Weil-McLain Boiler A158 Error — What It Means
A158 is not a verified standard Weil-McLain fault code in manufacturer documentation. It may be a model-specific internal code, a control-board identifier, or a misread display. Weil-McLain boilers use different fault-code families depending on the model and control type, so the exact meaning depends on your specific boiler series and installation manual.
Most Weil-McLain ignition-related faults (such as the documented E02 code on Ultra-series models) indicate the boiler tried to light but failed to establish or prove flame. Common underlying problems include no gas flow, a dirty or failed flame sensor, a bad ignitor, or a blower issue on power-vented models. The control will lock out after repeated failed attempts. Always confirm the exact model and control from the boiler nameplate and consult the service manual before diagnosing or resetting.
Before You Replace Anything
Homeowners often replace the ignitor or gas valve when the real problem is a dirty flame sensor or a closed gas shutoff valve. Before buying parts, verify gas supply and clean the flame sensor.
Common Causes
- Gas supply issue (~30%) Gas valve closed, inadequate supply pressure, or the gas valve not opening when commanded.
- Dirty or failed flame sensor (~25%) The flame sensor is coated with soot or carbon or has failed, preventing the control from proving flame after ignition.
- Bad ignitor or ignition circuit (~20%) The ignitor does not glow or spark, so the boiler cannot establish flame.
- Blower or inducer failure (~15%) On models with induced draft, the blower does not run or does not prove sufficient air movement, aborting the ignition sequence.
- Control board or ignition-control fault (~10%) The control itself has failed or is locked out, preventing the ignition sequence from completing even when all components are good.
Quick Diagnosis
Answer these to narrow it down fast.
Are all gas shutoff valves (main and boiler) fully open?
No: Open the valves fully, wait a moment for gas to purge air, then attempt to restart the boiler.
Does the ignitor glow orange or produce a spark during the start sequence?
No: Ignitor or its circuit has failed. Power off and test the ignitor for continuity or call a technician.
Does the blower (inducer fan) spin up before the ignition attempt?
No: Blower or draft-proving switch has failed. The control will not allow ignition without proven airflow.
Step-by-Step Fix
- Turn off power at the boiler switch and the circuit breaker.
- Verify the exact model and control type from the boiler nameplate and find the installation or service manual online to confirm what A158 means for your specific unit.
- Check all gas shutoff valves (street valve and boiler valve) and confirm they are fully open.
- Inspect the flame sensor (a thin rod near the burner). Remove it, clean it gently with fine steel wool or emery cloth, and reinstall.
- Check the ignitor for cracks or breaks. If it has a hot-surface ignitor, look for visible damage. If a spark ignitor, listen for a clicking spark during start-up.
- Observe the ignition sequence with power restored: inducer fan starts, ignitor glows or sparks, gas valve clicks open, flame lights, and flame sensor proves flame. Note where the sequence stops.
- If the boiler still will not light, call a qualified gas technician to test the gas valve, blower operation, and control board before replacing any parts.
Parts Often Needed
| Part | Notes |
|---|---|
| Flame sensor rod | Amazon | Order by model number. Often just needs cleaning, not replacement. |
| Hot-surface ignitor | Amazon | Fragile ceramic element. Match exact part number for your Weil-McLain model. |
| Gas valve | Amazon | Model-specific. Replacement requires gas-piping work and a licensed technician. |
| Inducer blower assembly | Amazon | For power-vented models. Match model and voltage from the nameplate. |
When to Call a Pro
Call a licensed heating technician immediately if you smell gas, if the boiler repeatedly locks out after resets, or if you are not comfortable working with gas appliances. Any work on the gas valve, gas piping, combustion air system, or control board should be done by a professional. Technicians have the correct manuals, diagnostic tools, and gas-detection equipment to safely trace ignition faults and verify proper combustion. Because A158 is not a standard published Weil-McLain code, a pro can cross-reference your exact model and control to find the manufacturer-defined meaning and follow the correct test sequence.
Rough cost: A pro service call runs about $150-350.