HeatingCodes

Weil-McLain Ultra boiler lockout codes

By Jason Dearmin · Last reviewed November 14, 2026

The Ultra boiler display shows a code when it locks out. A soft lockout clears itself. A hard lockout needs the RESET button on the display.

What this table coversLockout codes from the Weil-McLain Ultra Gas Series 1 and 2 control supplement and service guide (Ultra-80, 105, 155, 230 and 310, dated 2010). Other Weil-McLain boilers use different controls.

How to read the display

On a soft lockout the display flashes a 9 and then shows a lowercase b in the first position with the two-digit code steady. So "b18" is soft lockout code 18. On a hard lockout the display flashes the first digit, then an E, then the last two digits, so "E02" is hard lockout code 02.

Weil-McLain says pressing RESET should restart the boiler immediately after the cause is corrected. It also warns of electrical shock hazard and says to turn off power to the boiler when working on wiring or replacing a component, so these repairs belong with a heating technician.

Weil-McLain code table

CodeMeaningWhat the document says
b18 Soft lockout: high limit operation, burner off until the outlet water temperature drops below 203 degrees F minus the on differential The guide says this should only happen when heat demand is lower than the boiler lowest firing rate, such as a single small zone. Check system water, trapped air, piping and circulators. Full explanation
b19 Soft lockout: return water temperature higher than 203 degrees F, burner off Verify the direction of water flow, that the system is full, and circulator operation.
b24 Soft lockout: return temperature higher than supply temperature Burner off, waiting for the supply temperature to rise above the return temperature.
b25 Soft lockout: outlet water temperature rose too quickly Burner off for a 10 minute wait, increasing a minute each attempt to a maximum of 15 minutes.
b26 Soft lockout: external limit open on the limit wired to low-voltage terminals 6 and 7 Burner off for 150 seconds. If no external limit is connected, verify the green jumper wire is making good contact.
b29 Soft lockout: blower signal not zero when it should be Possible strong wind against the vent turning the blower wheel, a defective blower or a defective control module.
b30 Soft lockout: temperature rise across the boiler more than 45 degrees F A flow problem through the boiler. After repeated attempts the boiler goes to hard lockout.
b35 Soft lockout: short circuit across the flue temperature sensor terminals
b40 Soft lockout: open circuit across the flue temperature sensor terminals
b52 Soft lockout: flue temperature exceeded 216 degrees F but did not exceed 225 degrees F Hard lockout occurs above 225 degrees F.
b65 Soft lockout: waiting for the blower to start
E00 Hard lockout: flame detected on startup
E02 Hard lockout: ignition failed through 5 attempts The guide separates "spark occurs" from "spark occurs but no flame" with a long technician checklist. Full explanation
E03 Hard lockout: ignition failure or gas valve electrical problem
E04 Hard lockout: voltage lost after lockout occurred The control only restarts with a manual reset, even after a power loss.
E05 Hard lockout: internal control or display error
E06 Hard lockout: internal control failure
E07 Hard lockout: gas valve relay error
E08 Hard lockout: internal control failure
E09 Hard lockout: internal control failure
E11 Hard lockout: EEPROM error
E12 Hard lockout: external limit open (between terminals 6 and 8) Check the low water cut-off and limit controls wired between those terminals.
E13 Hard lockout: usually indicates a sensor problem
E14 Hard lockout: supply water sensor problem
E15 Hard lockout: internal control failure
E18 Hard lockout: thermal overrun condition, outlet The guide says supply or return water temperature reaching 210 degrees F causes a hard lockout. Full explanation
E19 Hard lockout: thermal overrun condition, return Full explanation
E25 Hard lockout: temperature rise greater than 58 degrees F
E28 Hard lockout: blower not running
E29 Hard lockout: blower signal not zero when it should be The guide says a vent facing a very windy side of the building can turn the blower and cause E-29.

Source: Weil-McLain 550-100-029/0810, Troubleshooting figures for soft and hard lockout codes.

Detailed explanations

SafetyThese tables describe what the manufacturer documents say. Gas, combustion, refrigerant and line-voltage repairs are for licensed professionals. If you smell gas, leave the building and call your gas utility or 911. Safety disclaimer.

Sources