What error codes does a Weil-McLain boiler display?

Modern Weil-McLain gas boilers — AquaBalance Series 2 (AB-80C, AB-120, AB-155), Ultra 80/105/155/230/310, ECO Tec, and Evergreen — use F-prefix fault codes on the control module LCD. The F0X–F1X range covers ignition, fan, and temperature-sensor faults; F2X covers exhaust temperature and pressure-switch warnings; F3X covers supply voltage, frequency, and low water pressure; F4X covers high-pressure, heat-exchanger protection, and pump-related faults; F5X covers low-water cutoff and high-limit thermostat inputs; F6X covers software and controller resets. Press Reset for 1.5 seconds to clear soft faults; persistent faults need diagnosis using the F-code descriptions in the boiler's installation and operation manual.

What does F07 mean on a Weil-McLain boiler and how do I fix it?

F07 indicates the exhaust gas temperature exceeded the maximum safe threshold for the installed vent material (176°F for PVC, higher limits for CPVC, Polypropylene, or AL29-4C). The controller shuts off heat demand for 15 minutes; if exhaust temperature returns to normal, the fault clears automatically. Top causes: air trapped in the heat exchanger restricting water-side heat absorption (listen for gurgling when the circulator starts and bleed thoroughly), CH supply setpoint set too high (drop from 190°F to 175–180°F), parameter P02 ramp slope set aggressively at the factory default of 11 (reduce to 6 to slow temperature ramp), undersized near-boiler piping on AB-155C models that requires full 1-inch pipe, or vent length exceeding manual specifications. Verify parameter b22 (vent material) matches the installed pipe type. Persistent F07 with everything correct usually means combustion is out of calibration and needs CO₂ verification with a combustion analyzer.

What does F12 mean on a Weil-McLain combi boiler?

F12 is a DHW (domestic hot water) temperature sensor fault — the sensor has gone outside its valid operating range due to an open or short circuit. Only DHW heat demand is disabled; central heating continues to operate normally, so cold water at hot taps is the giveaway symptom on a combi boiler. Common causes: sensor failed (open or shorted), wiring damaged where it routes near the heat exchanger, sensor connector corroded or improperly seated. Fix sequence: locate the DHW sensor on the domestic side of the heat exchanger (per the AquaBalance Series 2 IOM diagram), inspect the wiring run for pinching or heat damage, re-seat the connector at the control board, and measure sensor resistance with a multimeter at room temperature — an open or near-zero short reading confirms a failed sensor. Replace with a genuine Weil-McLain DHW sensor; if F12 still returns after replacement, the control board input circuit has likely failed.

What does F13 mean on a Weil-McLain boiler?

F13 indicates the exhaust gas temperature probe (flue probe) has gone outside its valid operating range due to an open or short circuit, and all heat demands are disabled while the fault is active. The fault typically appears after repeated F07 events because the probe sits in the hottest part of the flue path — sustained high exhaust temperatures degrade the wiring insulation or burn the probe element. Common causes: probe wiring shorted from contact with hot vent surfaces, probe disconnected at the body or control board, probe failed open or shorted, corroded probe connector. Diagnose by locating the probe on the flue outlet section of the boiler, inspecting for melted insulation or physical contact with the vent pipe wall, re-seating the connector, and measuring probe resistance at a known temperature against the IOM resistance table. Repeated F13 with replacement probes points to underlying combustion or venting problems that drive exhaust temperature too high in the first place.

What does F14 mean on a Weil-McLain boiler and is it different from F10?

F14 is a CH supply water temperature sensor 2 (the high-limit NTC sensor) fault — the secondary supply sensor has gone open or shorted, and all heat demands are disabled. F10 covers supply sensor 1 (the primary CH discharge sensor); F11 covers the return sensor; F14 covers the redundant high-limit sensor that protects against runaway temperatures. AquaBalance Series 2 boilers use both supply sensors so the controller can cross-check readings and shut down on disagreement, which is a safety feature against single-point sensor failure. Diagnose F14 the same way as F10/F11/F12: locate the sensor (on the discharge assembly per the IOM diagram), inspect wiring for damage, re-seat the connector, and measure resistance against the manual's temperature–resistance table. F14 appearing simultaneously with F10 indicates a control-board input-circuit failure rather than two sensor failures at once.

What does F39 mean on a Weil-McLain boiler?

F39 indicates the outdoor temperature sensor (used for outdoor temperature compensation / OTC) has gone outside its normal operating range of −22°F to +158°F due to a short circuit or open circuit. If the sensor open-circuits and an OTC curve is selected, the OTC function disables itself and the boiler runs at fixed setpoint without compensation; a short circuit sustained for over 10 seconds generates the fault. Common causes: outdoor sensor wiring damaged from physical pinching, moisture, or rodent damage along the cable run; sensor disconnected at the boiler after OTC was active; sensor failed open from age or water ingress; corroded connector terminals. Inspect the wiring along its full run, re-seat the connector at the control board, measure sensor resistance against the IOM table, and replace the sensor if defective. If the installation is exempt from OTC requirements, set the OTC Exempt parameter to 1 to suppress F39.

Are Weil-McLain Ultra series fault codes the same as AquaBalance?

Mostly yes — Weil-McLain's modern condensing platforms (AquaBalance Series 2, Ultra 80/105/155/230/310, ECO Tec) all share the F-prefix fault scheme on the SAGE2 / SOLA-family control module, and the high-traffic codes (F07 high exhaust temperature, F10–F14 sensor faults, F37/F40 pressure, F39 outdoor sensor) carry the same meaning across the range. However, specific code numbers and parameter addresses can differ between firmware revisions of the Ultra series — some early Ultra production used a slightly different F-code map for fan-speed and ignitor-circuit faults. Always cross-reference against the boiler's installation and operation manual for your specific model and serial-number range; the IOM is the source of truth. The Ultra series also uses parameter codes (b01–b22 configuration / P02 ramp slope) accessed by holding DHW+ and DHW– for 10 seconds (configuration) or Reset for 20 seconds (transparent parameters) — the same access pattern as AquaBalance.

What does F37 mean on a Weil-McLain boiler and how do I refill the system?

F37 is a low-water-pressure fault — system pressure has dropped below the low-pressure switch threshold (set at 9 psi ± 2 psi). Heat demand and the pumps shut off when F37 is active. Most common cause is a slow system leak that the automatic fill valve cannot keep up with; secondary causes are a waterlogged expansion tank (failed air pre-charge), a failed automatic fill valve that no longer adds makeup water, or a damaged water-pressure sensor. Refill procedure: read the pressure gauge (target 12–15 psi cold), walk the entire piping run to find drips or staining (circulators, zone valves, fittings, boiler body), repair any leaks before refilling, then slowly open the fill loop until pressure reads 12–15 psi and close the valves immediately. If pressure keeps dropping, the system has an active leak that needs pressure-testing to locate; do not keep refilling without finding the source.

What does F40 mean on a Weil-McLain boiler?

F40 is a high-water-pressure fault — system pressure has exceeded the high-pressure limit as detected by the water-pressure sensor for over 2 seconds, and the controller disables heat demand and shuts off the pumps. Common causes: pressure-reducing valve (fill valve) stuck open and continuously adding makeup water (close the fill-valve shutoff and watch — if pressure stops climbing, the valve is passing); waterlogged expansion tank with no air pre-charge so it cannot absorb thermal expansion (isolate, drain, and measure pre-charge — a tank with zero air must be replaced); relief valve seat fouled and not releasing excess pressure (test the lever); pressure-sensor wiring shorted producing a false high reading. F40 will recur within minutes of reset if the root pressure source is not corrected. After fixing the cause, press Reset for 1.5 seconds and watch the gauge through the next heating cycle.

When should I call a service technician for a Weil-McLain boiler fault?

Several Weil-McLain F-codes always require a licensed technician: F66 (software mismatch — control board needs reprogramming or replacement with model-specific b01–b12 parameter setup), persistent F64 (MicroCom controller resetting 5 times in 15 minutes after a power-cycle attempt), F53 with a confirmed open high-limit thermostat (locate why the boiler is overheating before resuming operation), and any F07 / F27 / F43 that persists after the homeowner-level fixes (air purge, lower setpoint, P02 ramp adjustment, vent material verification) because resolution requires combustion analysis with a calibrated CO₂ meter. Heat-exchanger protection (F43), repeated low-pressure F37 lockouts indicating an unfixed leak, and any code that returns immediately after replacing a sensor all indicate problems beyond simple component replacement — board-level diagnosis, hydraulic analysis, or pressure-testing requires the right tools and Weil-McLain service documentation.

Quick Answer

Weil-McLain gas and oil boilers (AquaBalance Series 2, Ultra 80/105/155/230/310, ECO Tec, Evergreen) display F-prefix fault codes on the front-panel control module. The most-searched codes — F07 (high exhaust temperature), F10–F14 (supply, return, DHW, and high-limit sensor faults), F15 (fan / blower speed), F37 (low water pressure), F39 (outdoor sensor), and F40 (high water pressure) — cover ignition, sensor, pressure, and venting failures across the residential and light-commercial range.

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By Octavian Ghindea

Co-founder & CTO · Reviewed May 2026

What error codes does Weil McLain display?

F07

High exhaust temperature — air in heat exchanger, setpoint too high, or P02 ramp too aggressive

F10

CH supply sensor 1 (primary discharge sensor) open or shorted

F11

CH return sensor open or shorted — all heat demands disabled

F12

DHW sensor open or shorted — cold water at hot taps on combi models

F13

Exhaust gas temperature probe fault — often follows repeated F07 events

F14

CH supply sensor 2 (high-limit NTC) open or shorted

F15

Fan / blower speed fault — check 8-pin connector, condensate trap, and 120 V power

F37

Low water pressure — system leak or failed automatic fill valve

F39

Outdoor sensor open or shorted — OTC compensation disabled until fixed

F40

High water pressure — stuck fill valve, waterlogged expansion tank, or sensor short

Common questions about Weil McLain equipment

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