Weil-McLain A37 Error Code — What It Means
An A37 error code cannot be verified as a Weil-McLain factory-defined fault code from manufacturer documentation. Weil-McLain boilers use model-specific control boards and diagnostic systems, and the same alphanumeric code can mean entirely different things depending on your exact boiler model and control platform. Before interpreting any fault, identify your boiler model number and control board type, then use the diagnostic or fault-history menu on the control to retrieve the stored lockout code.
If your display is showing A37 or a similar code, consult the service manual for your specific model to decode the fault. Common Weil-McLain lockouts involve ignition failure, flame-proving issues, pressure or sensor faults, or combustion-air problems, but without confirmation that A37 is a real code for your unit, follow the fault-history retrieval procedure in your manual rather than guessing at the cause.
Before You Replace Anything
Homeowners sometimes replace the igniter or flame sensor when the real problem is low gas pressure or a blocked condensate trap. Always check gas supply, system pressure, and condensate drainage before buying parts.
Common Causes
- Ignition or flame-proving failure (~30%) No gas flow, a weak igniter, or a dirty flame sensor prevents the boiler from proving flame after an ignition attempt, triggering a lockout.
- Low hydronic system pressure (~25%) A pressure gauge reading below 12 psi can cause the boiler to shut down on a low-water cutoff or pressure switch.
- Blocked venting or combustion air intake (~20%) Debris, ice, nests, or flue restrictions in the intake or exhaust prevent safe combustion and cause a safety lockout.
- Faulty temperature sensor or thermistor (~15%) A failed or misaligned temperature probe sends incorrect readings to the control, causing erratic lockouts or flame-loss faults.
- Blocked condensate trap or drain (~10%) On condensing models, a clogged condensate trap or drain line can create a pressure switch fault and shut down the boiler.
Quick Diagnosis
Answer these to narrow it down fast.
Does the pressure gauge read above 12 psi when the system is cold?
No: Low pressure is likely causing the lockout. Add water to the system to bring pressure to around 15 psi, then reset the boiler and monitor for leaks.
Can you hear the igniter clicking or see a spark during a call for heat?
No: No ignition attempt means a control, igniter, or gas-valve problem. Call a qualified technician to diagnose the control and ignition circuit.
Is the exhaust or intake vent clear of blockages, ice, or debris?
No: Clear the blockage, then reset the boiler and test. Repeat blockages require a vent-termination inspection by a pro.
Step-by-Step Fix
- Identify your boiler model and control. Locate the rating plate on the boiler jacket and note the exact model number and the control board or display type. Retrieve the stored fault history from the contractor or diagnostics menu using the procedure in your service manual.
- Shut off power and gas. Turn off the electrical supply at the service switch or breaker, and close the manual gas shutoff valve upstream of the boiler before inspecting any components.
- Check system pressure. Read the pressure gauge on the boiler. If it is below 12 psi, add water through the fill valve until the gauge reads around 15 psi, then restore power and gas and attempt a reset.
- Inspect gas supply and ignition. Confirm the manual gas valve is fully open. Restore power and initiate a call for heat. Listen for the igniter and watch for spark or glow. If ignition attempts but no flame appears, inspect the burner area for debris and check that gas is flowing.
- Clean or replace the flame sensor. If the boiler lights but shuts down after a few seconds, remove the flame sensor, clean it with fine abrasive cloth or steel wool, and reinstall it in correct alignment with the burner flame.
- Check venting and combustion air. Inspect the intake and exhaust vent terminations for blockages, ice, bird nests, or debris. On indoor models, confirm the burner compartment door is fully closed and sealed.
- Inspect the condensate trap and drain. On condensing boilers, remove and rinse the condensate trap, clear the drain line, and confirm water flows freely from the boiler to the drain.
- Test sensors and controls. Use a multimeter to check thermistor or temperature-sensor resistance against the values in your service manual. If readings are out of range, replace the sensor. If the fault persists, call a qualified technician to test the control board and pressure switches.
Parts Often Needed
| Part | Notes |
|---|---|
| Flame sensor rod | Amazon | Choose the OEM part number for your exact Weil-McLain model and control. |
| Igniter or spark electrode | Amazon | Match to your boiler’s ignition system, hot-surface or spark type. |
| Temperature sensor or thermistor | Amazon | Verify the resistance spec in your manual before ordering. |
When to Call a Pro
Call a qualified HVAC or boiler technician if you cannot retrieve a confirmed fault code from your control, if the boiler repeatedly locks out after you have checked pressure and gas supply, if you smell gas or see evidence of a gas leak, or if you need to test or replace the gas valve, control board, pressure switches, or any wired safety device. Professional diagnosis is also required when the fault history points to a combustion or flame-proving problem that persists after cleaning the flame sensor and burner area, or when the boiler model and control type do not match any code definition you can find in your manual.
Rough cost: A pro service call runs about $150-400.