Weil-McLain A157 Error Code — What It Means
A157 does not appear in verified Weil-McLain manufacturer documentation as a standard fault code. It may be specific to certain control boards, a historical display entry, or a misread of another code. Before attempting any repair, you must identify your exact boiler model and control type, then consult the correct manual or code table for that control family. Weil-McLain boilers use different control platforms depending on the model and year, and each platform has its own fault-code dictionary.
Because the code is unconfirmed, guessing at a single cause risks replacing the wrong part. The safest approach is to record the full display, check the fault history in the diagnostics menu if your control supports it, and match the code to your specific boiler. Common faults on Weil-McLain equipment include sensor failures (thermistors, low-water cutoff), ignition problems, low system pressure, air in the lines, and circulator issues, but without a verified code definition you cannot narrow the field responsibly.
Before You Replace Anything
Homeowners sometimes replace the ignition control or flame sensor when the real problem is a failed thermistor or low-water sensor giving false temperature readings. Test suspected sensors with a multimeter for resistance before swapping major controls.
Common Causes
- Unverified code or wrong control manual (~30%) The code may not exist for your control board, or you are looking at the wrong manual for your specific Weil-McLain model.
- Failed thermistor or temperature sensor (~25%) A faulty thermistor can send incorrect temperature data and trigger fault codes or prevent ignition on many Weil-McLain boilers.
- Low-water cutoff sensor fault (~20%) A stuck or failed low-water sensor may falsely signal unsafe conditions even when water level is correct.
- Low system pressure (~15%) If pressure drops below around 12 psi the boiler may lock out or display a fault, especially after a leak or air purge.
- Ignition or flame-sensor problem (~10%) Ignition components or a dirty flame sensor can prevent the burner from firing and log a fault code.
Quick Diagnosis
Answer these to narrow it down fast.
Is the system pressure gauge reading below 12 psi?
No: Pressure is adequate. Move on to checking the exact code definition for your control board.
Can you find A157 in the fault-code table in your boiler's installation and service manual?
No: The code may be control-specific or misread. Note your boiler model and control board type, take a photo of the display, and call a qualified technician who can match the code to your equipment.
Does the boiler have a diagnostics menu that shows fault history?
No: You will need to rely on the manual's code table or professional diagnostics to interpret A157.
Step-by-Step Fix
- Turn off power and fuel at the service switches and gas valve before inspecting the boiler.
- Record the full display exactly as shown, along with your boiler model number and serial/CP number from the data plate.
- Locate the correct installation and service manual for your model on the Weil-McLain website or with your installer, and look up A157 in the fault-code table.
- Check system pressure on the gauge. If it is below 12 psi, slowly refill through the fill valve until the gauge reads in the normal range, then close the valve.
- Enter the diagnostics or contractor menu if your control supports it and review the fault history and lockout log for additional codes or patterns.
- Test suspected sensors with a multimeter if the manual points to a thermistor, low-water cutoff, or other sensor. Compare resistance readings to the spec table in the manual.
- Replace the failed component identified by testing or by following the manual’s diagnostic flowchart, then restore power, purge air if the system was drained, and confirm stable operation and heat delivery.
Parts Often Needed
| Part | Notes |
|---|---|
| Thermistor / Temperature Sensor | Amazon | Verify the exact part number for your boiler model before ordering. |
| Low-Water Cutoff Sensor | Amazon | Match to your boiler’s control family and mounting configuration. |
When to Call a Pro
Call a licensed heating technician immediately if you cannot find A157 in your boiler’s manual, if you are uncomfortable working with gas or high-voltage controls, or if basic checks (pressure, power, thermostat) do not clear the code. Weil-McLain boilers require model-specific diagnostics, and the wrong repair can damage the control board or create a safety hazard. A qualified technician will identify your exact control type, pull the correct fault-code table, test sensors and ignition components with the right meters, and document the failed part number for any warranty claim. Do not guess at sensor or control replacements without verifying the code’s meaning first.
Rough cost: A pro service call runs about $150-400.