Weil-McLain A42 Error Code — What It Means
A42 is not a universal error code across all Weil-McLain boilers. The exact meaning depends on your specific model and control board. Weil-McLain uses different fault tables for different product lines, so A42 on one boiler may indicate a sensor fault, a limit condition, or a communication issue, while the same code on another model may mean something entirely different or not exist at all.
The only reliable way to know what A42 means on your boiler is to locate the service manual or control-board fault table for your exact model and serial number. Weil-McLain’s own guidance directs contractors to use model-specific documentation and contact professional support for troubleshooting rather than relying on generic assumptions.
Before You Replace Anything
Homeowners sometimes replace control boards or sensors without confirming the exact fault definition, wasting hundreds of dollars. Always pull the model-specific fault table and test the suspect component before ordering parts.
Common Causes
- Model-specific sensor fault (~30%) A42 may indicate a temperature, pressure, or flame sensor issue on your particular control board, but the exact sensor varies by model.
- High-limit or safety lockout (~25%) Some Weil-McLain controls use A-series codes for safety trips such as high-limit overheat or low-water cutoff.
- Wiring or connection problem (~20%) Loose or corroded connections to sensors or the control board can trigger undefined or intermittent fault codes.
- Control board firmware or internal fault (~15%) The control itself may have a software glitch or internal failure that logs an A42 code incorrectly.
- Pressure switch or air-proving device (~10%) On some models, A-codes relate to combustion-air proving, venting pressure, or exhaust blockage.
Quick Diagnosis
Answer these to narrow it down fast.
Does the boiler's display show any other active codes or recent fault history?
No: A42 may be an isolated fault. Note the exact model and call a qualified heating contractor with Weil-McLain experience to pull the correct fault definition.
Does the A42 code clear after a power cycle and stay off for several hours?
No: A persistent A42 indicates a real fault condition. Do not keep resetting the boiler. Call a pro to diagnose using the model-specific fault table.
Can you locate your boiler's service manual or control-board label with a fault-code table?
No: Contact Weil-McLain customer support or a local authorized service provider with your model and serial number. They can supply the correct fault definition and parts list.
Step-by-Step Fix
- Turn off power to the boiler at the service switch and wait 60 seconds, then restore power to see if A42 clears on its own.
- Record your boiler’s exact model and serial number from the data plate, usually found on the front jacket or inside the control panel door.
- Check the display or control panel for any additional fault codes, status indicators, or recent fault history stored in the diagnostics menu.
- Locate the service manual for your specific model online or request it from Weil-McLain, and find the fault-code table that defines A42 for your control board.
- Call a licensed heating contractor with the model, serial, and A42 code in hand so they can bring the correct diagnostic tools and parts for your boiler family.
- Do not attempt repairs involving gas piping, burner adjustments, or internal wiring unless you hold the proper licenses and training for gas-fired heating equipment.
- Follow the contractor’s diagnostics and approve only the parts and labor backed by the model-specific fault definition and component tests, not guesses.
Parts Often Needed
| Part | Notes |
|---|---|
| Temperature or pressure sensor (model-specific) | Amazon | Only order after confirming A42 points to this sensor in your boiler’s fault table. |
| Control board or display module (model-specific) | Amazon | Required if A42 is traced to an internal control fault or if the board cannot read sensors correctly. |
When to Call a Pro
Call a professional heating contractor immediately if you see A42 on your Weil-McLain boiler. Gas-fired boilers require licensed technicians for diagnostics and repair due to safety codes, combustion hazards, and warranty requirements. A qualified contractor will use your exact model and serial number to pull the correct fault definition, test the suspect components with calibrated meters, and replace only the parts confirmed by the manufacturer’s service procedure. Attempting DIY repairs on gas appliances without proper training and permits can void your warranty, create carbon-monoxide or explosion risks, and violate local building codes.
Rough cost: A pro service call runs about $200-450.