Abnormal Combustion
🔎What the code means
Error Code 101 (Abnormal Combustion) is a warning for a possible upcoming error 991 (improper combustion).
Because combustion issues with gas-fired, vented appliances can pose safety risks (carbon monoxide, improper venting, flame roll-out, etc), this code should not be ignored.
🔍 Possible Causes
Here are the common culprits for Error 101:
Blocked intake or exhaust venting — debris, nests, corrugated piping collapsed, etc.
Intake air terminal too close to exhaust terminal (leading to recirculated exhaust air, inadequate fresh air) or other intake/exhaust clearance issues.
Dirty burner, flame sensor, or fan motor causing improper combustion.
Incorrect gas pressure or wrong gas type (natural vs propane) causing a bad fuel/air mix.
Improper DIP switch settings for altitude, model type, installation setting (which affect combustion parameters).
Installation at too high altitude without proper adjustment (air density impacts combustion) or venting length outside spec.
🚫 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 101 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
🔍 Step-By-Step Procedure for Code 101
Here are the key things to look at. Some may be safe for you to check, others may require a licensed technician.
Step 1: Gas Supply & Type
Confirm the unit is set up for the correct gas type (Natural vs Propane) and that the supply pressure is correct. A mismatch or supply issue can yield improper combustion.
Check for any recent changes: has the gas type changed? Has something been serviced?
Using a manometer, check manifold gas pressure during operation (per manual specs). If outside tolerance, adjust or call pro.
Step 2: Inspect Venting / Air Supply
Remove/inspect intake air terminal for blockages (snow, leaves, bird-nest, debris).
Remove/inspect exhaust terminal for blockage or insulation/hose damage.
Confirm intake-to-exhaust spacing and conformity with installation manual vent termination clearances.
Check vent pipe slope and support (for condensation drainage) to ensure proper flow.
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. Code 101 can be tied to this.
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 101 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 HS199Con, 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: If Code 101 Persists
The manual states Code 101 is a WARNING for an upcoming Code 991: Abnormal Combustion.
At this point, consider replacing likely affected parts: air/fuel ratio rod/flame sensor, burner assembly, fan motor, pressure switch, or PCB if diagnostics show fault.
Also evaluate major system factors: vent length exceeded specification, gas supply undersized, vent termination improperly located, ambient air supply restricted, etc.
If, after replacing parts, the code still persists, there may be a more serious internal fault such as a failing board, incorrect vent design, or an impure installation environment (e.g., heavy grease, combustion contaminants). At this point a licensed technician should handle it.
Step 9: 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.