Boiler Error Codes

Ignition Device Fault

The ignition rod has failed or isn't making proper contact.

Action: Check the igniter electrode for damage or carbon buildup. Replace if cracked.

Ignition Failure (No Flame)

No flame was detected after three ignition attempts. One of the most common Navien faults.

Action: Confirm gas supply is on and other gas appliances work. If yes, the gas valve or flame sensor likely needs service.

Overheat Limit Tripped

Heat exchanger exceeded its maximum temperature. Often caused by low water flow.

Action: Check system pressure and pump operation. Clean strainer/filter. If it recurs, the expansion vessel or pump may need replacing.

Outlet Temperature Too High

Water leaving the boiler is above the safe upper limit.

Action: Check for blocked radiators or a failing circulation pump. Do not reset repeatedly.

Exhaust Temperature Too High

Flue gases are hotter than expected, which usually means a heat exchanger blockage.

Action: Shut down and call a technician. Running with a blocked exchanger causes permanent damage.

Cold Water Inlet Sensor Error

The inlet thermistor is reading outside its expected range.

Action: Check wiring connections to the thermistor. The sensor itself may need replacement.

Inlet Thermistor Error

Faulty or disconnected inlet temperature sensor.

Action: Inspect wiring harness and sensor. Replace thermistor if continuity test fails.

Abnormal Combustion

The burner is not firing cleanly — flame signal is unstable or too weak.

Action: Clean the burner and flame sensor. If the issue persists, gas valve or heat exchanger may be at fault.

Gas Pressure Fault

Inlet gas pressure is outside the acceptable operating range.

Action: Check your gas meter and other appliances. If pressure is fine elsewhere, the gas valve may be restricting flow.

Heat Exchanger Outlet Thermistor Error

The outlet temperature sensor is giving an out-of-range reading.

Action: Check thermistor wiring and resistance. Typically a sensor replacement job.

Ignition Lockout

No ignition after multiple attempts. One of the most frequent Viessmann Vitodens faults.

Action: Manually reset the boiler. If it locks out again, call a tech — repeated resets can damage the heat exchanger.

Flame Signal Fault

A flame is detected when there shouldn't be one — usually a faulty flame sensor.

Action: Do not operate. Gas may be leaking past the valve. Call a technician immediately.

Safety Limit Activated

The boiler shut down on its safety thermostat — caused by overheating or a blocked flue.

Action: Check for obvious flue obstructions outside. If clear, a technician needs to inspect the pump and heat exchanger.

Gas Valve Fault

The gas valve isn't responding correctly to the control board.

Action: This is a parts-and-labour repair. Call a licensed tech — don't attempt to bypass the valve.

Flame Loss During Operation

The burner lit but the flame went out unexpectedly mid-cycle.

Action: Check gas pressure stability. Could be a draughty flue, unstable gas supply, or a flame sensor needing cleaning.

Venting / Flue Blockage

Combustion air or exhaust is being restricted.

Action: Inspect the flue terminal outside for ice, debris, or bird activity. Common after heavy snowfall in Metro Vancouver.

Low System Pressure

System water pressure has dropped below 0.8 bar.

Action: Repressurise via the filling loop to 1.0–1.5 bar. If pressure drops repeatedly, have a tech find the leak.

Frost Protection Active

The boiler has fired automatically to prevent freezing. Not a fault — it's doing its job.

Action: No action required. If frost protection is triggering frequently, check insulation on external pipework.

Air Supply or Flue Blockage

Insufficient combustion air or blocked exhaust.

Action: Check the flue terminal for obstructions. Ensure vents aren't blocked by insulation or renovations.

No Ignition

The burner failed to light within the allowed attempts.

Action: Verify gas supply. If good, clean the igniter and flame sensor. Persistent faults need a technician.

Flame Failure During Operation

The flame dropped out unexpectedly during a heating or hot water cycle.

Action: Clean flame sensor rod. Check for gas pressure fluctuations — especially common on older BC gas mains.

Thermal Fuse Failure

The thermal fuse has blown — a one-time overheat protection device.

Action: The fuse must be replaced by a technician. Investigate why it overheated before returning to service.

Water Temperature Over 100°C

The heat exchanger outlet exceeded a safe threshold.

Action: Check flow rate and pump operation. Descale if limescale buildup is suspected.

Cold Water Inlet Sensor Error

The inlet temperature sensor is reading incorrectly.

Action: Inspect the sensor and wiring. Replacement is usually straightforward.

Combustion Fan Fault

The fan isn't running at the correct RPM.

Action: Don't operate the unit — exhaust gases could back-draft into the building. Call a technician to inspect the fan motor.

Water Flow Control Valve Fault

The internal flow valve isn't operating correctly.

Action: This is an internal component failure. A technician will need to test and likely replace the valve.

Solenoid Valve Fault

The gas solenoid valve is not opening or closing as commanded.

Action: Gas fault — do not attempt to bypass. Requires a licensed gas fitter.

Ignition Lockout

Failed to ignite after the maximum number of attempts.

Action: Reset once. If it recurs, check gas supply and call a technician — may be a PCB or gas valve issue.

Flame Loss

Flame detected initially but lost during the heating cycle.

Action: Clean the flame sensor. Check for draughts affecting the flue. Gas pressure instability can also cause this.

Fan Fault

Fan isn't running or not reaching the required speed before ignition.

Action: Check for obstructions in the flue. Fan motor may need replacement.

Low Water Pressure

System pressure has dropped below 0.5 bar.

Action: Repressurise to 1.0–1.5 bar using the filling loop. If pressure drops again, call a plumber to find the leak.

Gas Valve Fault

The PCB is not receiving the expected signal back from the gas valve.

Action: Requires a technician. Could be the valve itself or a wiring/PCB fault.

Ignition Failure

Burner failed to light. Common on IBC SL and DC series.

Action: Check gas supply and igniter. IBC units often need the flame sensor cleaned during annual servicing.

High Limit Fault

The high-temperature limit switch has opened.

Action: Check pump operation and ensure all zone valves are functioning. If pressure is fine, the high-limit may need replacing.

Exhaust Overtemperature

Exhaust gases are too hot — usually a scaled or fouled heat exchanger.

Action: Annual descaling is recommended. Continuing to run risks exchanger damage.

Low Water Cut-Off

The low-water level sensor has triggered. The boiler has shut down to prevent dry-firing.

Action: Repressurise the system. If the problem recurs, check for leaks or a failed expansion tank.

Supply Thermistor Fault

The supply temperature sensor is reading outside its expected range.

Action: Check the thermistor wiring and resistance. Replace if out of spec.

System Pressure Below 1 Bar

Water pressure in the heating circuit has dropped. The boiler won't fire reliably below about 0.8 bar.

Action: Add water via the filling loop until the gauge reads 1.0–1.5 bar. If pressure drops again within days, you have a leak or a failed expansion vessel.

System Pressure Above 3 Bar

Over-pressurisation — usually caused by a waterlogged or failed expansion vessel.

Action: Do not keep resetting. The pressure relief valve may vent to prevent damage. Have the expansion vessel tested and replaced if needed.

Repeated Lockout (Flashing Light)

The boiler keeps going into lockout after manual resets. This is the boiler telling you something is wrong — not just a fluke.

Action: Stop resetting and call a technician. Repeated lockouts can cause carbon buildup and heat exchanger damage.