Weil-McLain A90 Error — What It Means
A90 on Weil-McLain boilers is not universal across every model, but on systems that use alphanumeric alarm codes it commonly signals a flame-proving or ignition failure. The control board attempted to light the burner but did not confirm a stable flame, or the flame was lost during the run cycle and the boiler locked out. The exact definition depends on your specific model and control platform, so you must consult your unit’s manual and identify the model number and CP number from the rating plate to confirm the meaning. On models using this code, A90 indicates the ignition sequence did not reach or maintain proven flame, resulting in a lockout or fault condition.
Before You Replace Anything
Many technicians replace the igniter or flame sensor first without measuring gas pressure. Low inlet pressure or a closed service valve is often the real culprit and costs nothing to check.
Common Causes
- Weak or interrupted gas supply (~35%) Closed gas cock, low inlet pressure, undersized piping, failing gas valve, or utility interruption prevent the burner from lighting or staying lit.
- Dirty or misaligned flame sensor (~25%) Contamination, poor grounding, or incorrect position stops the control from confirming flame even when the burner lights.
- Failed igniter or ignition hardware (~20%) A cracked hot-surface igniter, poor connection, or incorrect spark-gap on spark systems prevents ignition.
- Burner contamination or improper combustion (~10%) Dirty burners, condensate debris, or combustion adjustment issues allow the flame to light but not remain stable.
- Airflow or venting problems (~7%) Blocked intake or exhaust, condensate-related restriction, or pressure-switch faults interrupt the ignition sequence or cause flame-loss.
- Control board or wiring faults (~3%) Loose harness, bad connector, failed control, or intermittent safety-circuit opening causes false lockout.
Quick Diagnosis
Answer these to narrow it down fast.
Is the gas service valve to the boiler fully open?
No: Open the valve completely and reset the boiler. If it still locks out, the problem lies elsewhere in the gas or ignition system.
Can you hear the igniter click or see it glow when the boiler tries to start?
No: The igniter may be failed, the wiring is open, or the control board is not commanding ignition. Call a pro.
Does the burner light but then go out after a few seconds?
No: No flame at all. Focus on gas supply, igniter function, and valve operation.
Step-by-Step Fix
- Identify your exact model and control platform by reading the rating label and model number. Write down the model number and CP number before consulting the manual or calling service.
- Verify the basics: confirm the boiler is calling for heat, electrical power is present, all manual gas valves are fully open, and the control is not in manual lockout. Reset the boiler per the manual’s procedure.
- Measure gas supply pressure at the boiler inlet and while the burner is firing. Compare readings to the rating plate and manual requirements. Low pressure or excessive drop under load is a common root cause and must be corrected before replacing parts.
- Inspect the igniter and flame sensor. Check for cracks, corrosion, loose wiring, and correct positioning. Clean the flame sensor with fine abrasive cloth only if the manual permits and the sensor is removable. Do not touch the hot-surface igniter ceramic with bare hands.
- Examine the burner and combustion chamber for dirt, condensate residue, debris, rust, or water intrusion. Clean only per the manufacturer’s procedure and verify burner door seals are intact.
- Check venting and pressure-switch circuits where applicable. Inspect intake and exhaust piping for obstructions, verify the condensate trap and drain are clear, and confirm all safety switches are making properly.
- Confirm flame establishment and flame signal with the control board diagnostics or a multimeter if you are a qualified technician. If the flame lights but the signal is weak or absent, suspect sensor grounding, contamination, or control-board fault. Replace only the component that tests bad.
Parts Often Needed
| Part | Notes |
|---|---|
| Flame sensor rod | Amazon | Model-specific; verify part number from manual or rating plate before ordering. |
| Hot-surface igniter | Amazon | Fragile ceramic; confirm correct voltage and mounting style for your control. |
| Gas valve | Amazon | Only replace after confirming inlet pressure is correct and the valve does not open on command. |
When to Call a Pro
Call a licensed technician immediately if you are not trained to work on gas appliances, if you smell gas, or if you cannot safely access the rating plate and manual. Gas pressure measurement, burner adjustment, flame-sensor diagnosis, and control-board troubleshooting require proper tools and combustion-analyzer testing. Weil-McLain requires the model number and CP number for warranty and parts support, so have that information ready. If the boiler has locked out multiple times, do not keep resetting it. Repeated ignition attempts without a confirmed fix can damage the control or create unsafe conditions. A qualified pro will measure gas pressure, test ignition components, verify flame signal, inspect venting, and consult the correct manual for your specific control platform before replacing parts.
Rough cost: A pro service call runs about $150–350.