Error Code	Symptom	Fault Type	Guide
A0	Indoor unit does not start; display shows A0 shortly after power-on	Indoor–Outdoor Communication Fault	View Details
A1	Indoor unit runs briefly then stops; A1 displayed on indoor unit	Indoor Room Temperature Sensor (NTC) Fault	View Details
A2	Indoor unit stops cooling; A2 error on display	Indoor Coil (Evaporator) Temperature Sensor Fault	View Details
A3	Indoor unit stops and A3 error appears; sometimes accompanied by water dripping from front panel	Condensate Drain Pump Fault / Float Switch Activation	View Details
A4	Indoor unit stops; A4 displayed; unit may short-cycle in heating mode (for heat pump models)	Indoor Heat Exchanger Overheat / Protection	View Details
A5	Indoor unit stops in heating mode; A5 error shown; outdoor unit may still run briefly	Indoor Heat Exchanger Freeze / Frost Protection	View Details
A6	Indoor unit attempts to start but fan does not run; A6 error displayed	Indoor Fan Motor Fault	View Details
C0	System does not respond to remote; C0 appears on display or service menu	Remote Control / Signal Reception Fault	View Details
C4	Unit runs but performance is unstable; C4 appears in diagnostics	Outdoor Coil Temperature Sensor Fault	View Details
C5	Outdoor unit stops unexpectedly; C5 error logged; may occur during defrost cycles	Outdoor Discharge Pipe Temperature Sensor Fault	View Details
C9	Outdoor unit fails to start or stops; C9 error in diagnostics	Outdoor Ambient Temperature Sensor Fault	View Details
E0	System does not start; E0 displayed on indoor unit	EEPROM / Configuration Data Error	View Details
E1	Indoor unit stops; E1 error displayed; may occur shortly after compressor start	High-Pressure Protection / Refrigerant Circuit Overpressure	View Details
E2	Unit stops cooling; E2 error; may be accompanied by frost on indoor coil	Low-Pressure Protection / Evaporator Freeze	View Details
E3	Outdoor unit does not start; E3 error displayed; compressor remains off	Outdoor Unit Overcurrent / Compressor Overload	View Details
E4	System stops; E4 error; may occur during heating mode on heat pump models	Discharge Temperature High Protection	View Details
E5	Outdoor unit shuts down; E5 error; may be intermittent	Inverter Module Overcurrent / IPM Protection	View Details
E6	Indoor unit shows E6; outdoor unit may not respond; no cooling or heating	Indoor–Outdoor Communication Error	View Details
E7	Indoor unit displays E7; louver may not move correctly	Indoor Louver / Stepper Motor Fault	View Details
E8	Indoor unit stops; E8 error; may be associated with very low indoor airflow	Indoor Fan Speed Feedback Error	View Details
F0	System stops; F0 error; may occur during cooling startup	Refrigerant Undercharge / Low Refrigerant Protection	View Details
F1	Indoor unit shows F1; unit may run with poor temperature control	Indoor Room Temperature Sensor Out of Range	View Details
F2	Indoor unit shows F2; coil temperature reading appears abnormal in diagnostics	Indoor Coil Sensor Out of Range	View Details
F3	Outdoor unit shows F3; poor heating performance on heat pump models	Outdoor Coil Sensor Out of Range	View Details
H0	System locked out; H0 or similar lock code displayed; cannot change settings	Child Lock / Keypad Lock Activated	View Details
H1	Heating mode delayed; H1 displayed; outdoor unit not starting immediately	Defrost / Anti-Cold Air Protection (Informational)	View Details
P0	System stops; P0 error; may occur under heavy load	DC Bus Voltage Protection	View Details
P1	System stops; P1 error; may be intermittent with power fluctuations	Low Voltage Protection	View Details
P2	System stops; P2 error; often during high ambient temperatures	High Temperature Protection of Compressor or Inverter	View Details
P4	System stops; P4 error; may be associated with abnormal compressor behavior	Inverter Module Temperature Sensor Fault	View Details
–	Burning or electrical smell from indoor or outdoor unit	Overheating Electrical Component	View Details
–	Indoor unit blows warm air in cooling mode even though compressor is running	Refrigerant Flow Misrouting / 4-Way Valve Stuck (Heat Pump)	View Details
–	Indoor unit displays temperature but does not respond to remote commands	Remote Signal Reception Failure	View Details
–	Indoor unit fan runs but outdoor unit does not start; no error code shown	Outdoor Unit Not Responding	View Details
–	Indoor unit leaking water from front panel or dripping onto wall	Condensate Drain Blockage	View Details
–	Indoor unit louver does not move or is stuck in one position	Louver Mechanism Jam	View Details
–	Indoor unit makes loud rattling or buzzing noise during operation	Indoor Mechanical Vibration / Loose Components	View Details
–	Indoor unit runs but no cooling; air temperature hardly changes	Loss of Cooling Capacity	View Details
–	Outdoor unit runs but indoor fan does not start; coil gets cold and may ice up	Indoor Fan Not Running	View Details
–	Outdoor unit very noisy (grinding or scraping) when running	Outdoor Fan or Compressor Mechanical Noise	View Details
–	Unit frequently trips the circuit breaker when starting	Excessive Inrush Current / Electrical Overload	View Details
–	Unit starts then stops after a few minutes; short cycling without error code	Short Cycling / Incorrect Capacity Control	View Details
–	Unit will not start at all; no lights on indoor display	No Power / Power Supply Failure	View Details
–	Unpleasant musty or moldy smell when the air conditioner starts	Biological Growth on Evaporator and Drain Pan	View Details

Viessmann Air Conditioners Error Codes

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