What does Lochinvar APS mean?

The APS fault on Lochinvar Armor and Shield water heaters indicates that the air pressure switch contacts are open. The control module monitors this switch to confirm the combustion air blower is moving adequate airflow before allowing ignition.

What does Lochinvar F2 mean?

The F2 fault on Lochinvar Shield water heaters indicates that fuse F2 on the main control board has blown. Fuse F2 is a 3.15-amp slow-blow fuse that protects the 24-volt control circuit supply. When this fuse opens, the 24V control circuit loses power and the unit cannot operate or display normal status.

What does Lochinvar F3 mean?

The F3 fault on Lochinvar Shield water heaters signals that fuse F3 on the main control board has blown. Fuse F3 is a 1.25-amp slow-blow fuse that protects the main control board's internal logic circuits.

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 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.

HVAC

Lochinvar error codes, decoded.

Look up any Lochinvar fault code below. Each one is grounded in Lochinvar's own service documentation — the meaning, the symptoms, the causes, and the steps to clear it.

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.

By Octavian Ghindea

Co-founder & CTO · Reviewed May 2026

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What error codes does Lochinvar display?

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APS

active not Closed EEPROM code APS active not Open 236= EEPROM code flame out of

on the control board, see page 68 of this manual. · Check water heater pump oper

on the control board, see page 68 Blown fuse. of this manual. · Verify flame is

on the control board, see page 68 of this manual. No Pump - Faulty pump. · Repla

, F2, F3, and "24V DC SUPPLY." On the right, the "RELAY BOARD" section consists

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