Weil-McLain A79 Error Code — What It Means
A79 does not appear in verified Weil-McLain boiler documentation for any common series including Aqua Balance, Ultra, or EG models. The closest documented code is A01, which indicates no burner ignition after a call for heat. If your display truly shows A79, consult the installation and operation manual specific to your boiler model and control board type, because code meanings vary by series and control generation. If the code is actually A01, the boiler’s ignition system has failed to light the gas burner within the allowed time window.
Because no manufacturer source confirms A79, this guide cannot state a specific meaning or probable cause for that exact code. If you see A01 instead, Weil-McLain lists gas supply problems, ignition or detection electrode faults, gas valve failure, insufficient gas pressure (below 3.5 in. w.c. or above 11 in. w.c.), and dirty heat exchangers as the documented causes. Always verify the code on your display against the fault table printed inside the boiler jacket or in the manual that shipped with your unit.
Before You Replace Anything
Technicians sometimes replace the gas valve or ignition electrode without first confirming gas supply pressure and purging the line on new installs. Measure inlet gas pressure with a manometer and verify the manual gas valve is fully open before ordering parts.
Common Causes
- Code not documented by manufacturer (~40%) A79 does not appear in verified Weil-McLain fault tables, so the real issue may be a different code, a mis-read display, or a control board from a third-party retrofit.
- No gas supply or unpurged line (~25%) On new installations or after service work the gas line may still contain air, or the manual valve may be closed or partially open.
- Ignition or detection electrode fault (~15%) The electrode may be dirty, corroded, cracked, or positioned outside the flame envelope so it cannot sense the burner.
- Faulty gas valve (~10%) The gas valve may fail to open when energized, or internal components may stick and prevent gas flow to the burner.
- Insufficient gas supply pressure (~5%) Inlet pressure below 3.5 in. w.c. or above 11 in. w.c. can prevent ignition or cause the control to lock out.
- Dirty heat exchanger or overdue maintenance (~5%) Soot buildup or debris in the heat exchanger restricts airflow and can prevent stable ignition or flame proving.
Quick Diagnosis
Answer these to narrow it down fast.
Does the display show A79 exactly, or could it be A01 or another code?
No: If the code is A01, proceed with ignition diagnostics. If it is a different code, look up that code in your model's manual before taking action.
Is the manual gas shutoff valve upstream of the boiler fully open and has the gas line been purged if this is a new installation?
No: Open the manual valve fully and purge the line at the union or test port, then reset the control and retry the call for heat.
Do you hear the igniter clicking or see a spark at the burner when the boiler calls for heat?
No: No spark means a wiring fault, failed ignition module, or no power to the control. Check line voltage and control board connections before replacing parts.
Step-by-Step Fix
- Shut off power at the boiler switch and the main circuit breaker, then turn the manual gas valve to the off position.
- Locate the fault code on the control display and write down the exact characters. Compare the code to the fault table printed on the inside of the boiler jacket or in the installation manual for your specific model and control type.
- If the code is A01 or similar ignition fault, restore power and gas, then watch the ignition sequence. Listen for the igniter clicking and look through the sight glass for a spark at the burner.
- Measure inlet gas pressure using a manometer connected to the test port on the gas valve. Verify the reading stays between 3.5 and 11 in. w.c. during the call for heat.
- Inspect the ignition and detection electrode for cracks, corrosion, or soot buildup. Clean the ceramic insulator and metal rod with a soft brush and confirm the electrode gap and position match the installation manual specifications.
- Check the gas valve wiring and verify the control board is sending voltage to the valve coil during the call for heat. If the valve does not open, replace the gas valve.
- Reset the control by pressing and holding the reset button for 1.5 seconds, then initiate a new call for heat and observe whether the burner lights and the fault clears.
Parts Often Needed
| Part | Notes |
|---|---|
| Ignition / detection electrode assembly | Amazon | Order by model number from the boiler data plate. Verify ceramic insulator is intact and electrode tip reaches the correct position over the burner. |
| Gas valve | Amazon | Match the voltage and inlet/outlet size to your boiler. Confirm the valve does not open before replacing. |
When to Call a Pro
Call a licensed heating technician if you are not familiar with gas appliance work, if you cannot locate the fault code in your boiler manual, or if you see A79 and cannot confirm its meaning from manufacturer documentation. Gas boiler repair requires measuring gas pressure with a manometer, testing ignition voltage with a multimeter, and working safely around live gas lines and high-voltage controls. A technician will verify the code against the correct manual for your control board, measure supply pressure, inspect the ignition system, and replace the gas valve or electrode only after confirming the diagnosis. If the code turns out to be a mis-read or a control board issue, a pro can identify the real fault and avoid unnecessary part replacement.
Rough cost: A pro service call runs about $180-350.