Air-Fuel Ratio Failure
🔎What the code means
The heater has detected an abnormal or failed thermistor (temperature sensor) reading. The thermistor monitors temperature in the outlet or heat exchanger; when its resistance value is out of spec (open, shorted, out of range), the control board sets Code 311.
🚫 Safety & Compliance Reminders
Because this is a gas-fired condensing unit, unsafe conditions (e.g., overheating, flame/combustion problems) could exist. If you are not qualified, call a licensed technician.
- Make sure to turn off the gas and power before opening the unit or doing maintenance.
- Even though the code is specific to the AFR rod, it’s almost always a system-condition issue (venting/gas/burner) rather than simply “replace the rod and done”.
- If you’re in a high altitude installation, or the system has been modified, ensure that altitude/gas-type settings are correct.
- Routine maintenance helps: cleaning burners, checking filters, verifying venting integrity will help prevent recurrence.
Quick Links
🧰 STEP 1 — Power down the unit and verify safety (shut off gas, water, power) before any inspection.
🧰 STEP 2 — Inspect the AFR rod and wiring
Locate the AFR rod (sometimes labelled “AFR rod” or “air-fuel ratio rod”) in the combustion chamber, as shown in the manual.
Check for damage, soot build‐up, or loose/poor connections on the wiring.
Make sure the rod’s alignment and mounting are correct (no excessive bending or corrosion).
If suspect, replacing the AFR rod is a common corrective action.
🧰 STEP 3 — Examine combustion & venting conditions
The AFR rod monitors flame quality; if ventilation is restricted or combustion air is insufficient, the rod may read “out of range” and trigger the fault. For example, one forum report: “The unit keeps tripping and showing code 391 with two green light flashes, indicating a problem with the air-fuel ratio rod.”
Inspect air intake and flue/exhaust: ensure no blockages, restrictions, or alternate air-routing issues.
Check that venting meets spec (length, diameter, termination location) and that combustion air supply is adequate.
🧰 STEP 4 — Check gas supply and burner condition
If the gas pressure is too low or too high, or if burner ports are clogged/fouled, the combustion may be abnormal causing the AFR rod to detect an issue.
Clean the burner assembly (remove soot, debris) and check that manifold/gas valve pressures are within spec. The service manual includes charts for manifold gas pressure checks.
Confirm the correct gas type is being used (natural gas vs propane) and that DIP/altitude switches are set properly (though more for other fault codes, but still part of overall system health).
🧰 STEP 5 — Check the control board and sensor calibration
If wiring, rod, burner, venting and gas supply all check out, it’s possible the control board is misreading or there is a sensor fault elsewhere.
Review the wiring diagram/checkpoints in the manual for proper sensor/resistor values.
Consider whether the unit has had any electrical disturbances or modifications (which might affect sensor readings).
🧰 STEP 6 — Replace Sensor if Defective
🧰 STEP 7 — Clear Fault Code
(typically by cycling power) and monitor the unit under operation to verify code does not return.
If after replacing sensor and checking wiring, the code persists → suspect the control board or sensor read circuit. At that point, board replacement may be required.