What error codes does a Lochinvar boiler display?

Lochinvar's SMART SYSTEM control on Armor, Shield, Knight, Crest, FTXL, and Hellcat models uses a two-tier hierarchy. Hard-lockout faults that require manual reset include No Flame Ign (failed ignition after four attempts — one retry on Armor 500–800), APS Open (air pressure switch contacts open), Manual Reset High Limit (outlet water exceeded fixed safety setting), Sensor Open / Sensor Shorted (inlet or outlet temperature sensor failed), Flame Sequence (flame detected with no call), GV/Relay Fail (gas-valve circuit fault), Flow Switch (no flow during a heat call), Blocked Drain SW (condensate buildup), Gas Pressure SW (manual-reset pressure switch tripped), Aux. Proving (remote proving switch open), Temp O/Shoot (flue temperature exceeded set parameters), and Fan Low / Fan High (RPM more than 30 % off setpoint). Soft conditions (display blank, no burner operation) usually trace back to blown fuses F1–F5 on the main control board. Knight commercial boilers add F-numbered codes (F213 outdoor sensor / operational, F303 ignition family) on the WHB055N–399 platforms.

What does F5 mean on a Lochinvar Shield water heater?

F5 indicates that fuse F5 — the 5-amp slow-blow fuse on the main control board — has blown. F5 protects the circulation pump circuit. When the fuse opens, the pump cannot energize, and the unit locks out to prevent firing without water flow. Common causes: a seized pump motor drawing locked-rotor current, a short in the pump wiring harness or contactor, a failed pump relay on the control board, an oversized replacement pump, or a weakened fuse failing on a normal start. Diagnosis sequence: disconnect power, confirm F5 is open with a multimeter (slow-blow fuses can fail without visible damage), inspect pump wiring for shorts and pinching, and manually test the pump motor with 120 VAC per the service-manual procedure. Replace F5 only with the exact 5-amp slow-blow specification; never substitute a fast-blow fuse or jumper the holder. If the replacement opens immediately or on the next call for heat, the pump relay on the control board has an internal fault and the board needs replacement (page 62 of the SWR125N service manual).

What does F303 mean on a Lochinvar Knight boiler?

F303 belongs to Lochinvar's Knight Fire Tube wall-mount platform (WHB055N–399) F-numbered fault family. The F3xx range covers ignition and gas-valve faults; F303 specifically appears in the WHB055N IOM index alongside ignition-supervisory codes. The most reliable diagnostic path is to read the actual fault description from the SMART TOUCH display rather than rely on a generic decode — firmware revisions across the Knight FT family can shift sub-code meanings, and the IOM for your specific model (WHB055N, WHB085N, WHB110N, WHB155N, WHB205N, WHB285N, WHB399N) is the source of truth. Generally, F3xx-family faults point to: failed ignition after retries (inspect spark electrode, flame rod, and gas-valve wiring), gas-valve circuit failure (check 24 VAC at the 2-pin valve connector during the ignition attempt), or an unstable flame signal. If the display shows F303 alongside the No Flame Ign or Flame Sequence message, follow the No Flame Ign troubleshooting checklist: verify gas pressure (4–14 in. w.c. natural, 8–14 in. w.c. LP), inspect spark electrode and flame rod for fouling, check vent and air-intake pipe lengths against manual specifications, and confirm 24 VAC reaches the gas valve.

What is the difference between a Lochinvar lockout code and a warning code?

Lochinvar SMART SYSTEM faults split into two tiers. Lockout faults stop the unit until a technician investigates and presses the RESET button on the SMART SYSTEM display — the unit will not retry on its own, even if the underlying condition clears. Examples: No Flame Ign, APS Open, Manual Reset High Limit, Sensor Open, Sensor Shorted, GV/Relay Fail, Gas Pressure SW, Blocked Drain SW, Flow Switch, Aux. Proving, Temp O/Shoot, Flame Sequence, and Fan Low / Fan High on Armor and Shield series. Warning or blocking codes (sometimes labeled "AI" Boiler Blocking Code over Modbus / BACnet) suspend operation only while the condition is active, and the boiler resumes automatically once readings return to normal — typical of soft sensor drift, transient pressure dips, or short-cycle protection. The Modbus / BACnet register map exposes both as separate fault registers (Boiler Blocking Code AI vs. Boiler Lockout Code) for BMS supervision.

How are KNIGHT and CREST different on Lochinvar's commercial boilers?

Both KNIGHT and CREST are Lochinvar commercial-grade condensing boiler families, but they target different Btu/hr ranges and applications. KNIGHT covers the smaller commercial range — Knight Fire Tube wall-mount (WHB055N–399, 55,000–399,000 Btu/hr) and Knight floor-standing (KBN, KHB) — with up to 95 % thermal efficiency, modulating burners (10:1 turndown), and stainless-steel fire-tube heat exchangers. CREST is the high-mass, high-output line (1.5–16 MBH) for large commercial and institutional jobs, with 96 % thermal efficiency and a higher turndown ratio. Crest CFC adds combi domestic-hot-water capability for hospitality and multi-family applications, and Hellcat extends the platform to combined boiler / water heater duty. All four families share the SMART SYSTEM / SMART TOUCH controller and Modbus / BACnet communication option (WAN125 module), so the Boiler Blocking Code AI and Boiler Lockout Code register pattern is consistent across the line. The fault descriptions in the IOM are model-specific — always cross-reference the IOM for the exact model and serial-number range.

What does APS Open mean on a Lochinvar Armor or Shield water heater?

APS Open is a hard lockout indicating the air pressure switch contacts have failed to close. The switch monitors combustion-air blower differential pressure to confirm adequate airflow before each ignition attempt. The unit locks out and will not retry until a technician resolves the condition and presses RESET on the SMART SYSTEM display. Top causes (in service-manual order): wires not connected to the common and normally-closed terminals on the switch, vent or air-intake pipe lengths exceeding the maximum allowed in Section 3 of the IOM (each elbow adds equivalent length), obstruction or blockage at the intake pipe or termination cap (bird nests, ice, debris), kinked or condensate-logged reference hose between the switch and blower housing, fouled burner or scaled heat exchanger raising combustion-air resistance, or a defective switch that simply will not close at the rated differential pressure. Repeat APS Open after replacing the switch usually points to the combustion air blower itself or excessive vent restriction.

What does No Flame Ign mean on a Lochinvar boiler and how do I fix it?

No Flame Ign is a hard lockout indicating the unit failed to prove main-burner ignition after four attempts (Armor 500–800 lock out after one retry). All heat demands stop until manual reset. Diagnosis sequence per the Armor and Shield service manuals: inspect the spark electrode and flame rod for fouling or wiring damage and clean or replace per page 19 of the IOM; verify proper electrical grounding of the unit; check incoming gas pressure (4–14 in. w.c. natural, 8–14 in. w.c. LP); confirm the plastic hose from the gas valve to the air inlet is connected and undamaged; verify vent and air-intake pipes are correctly installed and unobstructed; measure 24 VAC at the 2-pin gas-valve connector on the side of the main control board during the ignition attempt (no voltage means replace the control board); if 24 VAC is present, inspect the gas-valve wiring; with a manometer at the outlet tap, confirm the valve actually opens during the trial-for-ignition window. If everything passes and No Flame Ign returns, replace the gas valve, then the main control board.

What does Manual Reset High Limit mean on a Lochinvar water heater?

Manual Reset High Limit is a hard lockout triggered when outlet-water temperature exceeds the fixed setting of the manual-reset high-limit thermostat (a separate safety device from the operating control). The unit shuts down and will not restart until both the high limit's mechanical reset button is pressed and the SMART SYSTEM RESET is acknowledged. Root-cause checklist: verify the system is full of water and properly purged of air (low flow through the heat exchanger drives outlet temperature up fast); confirm the boiler is piped per Section 6 of the IOM (incorrect primary / secondary loop separation is a common installer error); check 120 VAC to the pump motor on a call for heat — if voltage is present but the pump is not running, replace the pump; if voltage is missing, trace wiring back to the main control board; verify the manual-reset high limit setting and mechanical condition; measure water-sensor resistance against the IOM tables (Table 3B on Armor) and replace any sensor outside spec; if the high limit itself has tripped due to age or contact wear, replace the device.

What do fuses F1, F2, and F3 on the Lochinvar control board protect?

On Lochinvar Armor and Shield SMART SYSTEM control boards, F1–F5 protect specific circuits and a blown fuse stops the corresponding section without necessarily displaying a code. F1 protects the 24 V DC supply or relay-board section — a blown F1 leaves the display working but relays do not energize, so connected loads (pump, zone valves) silently fail. F2 (3.15 A slow-blow) protects the 24 V control circuit; F2 open kills the gas valve and zone-control supply, and the unit goes completely non-responsive with no display. F3 (1.25 A slow-blow) protects the main control board internal logic; F3 open blanks the display entirely with no indicator lights and the unit will not respond to any input. F4 protects the fan circuit — a blown F4 manifests as a Fan Speed Low or fan-non-running condition. F5 (5 A slow-blow) protects the pump circuit. Always replace with the exact specified type (slow-blow) and amperage; never jumper or substitute fast-blow. If a fuse blows immediately after replacement, a sustained short exists in the protected circuit.

When should I call a service technician for a Lochinvar boiler fault?

Call a licensed technician when: a hard-lockout fault returns immediately after manual reset (No Flame Ign, APS Open, Manual Reset High Limit, GV/Relay Fail) because the condition has not been resolved and combustion or gas-side work is required; any fuse blows immediately after replacement (sustained short circuit); Sensor Open or Sensor Shorted persists with new sensors installed (control board input-circuit failure); Temp O/Shoot recurs after heat-exchanger cleaning (combustion calibration with a CO₂ / O₂ analyzer is required); Flame Sequence appears with no obvious external wiring problem (board replacement); high or low gas-pressure switches need verification with a calibrated manometer; or the F-numbered codes on Knight (F213, F303) recur after the homeowner-level fixes. Combustion analysis, gas-valve adjustment, and control-board parameter setup all require Lochinvar service documentation, the proper instruments, and the technician credentials to operate gas-fired equipment safely.

Quick Answer

Lochinvar gas boilers and water heaters — KNIGHT (commercial), CREST (high-efficiency), Crest CFC (combi), Armor, Shield, Hellcat, and FTXL — use a two-tier alert hierarchy on the SMART SYSTEM display. Lockout faults (No Flame Ign, APS Open, Manual Reset High Limit, Sensor Open / Shorted, Flame Sequence, GV/Relay Fail) require manual reset; warnings or blocking codes self-clear when the condition resolves. Fuses F1–F5 on the control board protect specific circuits. Knight commercial boilers add F-numbered fault codes (F213, F303) for combustion-air, ignition, and pressure faults.

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What error codes does Lochinvar display?

No Flame Ign

Ignition failure after four attempts — inspect electrode, gas pressure, and 24 VAC at valve

APS Open

Air pressure switch contacts open — vent length, intake blockage, or switch failure

Manual Reset High Limit

Outlet temperature exceeded fixed limit — check pump operation and water flow

Sensor Open

Inlet or outlet water sensor disconnected — inspect wiring and resistance

Sensor Shorted

Inlet or outlet sensor shorted — measure against Table 3B resistance values

Flame Sequence

Flame signal detected with no call — supply voltage, flame rod, or board fault

GV/Relay Fail

Gas-valve circuit not detected — check 2-pin connector and wiring harness

Relay-board / 24 V DC supply fuse open — inspect for shorts on the relay rail

24 V control-circuit fuse (3.15 A slow-blow) blown — unit goes non-responsive

Main control board logic fuse (1.25 A slow-blow) blown — display blank, no indicators

Common questions about Lochinvar equipment

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