Weil-McLain Boiler A138 Error — What It Means
The A138 code is not a manufacturer-universal fault code documented in standard Weil-McLain service manuals. It is likely a model-specific or control-platform-specific service code or history entry. Weil-McLain boilers use different control systems across their product lines, and each controller can display unique diagnostic codes. The exact meaning of A138 must be confirmed from your specific boiler’s installation and service manual or from the control manufacturer’s documentation before you attempt any repair.
Because the code is not universally defined, the root cause could involve a temperature sensor, flame detection issue, stuck relay, circulation problem, or condensate drainage fault. These are common fault categories on Weil-McLain boilers, but you should not replace parts based on guesswork. Retrieve the fault history from the diagnostic menu, record the operating conditions when the fault occurred, and cross-reference the code in your model’s manual to identify the failed component.
Before You Replace Anything
Many technicians replace the control board or flame sensor first when they see an unfamiliar code. Pull the fault history from the diagnostic menu and test the suspected sensor or relay electrically with a meter before ordering parts.
Common Causes
- Model manual not consulted (~40%) The A138 code is not a standard Weil-McLain fault and its meaning varies by control platform, so repair work without the correct manual often replaces the wrong part.
- Failed or misreading temperature sensor (~25%) A faulty thermistor or temperature sensor can cause false overheating or no-heat behavior and trigger service codes on many Weil-McLain models.
- Stuck relay or control output (~15%) A mechanically stuck relay can make the boiler run continuously or fail to stop, and Weil-McLain service cases document this as a real root cause.
- Flame sensor or ignition fault (~10%) A dirty or failing flame sensor or ignition lockout can produce codes on some Weil-McLain boilers and prevent the burner from lighting.
- Circulation or low-water issue (~10%) A faulty circulator pump, trapped air, or low system pressure are common boiler faults mentioned in Weil-McLain troubleshooting materials.
Quick Diagnosis
Answer these to narrow it down fast.
Is the system water pressure below 12 psi on the gauge?
No: Pressure is normal. Continue to the next check.
Can you enter the diagnostic menu and see a fault history or lockout list?
No: You may not have access to the advanced menu. Call a technician who can retrieve the history and interpret the codes correctly.
Does the boiler run continuously or fail to shut off when the thermostat is satisfied?
No: The fault is probably intermittent or related to a sensor. Pull the fault history and test the suspected sensor before replacing it.
Step-by-Step Fix
- Identify your exact boiler model and control type from the rating plate or control display before doing anything else.
- Enter the contractor or diagnostic menu following the procedure in your model’s manual and retrieve the past faults and lockout history.
- Record the A138 code, any other active codes, the time stamps, and the operating conditions when each fault occurred.
- Look up each code in your boiler’s installation and service manual or contact Weil-McLain technical support to get the verified fault name and recommended corrective action.
- Check the obvious system conditions first: thermostat demand, power, gas supply, water pressure (target about 15 psi cold), circulator operation, and any blocked intake, exhaust, or condensate drain.
- Test the suspect component electrically with a multimeter if the fault points to a temperature sensor, low-water sensor, or relay. Compare your readings to the values in the service manual.
- Replace only the verified failed component using the correct part number from the manual or parts diagram, then clear the fault, restore operation, and verify a full heat cycle under load.
Parts Often Needed
| Part | Notes |
|---|---|
| Temperature sensor or thermistor | Amazon | Match the part number to your exact boiler model and control platform. |
| Flame sensor or igniter assembly | Amazon | Clean the sensor first. Replace only if testing confirms failure. |
| Control relay or control board | Amazon | Weil-McLain service example uses 24 V relay part number 510-35223 for some models. Verify your model’s part number before ordering. |
When to Call a Pro
Call a qualified heating technician if you cannot access the diagnostic menu, if the fault history shows multiple unrelated codes, if you do not have the service manual for your specific model, or if electrical testing and relay or sensor replacement are beyond your skill level. Gas-fired boiler work involves combustion safety, venting, and electrical controls that require proper training and test equipment. Incorrect diagnosis or part replacement can leave the system unsafe or cause repeated failures. A technician can retrieve the fault history, cross-reference the A138 code in the correct manual, test the suspected component, and replace only the failed part with the manufacturer-approved part number.
Rough cost: A pro service call runs about $200-450.