IVT Furnaces Error Codes

A01

Furnace will not start; no burner ignition, lockout indicated on IVT control display

Ignition failure lockout

A02

Burner lights briefly then shuts down; multiple failed ignition attempts before lockout

Flame detection failure

A03

Furnace attempts to start but shuts down during pre-ignition; combustion fan runs briefly then stops

Combustion air fan fault

A04

Furnace shuts down shortly after ignition; display indicates overheat or high limit trip

Primary heat exchanger overheat

A05

Furnace display shows low water pressure; burner does not start (for IVT hydronic furnace/boiler models)

System water pressure low

A06

Furnace display shows high water pressure; safety valve may discharge water (hydronic models)

System water overpressure

A07

Furnace locks out with flue gas temperature error; burner stops during operation

Flue gas overtemperature

A08

Furnace display indicates return temperature sensor fault; unit may run at reduced output or shut down

Return water temperature sensor failure

A09

Furnace display indicates flow temperature sensor fault; burner may not modulate correctly

Flow water temperature sensor failure

A10

Furnace shows outdoor sensor error; weather-compensated control not functioning correctly

Outdoor temperature sensor failure

A11

Furnace indicates circulation pump fault; no flow through system, radiators remain cold

Heating circulation pump failure

A12

Furnace display shows domestic hot water (DHW) sensor error; unstable or no hot water on combi models

DHW temperature sensor failure

A13

Furnace shows gas valve error; no gas flow to burner despite ignition attempts

Gas valve actuation failure

A14

Furnace indicates air pressure switch error; combustion fan runs but burner does not start

Air pressure switch / differential pressure fault

A15

Furnace display shows flue gas sensor error; unit may shut down or run at reduced output

Flue gas sensor failure

A16

Furnace indicates condensate fault; unit shuts down during operation (condensing models)

Condensate drainage failure

A17

Furnace shows communication error between user interface and main control; display may be blank or show dashes

Control communication failure

A18

Furnace indicates thermostat/room sensor error; heating does not respond correctly to room temperature

Room thermostat or room sensor fault

A19

Furnace shows power supply error; random resets or control lockups

Control power supply instability

A20

Furnace indicates safety chain open; unit will not start

Safety interlock circuit open

A21

Furnace shows fan speed error; circulation blower not reaching commanded speed

Circulation fan speed control fault

A22

Furnace indicates frost protection / anti-freeze error; system may not protect against freezing

Frost protection function failure

A23

Furnace shows sensor calibration error; temperatures or pressures appear unrealistic

Sensor calibration / plausibility fault

A24

Furnace indicates service interval exceeded; reminder for maintenance

Scheduled maintenance reminder

A25

Furnace indicates flue gas recirculation or CO risk; unit shuts down

Flue gas recirculation / combustion air contamination

Symptom

Domestic hot water temperature fluctuates or runs hot then cold (IVT combi/hydronic with DHW)

Unstable DHW temperature control

Symptom

Furnace causes a metallic or chemical smell in the home

Contaminated combustion air or heat exchanger issue

Symptom

Furnace emits a burning smell when running

Overheating components or dust burn-off

Symptom

Furnace fan runs but burner does not light and no error code is displayed

No ignition without fault reporting

Symptom

Furnace frequently trips the household circuit breaker

Electrical overcurrent / short circuit

Symptom

Furnace is unusually noisy in flue area; whistling or roaring from vent

Flue gas flow noise / resonance

Symptom

Furnace leaks water around the base during operation

Condensate leakage

Symptom

Furnace makes loud banging or booming noise at startup

Delayed ignition / burner light-off noise

Symptom

Furnace produces a high-pitched whine or squeal during operation

Blower or combustion fan bearing noise

Symptom

Furnace runs but produces no heat; supply air is cold

Burner not firing with fan-only operation

Symptom

Furnace runs continuously and overheats the house

Runaway heating / thermostat control failure

Symptom

Furnace short cycles; starts and stops frequently without reaching set temperature

Short cycling due to control or airflow issues

Symptom

Furnace starts but shuts down after a few minutes; radiators or ducts only lukewarm

Premature shutdown due to overheating or sensor issues

Symptom

Furnace vibrates excessively and causes rattling noises in ducts

Mechanical vibration / imbalance

Symptom

Furnace will not start at all; no lights or display

No power to furnace

Symptom

Radiators on upper floors stay cold while lower floors heat normally (hydronic IVT furnace/boiler)

Air trapped in heating system