Improper Combustion
🔎What the code means
Error Code 991 (Improper Combustion / abnormal combustion conditions) is a maintenance / warning code and not always a full shut-down fault. Basically, the unit is detecting that the mixture of gas + air + exhaust conditions are out of tolerance.
🚫 Safety & Compliance Reminders
Ensure power and gas supply are shut off before performing service.
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.
Because Code 991 relates to combustion, it represents a potential safety hazard (poor combustion can mean carbon monoxide, flame roll-out, or venting failure). If you are not qualified, call a licensed technician.
Quick Links
🔍What to Check when you see Code 991
Since it’s a combustion/venting related issue, you should inspect the following system-areas (ideally a service technician should do this):
Step 1: Check Venting
Inspect exhaust and intake paths for blockage, incorrect termination, reverse airflow, or intake too near exhaust.
The majority of the time when you see a 991 error code, it will be from a blockage or windy conditions coming down into the vent. Once you've checked for blockages and everything looks clear. Reset the code.
Reset Steps: Press and hold the two bottom buttons as indicated by the green arrows for 4-5 seconds. Release and then power off and then power back on.
Monitor and if the error code appears again, follow the steps below.
Step 2: Air Supply
Ensure adequate fresh air for combustion, especially if the unit is in a sealed room or the venting has been modified.
Step 3: Altitude / Dip Switch Settings
If the unit is installed at a high altitude or the DIP switches (or config settings) were set incorrectly for altitude, the combustion mixture may be off.
Verify the altitude switch/settings match your installation location.
Reset the unit and clear the stored error code in the controller. Then observe if Code 991 reoccurs.
Step 4: Clean Burner & Fan
- Remove front cover and inspect burner assembly (Part #101 in manual) for grease build-up, soot, corrosion.
- Remove/inspect fan (Part #103) for lint, dust, any obstruction or motor bearing wear.
- Clean or replace as needed to restore proper air/fuel combustion.
Step 5: Condensate/Drain & Heat Exchanger
For condensing models like the HS120Plus, ensure the condensate drain is open, piping is clear, and there is no blockage of the heat exchanger causing back-pressure or improper exhaust flow.
If the exchanger is restricted, the unit may struggle to draw or exhaust properly leading to improper combustion.
Step 6: Check Manifold Gas Pressure
Using pressure gauge, measure inlet and manifold gas pressures per the installation & service manual specs.
Adjust minimum and maximum manifold pressure if out of spec.
Verify that no gas supply restrictions, undersized gas line, or sagging pressure is present.
Step 7: Monitor Combustion Stability
- After cleaning and adjustments, run the unit and monitor for normal ignition and flame establishment.
- If the flame appears unstable, noisy, flickering, or the unit trips back into Code 101 or Code 991 (Abnormal Combustion) — further inspection is required (sensor, burner, fan motor, etc).
Step 8: Record Maintenance & Reset Code
- Once service is complete, document all findings, parts replaced, vent/gas corrections.
- Clear fault code, run the unit under load for a period and verify stable operation with no error codes returning.