Confirm venting meets the appropriate specs:
max equivalent lengths
number of elbows
correct materials
If the unit pulls in its own exhaust, error 121 is extremely common.
🧰 STEP 4 — Clean Burner & Fan
-
If the burner flame becomes uneven, it can drop out and trigger 121.
Inspect for:Carbon/debris on burner surface
Dust buildup on fan
Loose or misaligned burner assembly
- Actions:
Use compressed air to clean the burner
Check fan for smooth rotation or obstructions
Make sure burner is properly seated
🧰 STEP 5 — Check Water Flow & Heat Exchanger
If flow is too low, the flame will modulate down too far and extinguish.
Check:
Clean cold-water inlet filter screen
Inspect for clogged aerators or partially closed valves
Ensure flow is above minimum activation (≈0.5–0.75 GPM)
A partially scaled heat exchanger can cause flame instability—especially at low fire.
🧰 STEP 6 — Electrical Grounds, Wiring & PCB
A weak ground or loose wire = poor flame sensing.
Verify:
Unit is properly grounded
Flame rod wire is fully seated
No moisture on PCB
No burnt connectors
PC board screws (ground reference) are tight
A failing flame-sense circuit on the board is rare but possible.
🧰 STEP 7 — Reset Unit
After correcting issues:
Turn off power for 30 seconds
Restore power
Watch for stable flame behavior.