Lochinvar Boiler Error Codes — Meanings, Causes & Repair in Greater Vancouver

Seeing a fault code on your Lochinvar boiler? This guide explains what the most common Lochinvar boiler error codes mean, what’s safe to check yourself, and when to call HeatLand’s licensed, TSBC-certified gas technicians for Lochinvar boiler repair across Metro Vancouver and the Fraser Valley.

The codes below apply across common Lochinvar models (KNIGHT, WHN, Noble NKB). Meanings follow Lochinvar’s documented fault codes — always confirm against your unit’s own manual, because exact wording can vary by model, control board and firmware.

How to read Lochinvar boiler error codes

Lochinvar residential condensing/combi boilers (KNIGHT Fire Tube/WHN, Noble NKB/NKC) use the SMART SYSTEM / SMART TOUCH control module. Rather than short numeric codes, the control shows a plain-text FAULT MESSAGE on the display window/touchscreen (for example 'Blocked Drain SW', 'No Flame Ign', 'Gas Pressure SW', 'Sensor Open'). Most faults are hard lockouts: once the underlying condition is corrected, the burner is restarted by pressing the RESET button on the SMART SYSTEM/SMART TOUCH display. Some messages (Anti-cycling, Parameters Progr) are not faults but informational/auto-clearing states. Exact wording can vary slightly by model and firmware (e.g. 'No Flame Ign' vs 'Flame Fail Ign', 'Fan Low' vs 'Fan Speed Low', 'Auto Reset High Limit' vs 'Outlet ARHL'). Fault definitions below are taken from the official Lochinvar KNIGHT service manual fault-message chart (Table 10) and the Noble NKB/NKC manuals.

Lochinvar boiler error codes & what they mean

These are the Lochinvar boiler codes our Greater Vancouver technicians see most. Anything marked pro repair involves gas, electrical, sealed-combustion or control-board work that must be done by a licensed gas technician (TSBC-certified in BC) — never open gas or electrical components yourself.

Code	What it usually means	Common causes	Safe to check yourself
Blocked Drain SW	The blocked drain switch has detected excessive condensate build-up inside the unit (condensate is not draining freely). Manual reset lockout pro repair	Clogged or kinked condensate drain tube or trap; Frozen condensate line; Blocked floor drain / condensate pump not working	Check that the condensate drain hose/trap is not kinked, clogged, or frozen and that it drains to the floor drain freely; If a condensate pump is used, confirm it has power and is running; After clearing a visible blockage, press RESET once on the SMART SYSTEM/SMART TOUCH display
Gas Pressure SW	The optional manual-reset low gas pressure switch or high gas pressure switch has tripped (or a required jumper is missing). Manual reset lockout pro repair	Low or interrupted gas supply pressure (NG should be 4-14 in. w.c., LP 8-14 in. w.c.); Gas meter/regulator or supplier issue; Tripped manual-reset pressure switch	Confirm the manual gas shutoff valve to the boiler is open; Confirm other gas appliances in the home work (so gas is supplied); Press RESET once on the SMART SYSTEM/SMART TOUCH display
Flow Switch/LWCO	The optional flow switch or low water cutoff is not making proper contact (inadequate flow / low water), or fuse F2 is blown. Manual reset lockout pro repair	Insufficient system flow (pump not running, air-bound system); Closed isolation valve or obstruction in piping; Low system water / low water cutoff tripped	Read the system pressure gauge – confirm cold-fill pressure is roughly 12-15 psi, not zero; Confirm any manual isolation valves on the boiler piping are open; Press RESET once on the SMART SYSTEM/SMART TOUCH display
No Flame Ign	The unit failed to prove main burner ignition after four (4) attempts (no flame established). Manual reset lockout. Also shown as 'Flame Fail Ign' on some models pro repair	Gas off, air in the gas line, or low gas pressure; Dirty, worn, or misaligned spark electrode / flame sensor; Gas valve or its wiring failure	Confirm the gas shutoff valve to the boiler is open and other gas appliances work; Confirm the vent and air-intake terminations outside are clear of snow, debris, or nests; Press RESET once on the SMART SYSTEM/SMART TOUCH display
No Flame Running	The unit was running and lost the flame signal; this occurred four (4) straight times. Manual reset lockout. Also shown as 'Flame Fail Running' pro repair	Marginal gas supply / pressure dropping under load; Dirty or degraded flame sensor or spark electrode; Vent/air-intake partial obstruction	Confirm other gas appliances run normally (steady gas supply); Confirm vent and air-intake terminations outside are clear; Press RESET once on the SMART SYSTEM/SMART TOUCH display
Flame Sequence	The flame detector circuit is seeing a flame signal while no flame should be present (false flame / flame-out-of-sequence). Manual reset lockout pro repair	Incorrect supply-voltage polarity (line/neutral reversed); External wiring voltage feedback; Contaminated/wet flame rod	Press RESET once on the SMART SYSTEM/SMART TOUCH display; If it returns, leave it for a technician – this fault usually involves wiring/polarity and is not a homeowner fix
APS Open	Air pressure switch contacts are open – the control did not see the expected pressure proof during the venting/purge sequence. Manual reset lockout pro repair	Vent or air-intake obstruction or excessive vent length; Blocked or disconnected pressure-switch reference hose; Dirty burner or heat exchanger	Confirm the vent and air-intake terminations outside are clear of snow, debris, leaves, or nests; Press RESET once on the SMART SYSTEM/SMART TOUCH display
Anti-cycling	Informational, not a fault. The control received a call for heat too soon after the previous call ended; it holds off the burner for a set time (or until the outlet temperature drops) to prevent short-cycling, then resumes automatically DIY-checkable	Frequent short calls for heat; Oversized boiler for a small load; Thermostat differential set too tight	No action needed – the boiler resumes on its own once the anti-cycle timer expires or the water cools; No reset required
Auto Reset High Limit	The outlet water temperature exceeded the fixed automatic-reset high-limit setting (or an optional manual-reset high limit). Often displayed as 'Outlet ARHL'. Indicates overheating, usually from poor flow pro repair	Boiler pump not running or seized; Air-bound system / low flow; Closed isolation valve	Read the system pressure gauge and confirm there is adequate pressure (roughly 12-15 psi cold); Confirm radiators/zones are not all shut off (so heat has somewhere to go); Press RESET once on the SMART SYSTEM/SMART TOUCH display
Outlet MRHL	The optional manual-reset high limit on the outlet has tripped due to excessive outlet water temperature; requires manual reset of both the device and the control pro repair	Loss of flow (pump failure, air, closed valve); Overfiring or control fault; Tripped/failed manual-reset limit	Confirm zones/radiators are open so heat can dissipate; Read the system pressure gauge for adequate pressure; Press RESET once on the SMART SYSTEM/SMART TOUCH display
Fan Low	Actual combustion fan RPM is about 30% lower than the commanded speed (also 'Fan Speed Low'); can also indicate a blown fan fuse F4. Manual reset lockout pro repair	Vent/air-intake obstruction or excessive length; Loose fan wiring connections; Failing combustion fan	Confirm the vent and air-intake terminations outside are clear; Press RESET once on the SMART SYSTEM/SMART TOUCH display
Fan High	Actual combustion fan RPM is about 30% higher than the commanded speed (also 'Fan Speed High'). Manual reset lockout pro repair	Vent/air-intake obstruction or back-pressure; Faulty fan or fan tachometer feedback; Loose fan wiring	Confirm the vent and air-intake terminations outside are clear; Press RESET once on the SMART SYSTEM/SMART TOUCH display
GV/Relay Fail	The main control board did not detect the gas valve (gas valve / relay fault). Often shown as 'Gas Valve' or 'Gas Valve Fail'. Manual reset lockout pro repair	Disconnected or damaged gas-valve wiring harness; Failed gas valve; Control board fault	Press RESET once on the SMART SYSTEM/SMART TOUCH display; If it returns, call a technician – this is an internal gas-valve/wiring fault, not a homeowner fix
Sensor Open	Either the inlet water or outlet water temperature sensor reads as open/disconnected. Manual reset lockout pro repair	Disconnected or damaged sensor wiring; Failed (open-circuit) temperature sensor	Press RESET once on the SMART SYSTEM/SMART TOUCH display; If it returns, call a technician – sensor diagnosis requires opening the control area
Sensor Shorted	Either the inlet water or outlet water temperature sensor reads as shorted. Manual reset lockout pro repair	Pinched/shorted sensor wiring; Failed (short-circuit) temperature sensor	Press RESET once on the SMART SYSTEM/SMART TOUCH display; If it returns, call a technician – sensor diagnosis requires opening the control area
Outlet Sensor Fault	Fault with the outlet (supply) water temperature sensor – open, shorted, or out of range. On Noble combi models a related 'Combi Sensor Fault' message refers to this outlet sensor circuit. Manual reset lockout pro repair	Disconnected, pinched, or corroded outlet/combi sensor wiring; Failed outlet temperature sensor	Press RESET once on the SMART SYSTEM/SMART TOUCH display; If it returns, call a technician
Inlet Sensor Fault	Fault with the inlet (return) water temperature sensor – open, shorted, or out of range. Manual reset lockout pro repair	Disconnected or damaged inlet sensor wiring; Failed inlet temperature sensor	Press RESET once on the SMART SYSTEM/SMART TOUCH display; If it returns, call a technician
DHW Sensor Fault	Fault with the domestic-hot-water sensor (combi models only) – disconnected, shorted, or out of range; affects hot-water (DHW) operation pro repair	Disconnected or damaged DHW sensor wiring; Failed DHW sensor	Press RESET once on the SMART SYSTEM/SMART TOUCH display; If it returns, call a technician
Flue Sensor Fault	Fault with the flue (stack) temperature sensor – open, shorted, or out of range. Manual reset lockout. (Also a 'Flue sensor open' condition can cause a no-display state on some KNIGHT models.) pro repair	Disconnected or damaged flue sensor wiring; Flue sensor not seated in the flue outlet; Failed flue temperature sensor	Press RESET once on the SMART SYSTEM/SMART TOUCH display; If it returns, call a technician – the flue sensor must not be left out (flue-gas spillage risk)
Flue Temp Limit	The stack (flue) temperature has exceeded the set parameters for the boiler – usually indicating a dirty heat exchanger or vent obstruction. Manual reset lockout pro repair	Sooted/fouled flue side of the heat exchanger; Vent/air-intake obstruction; Faulty flue sensor	Confirm the vent and air-intake terminations outside are clear; Press RESET once on the SMART SYSTEM/SMART TOUCH display; If it returns, call a technician for heat-exchanger inspection/cleaning
Temp O/Shoot	Outlet water temperature has exceeded the maximum allowed outlet temperature (temperature overshoot). Manual reset lockout. Indicates inadequate flow/heat dissipation pro repair	Boiler pump not running or wrong speed/size; Air-bound or improperly purged system; Improper near-boiler piping / low flow	Read the system pressure gauge and confirm adequate pressure (roughly 12-15 psi cold); Confirm zones/radiators are open so heat can dissipate; Press RESET once on the SMART SYSTEM/SMART TOUCH display
Delta T High	The temperature rise (delta-T) across the heat exchanger has exceeded the set parameters for the boiler – typically a low-flow condition pro repair	Boiler pump not running, wrong speed, or undersized; Air in the system / not fully purged; Closed valve or obstruction reducing flow	Read the system pressure gauge and confirm adequate pressure; Confirm boiler isolation valves are open; Press RESET once on the SMART SYSTEM/SMART TOUCH display
Low 24 vac	The 120 VAC input to the main control board dropped below about 80 VAC, leaving inadequate 24 VAC control voltage pro repair	Low or unstable incoming line voltage; Loose low-voltage terminal connections; Undersized/too-long wiring to remote devices	Confirm the boiler's electrical disconnect/breaker is fully on and the On/Off switch is on; Press RESET once on the SMART SYSTEM/SMART TOUCH display; If it returns, call a technician – voltage/transformer diagnosis is not a homeowner task
Rem Ctrl Flt	The external/remote control is cycling the boiler too often (remote control fault) pro repair	External control set point or differential set incorrectly; Wiring problem between external control and boiler; Failed external control	Check the thermostat/external control set point looks reasonable and is not rapidly cycling; Press RESET once on the SMART SYSTEM/SMART TOUCH display
Tank Open	On sensors with an internal limit (such as the Lochinvar Squire indirect tank), the limit has opened due to temperature (~195F) or the tank sensor has become disconnected pro repair	Tank over-temperature; Disconnected or damaged tank sensor wiring; Failed tank sensor	Press RESET once on the SMART SYSTEM/SMART TOUCH display; If it returns, call a technician to check the tank sensor
Aux. Proving	An optional remote/auxiliary proving switch is not making (its contacts did not close). Manual reset lockout pro repair	Remote proving device (e.g. louver/damper end switch) not proving; Loose or misplaced jumper if no auxiliary switch is installed; Wiring fault	Press RESET once on the SMART SYSTEM/SMART TOUCH display; If it returns, call a technician
Parameters Progr	Informational. After parameters were downloaded to the control (e.g. from a laptop during service), the main control board must be reset by pressing ENTER/RESET. Not a fault DIY-checkable	Control parameters were just programmed/updated during service	Press the ENTER/RESET button once on the SMART SYSTEM/SMART TOUCH display panel
Watch Dog Error	The main control board has detected an internal fault (watchdog). Indicates a control board problem pro repair	Main control board internal fault	Try a single power-off / power-on of the boiler and one RESET; If it persists, call a technician – this is an internal control-board fault
Fatal Error	The main control board has detected an internal/non-recoverable fault (also reported as Write EEProm, CRC Parameters, or No Error Stored). Generally requires control board replacement pro repair	Main control board memory/internal fault	Try a single power-off / power-on of the boiler; If it persists, call a technician – the control board likely needs service/replacement

Lochinvar boiler codes we get called for most

No Flame Ign — The unit failed to prove main burner ignition after four (4) attempts (no flame established). Manual reset lockout. Also shown as 'Flame Fail Ign' on some models

Typical causes include gas off, air in the gas line, or low gas pressure; dirty, worn, or misaligned spark electrode / flame sensor; gas valve or its wiring failure. A few things are safe to check yourself (Confirm the gas shutoff valve to the boiler is open and other gas appliances work; Confirm the vent and air-intake terminations outside are clear of snow, debris, or nests), but the repair is a licensed, TSBC-certified gas technician job. HeatLand diagnoses and fixes No Flame Ign on Lochinvar boilers throughout Greater Vancouver.

Gas Pressure SW — The optional manual-reset low gas pressure switch or high gas pressure switch has tripped (or a required jumper is missing). Manual reset lockout

Typical causes include low or interrupted gas supply pressure (ng should be 4-14 in. w.c., lp 8-14 in. w.c.); gas meter/regulator or supplier issue; tripped manual-reset pressure switch. A few things are safe to check yourself (Confirm the manual gas shutoff valve to the boiler is open; Confirm other gas appliances in the home work (so gas is supplied)), but the repair is a licensed, TSBC-certified gas technician job. HeatLand diagnoses and fixes Gas Pressure SW on Lochinvar boilers throughout Greater Vancouver.

Auto Reset High Limit — The outlet water temperature exceeded the fixed automatic-reset high-limit setting (or an optional manual-reset high limit). Often displayed as 'Outlet ARHL'. Indicates overheating, usually from poor flow

Typical causes include boiler pump not running or seized; air-bound system / low flow; closed isolation valve. A few things are safe to check yourself (Read the system pressure gauge and confirm there is adequate pressure (roughly 12-15 psi cold); Confirm radiators/zones are not all shut off (so heat has somewhere to go)), but the repair is a licensed, TSBC-certified gas technician job. HeatLand diagnoses and fixes Auto Reset High Limit on Lochinvar boilers throughout Greater Vancouver.

Fan Low — Actual combustion fan RPM is about 30% lower than the commanded speed (also 'Fan Speed Low'); can also indicate a blown fan fuse F4. Manual reset lockout

Typical causes include vent/air-intake obstruction or excessive length; loose fan wiring connections; failing combustion fan. A few things are safe to check yourself (Confirm the vent and air-intake terminations outside are clear; Press RESET once on the SMART SYSTEM/SMART TOUCH display), but the repair is a licensed, TSBC-certified gas technician job. HeatLand diagnoses and fixes Fan Low on Lochinvar boilers throughout Greater Vancouver.

Is it safe to keep running a Lochinvar boiler with a fault code?

Most Lochinvar fault codes put the boiler into a safe lockout for a reason. You can safely check the basics — gas supply on, power on, the thermostat calling for heat, system pressure in the normal range, and the condensate drain not frozen or blocked (a common cause in Vancouver cold snaps) — and restart it once. If the code returns, stop and book a technician. If you ever smell gas, leave the building and call your gas utility’s emergency line or 911 first. If a carbon-monoxide alarm sounds, get everyone out and call 911.

Lochinvar boiler repair across Greater Vancouver

HeatLand has kept Metro Vancouver homes comfortable since 1999. Our licensed, insured technicians diagnose and repair Lochinvar boilers — ignition faults, flame and sensor errors, low-pressure and pump problems, fan and flue faults, and control-board issues — including units we didn’t install. We’re brand-agnostic, carry common parts, and give you an honest repair-or-replace assessment with upfront pricing and a free estimate. If your boiler is beyond economical repair, we’ll show you right-sized, high-efficiency replacement options and the rebates available.

Need it sorted fast? We offer priority emergency boiler repair. Explore Boiler services, browse all boiler error codes by brand, or request a free estimate.

Lochinvar boiler service areas

We repair Lochinvar boilers across Greater Vancouver and the Fraser Valley, including:

Lochinvar boiler error code FAQs

Can I reset my Lochinvar boiler myself?

Yes — power-cycling the reset button once is generally safe after you’ve checked the basics (gas supply on, power on, the thermostat calling for heat, system pressure in the normal range, and the condensate drain not frozen or blocked (a common cause in Vancouver cold snaps)). If the boiler faults again right away, stop and note the code — repeatedly resetting a boiler that keeps locking out can mask a real problem. Call a licensed, TSBC-certified gas technician to diagnose it.

What does the Lochinvar No Flame Ign code mean?

On Lochinvar boilers, No Flame Ign commonly indicates the unit failed to prove main burner ignition after four (4) attempts (no flame established). manual reset lockout. also shown as 'flame fail ign' on some models. Confirm the exact meaning against your unit’s manual — codes can vary by model and series — and have the repair done by a licensed, TSBC-certified gas technician. HeatLand fixes this across Greater Vancouver.

Do you repair Lochinvar boilers you didn’t install?

Yes. HeatLand is brand-agnostic and services Lochinvar boilers throughout Metro Vancouver and the Fraser Valley, whether or not we installed the unit. We carry common parts and give you an honest repair-or-replace assessment.

Is a Lochinvar boiler fault code an emergency?

No heat in cold weather is worth treating urgently. If you ever smell gas, leave the building and call your gas utility’s emergency line or 911 first. If a carbon-monoxide alarm sounds, get everyone out and call 911. For priority boiler repair across Greater Vancouver, call (604) 330-3812.

How much does Lochinvar boiler repair cost in Vancouver?

It depends on the fault. Many Lochinvar codes point to sensors, ignition or pump parts — usually far cheaper than replacement. We give upfront pricing and a free estimate before any work.

More Boiler error-code guides

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