Lochinvar Knight Boiler Error Codes: What They Mean

By Ray Bennett · Updated 24 October 2026 · 8 min read · Checked against the maker’s manual

Lochinvar Knight Boiler Error Codes: What They Mean

Short answer

Lochinvar Knight boiler error codes depend on the model. On the KHB, Flame Fail Ign means ignition was not proved, while Fan Speed means the fan missed its target speed. Older Knights use different wording. Match your model below before deciding what the message needs.

Lochinvar Knight boiler error codes tell you what stopped the boiler, but the wording changes with the model. Lochinvar’s KHB manual calls an ignition failure “Flame Fail Ign”; its older Knight 80 to 285 manual calls it “No Flame Ign.”

Start with the model and the whole message. The tables below explain the faults, then the checks separate what you can see outside the boiler from the tests a heating technician needs to make.

Before you start

What to carry: a torch, a pen and your boiler’s user manual. These are checks with the cabinet closed, so you do not need tools for taking it apart.

Gas or electric? The Lochinvar manuals used here cover gas boilers. An electric boiler, a Cadet or a Noble needs its own instructions, even if Lochinvar made it.

Renting? Send the fault details to the landlord or the person who arranges your heating repairs before agreeing to work.

Deal with danger before trying a reset.
Overheating, or gas that will not shut off: Lochinvar’s Knight, KHB and Knight XL user manuals say to keep electrical power to the circulator on and shut off the gas at a valve outside the appliance. Do not use a power reset in that situation.

Smell gas? Those Lochinvar manuals say not to light an appliance, touch an electrical switch or use a phone inside the building. Leave and immediately call the gas supplier from outside. Follow the supplier’s instructions. If you cannot reach the supplier, call the fire department.

Keep internal repairs with a qualified heating technician. Lochinvar reserves these service manuals for that work. The gas valve, burner, live wiring and safety switches are not owner checks. Do not bypass a switch to get the boiler running.

A flooded boiler: Lochinvar’s user manuals say a boiler that has been under water must be replaced. Do not try to restart it.

Match your Knight to the right chart

Check 1No parts to buy for this check

Keep the model with the message

Write down the model, series, serial number and exact fault wording, plus whether it happened before ignition or after the burner ran. Lochinvar asks for the model and serial number when you contact it about the boiler.

This page uses three specific Lochinvar service manuals:

KHB Fire Tube, models 55 to 285: Rev E, covering Series 100 and 101, 110 and 111, and 120 and 121.
Older Knight, models 80 to 285: KB-SER-08, starting serial number H07H10040039.
Knight XL, models 400 to 801: KBXII-SER Rev M.

Wall mounted WH, WHN and WHB units, other Knight sizes and other control revisions are outside these charts. If your plate does not match, use the manual supplied for that boiler. A similar looking screen is not enough.

Keep the model and the message together. These are simplified examples of the wording in Lochinvar's manuals, not replicas of the screens.

KHB Fire Tube: what the messages mean

Lochinvar’s KHB Rev E service manual gives these meanings for the models listed above. The message identifies a condition; the repair still depends on testing.

KHB message	Meaning in Lochinvar’s chart
Flame Fail Ign	Main burner ignition was not proved after several attempts
Flame Sequence	The detector reports flame while the gas valve is off
Fan Speed	Actual fan speed is 30% below or above its target; a blown fuse is another listed cause
Flow Switch/LWCO	The optional flow switch or low water cutoff is not closing; a blown fuse is also listed
Air Pressure SW	Air pressure switch contacts are open
Gas Pressure SW	Gas pressure switch contacts on the low voltage connection board are open
Too Many Recyclings	A limit switch opened more than five times during the last heat demand
Outlet ARHL / Outlet MRHL	Outlet water exceeded the automatic / manual reset high limit setting
Inlet, Outlet, or Flue Sensor Fault	The named temperature sensor is disconnected or shorted
Outlet Temp Diff	The two outlet sensors disagree too much
Delta T High / Delta T Limit	Temperature rise through the heat exchanger exceeds the allowed setting
Flue Temp High / Flue Temp Limit	Exhaust temperature exceeds the allowed setting
Outlet Temp High / Outlet Temp Limit	Outlet water exceeds its maximum temperature
AUX Contact Sw	An optional remote proving switch is not closing

Check 2A diagnostic visit; parts depend on the tests

Separate ignition, circulation and sensor faults

For Flame Fail Ign, Lochinvar’s KHB checks include the ignition electrode, grounding, gas supply, venting and flame sensor. Tell the repairer whether you heard ignition attempts or the boiler never got that far.

For Delta T High or Delta T Limit, Lochinvar’s KHB chart sends the technician to the system water, trapped air, piping and pump operation. For Outlet Temp Diff, it instead calls for sensor wiring and resistance checks. These messages describe different problems.

If the temperature fault keeps returning, see boiler circulator pump problems for the circulation side of the investigation. Do not raise a temperature limit to make the message disappear.

Older Knight and Knight XL differences

For the older Knight 80 to 285 starting at serial H07H10040039, Lochinvar’s KB-SER-08 uses the following wording. Its ignition count belongs to this model range, not every Knight.

Older Knight message	Meaning in Lochinvar’s KB-SER-08
No Flame Ign	Ignition was not proved after four attempts
No Flame Running	Flame signal was lost while running four consecutive times
Flame Sequence	The circuit reports flame when none is present
Blocked Drain SW	The drain switch detected excessive condensate buildup inside the unit
APS Open	Air pressure switch contacts are open
Sensor Open / Sensor Shorted	An inlet or outlet water sensor is disconnected / shorted
GV/Relay Fail	The control did not detect the gas valve
Temp O/Shoot	Excessive exhaust temperature, heat exchanger temperature rise or outlet water temperature

For Knight XL 400 to 801, Lochinvar’s KBXII-SER Rev M calls four failed ignition attempts Flame Fail Ign. It also has Blocked Drain SW for condensate buildup and Flue Temp Limit for exhaust temperature above its limit.

APS / HEX Temp Sw Open needs particular care on that XL. Lochinvar says it can mean an open air pressure switch, heat exchanger temperature switch or burner door temperature switch. Its manual requires a qualified boiler technician to find and correct the cause before resetting a tripped burner door switch. It also warns that the door and switch will be extremely hot; leave them untouched.

Lochinvar Knight fan speed errors

Check 3Electrical and airflow testing before buying a fan

Read Low, High or Fan Speed exactly

Lochinvar’s older Knight KB-SER-08 defines Fan Low / Fan Speed Low as fan speed 30% below the requested speed. It defines Fan High / Fan Speed High as 30% above it. The KHB Rev E combines both directions under Fan Speed.

For those two ranges, Lochinvar lists excessive vent or intake length, obstructions, fan wiring, the fan and the control board in its investigation. It also lists a fuse fault with the older Knight low speed message and with KHB Fan Speed. A fan error alone does not tell you which part to buy.

Knight XL 400 to 801: Rev M groups “Fan Low/High” and “Fan Speed Low/High” together, but its description says only 30% below the requested speed. This manual does not say how far above the target speed triggers a high speed fault.

Lochinvar's older Knight and KHB charts include the air path, wiring and fan in the investigation. Internal tests belong to the technician. The drawing is a simplified layout.

Check 4No parts for easily removable debris; service for a hidden blockage

Look at the accessible air and vent openings

Lochinvar’s older Knight, KHB Series 100 and 101, and Knight XL user manuals allow removal of an air or vent blockage only when it is easily accessible and removable. Look from a safe place on the ground. Leave fixed vent joints together and do not reach into the boiler.

If the blockage is hidden or cannot be removed, those manuals call for a qualified service technician. They also require prompt attention to vent piping that leaks or deteriorates. Report any visible condensate leakage at the same visit; leave trap dismantling out of this check.

Resetting it, and what to watch afterwards

Check 5No parts needed for the reset; repair work depends on the cause

Correct the cause before clearing a lockout

For the ignition and fan faults described here, Lochinvar’s three service charts require a manual reset after the condition has been corrected, using RESET on the SMART SYSTEM display. A reset is the last step of that repair, not a test to repeat while the same fault remains.

If your screen instead says Anti-cycling, Lochinvar describes a waiting period after the previous burner run in the KHB chart. The control releases the next heat call after its set time, or if outlet temperature drops quickly enough. Let that sequence finish.

After a corrected fault, check whether heating returns and whether the same message comes back. Keep that result with the details from check 1 for the repairer. If the boiler runs briefly but the house stays cold, the next guide is a boiler that keeps turning off.

What each one takes

Finding	Work involved
Accessible loose debris at an air or vent opening	An owner check within the limits above; service if the boiler still will not run
Ignition fault	A heating technician testing ignition, fuel supply and flame detection
Fan speed fault	Air path and electrical checks, then a part only if testing identifies it
KHB temperature rise or high limit fault	Checking water circulation, piping and controls
Sensor fault	Wiring and resistance tests before replacing the sensor
XL APS / HEX Temp Sw Open	A boiler technician identifying which protection opened and why

Questions people ask

Is BLOCKED a complete error code on a KHB?

No. Lochinvar’s KHB status chart uses BLOCKED to mean a condition interrupted the heat call. Keep the accompanying fault wording as well as that status so the repairer knows what stopped it.

Why does my KHB say STANDBY when the house is cold?

Lochinvar defines STANDBY as no thermostat or domestic hot water heat demand received. Start with the thermostat and its heat request, rather than treating STANDBY as an ignition failure. See boiler not working for those first checks.

What does Low 24 vac mean on a KHB?

Lochinvar’s KHB Rev E chart says the control board’s 120 volt input has dropped below 80 volts. Its technician checks include supply power, connections and the transformer. The label does not tell you to change the thermostat batteries.

What is Memory Error on a KHB?

Lochinvar’s KHB chart associates Memory Error with a lockout occurring at the same time as a power outage. It separately describes Fatal Error as corrupted control settings.

Does a cleared display mean the old fault record is gone?

Lochinvar’s KHB control has a last ten errors log and a separate function to clear it. Ask the technician to review that history when a fault comes and goes; do not erase it while trying to diagnose the problem.

The other boiler guides: boiler not working, boiler pressure, and boiler keeps turning off.

Sources: Lochinvar Knight Fire Tube KHB service manual 100305279_2000558130 Rev E, models 55 to 285, Series 100-101, 110-111 and 120-121, pp. 1, 3, 6, 21, 42 to 48; KHB User's Information Manual 100305278_2000558127 Rev B, Series 100 and 101, pp. 1, 3, 6; Knight service manual KB-SER-08, models 80 to 285 starting serial H07H10040039, pp. 1, 32 to 39; Knight User's Information Manual KB-USER-07, same model range, pp. 1, 3, 6; Knight XL service manual KBXII-SER 100161490_2000013413 Rev M, models 400 to 801, pp. 1, 37 to 45; Knight XL User's Information Manual KBXII-USER 100161491_2000013415 Rev H, pp. 1, 3, 6.