Viessmann Boiler Fault Codes

If an issue occurs with your Viessmann boiler a display fault code on the control panel may show.

The fault codes will include letters or numbers or both.

The fault codes are used to help you and your heating engineer identify and solve issues over the phone or in person.

Viessmann fault codes apply to all Viessmann boilers or some will only apply to certain models.

 

If a fault code has appeared on your Viessmann boiler display, see the table below to identify the issue:

 

Fault Code 5C, 5D, 5E and 5F     

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOTRONIC 300-K WITH VITOTRONIC 200	CONTROL MODE – COMMUNICATION FAULT – VITOTRONIC 100 OF BOILER 5-8

 

Fault Code 10          

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 200-W / 222-F / VITOTRONIC 300 / 200 / 200-H / 050/200-H / 300-K / VITOCONTROL-S	BOILER OPERATES BASED ON OUTDOOR TEMPERATURE OF 32°F / 0°C SHORT CIRCUIT – OUTDOOR TEMPERATURE SENSOR
VITOTRONIC 300-K, MW2S WITH VITOTRONIC 100 / VITODENS 100-W (COMBIPLUS) / 200-W / VITOCONTROL-S WITH VITOTRONIC 100	CONTROLS AS IF THE OUTDOOR TEMPERATURE / CONSTANT MODE OUTDOOR TEMPERATURE SENSOR SHORTED OUT

 

Fault Code D4         

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOTRONIC 300 / VITOTRONIC 300-K WITH VITOTRONIC 300	BOILER COOLS DOWN FAULT, SAFET EQUIPMENT AT X2 / X3 / X7 OF PLUG-IN ADAPTOR 2 FOR EXTERNAL SAFETY EQUIPMENT
VITODENS 200	BOILER OPERATES IN EMERGENCY MODE (AFTER APPROX. 30MIN. BUS FAULT) OR BOILER COOLS DOWN INTERNAL BUS FAULT OR OPEN CIRCUIT – INTERNAL BUS CONNECTION

 

Fault Code 18 and D1       

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 100 / 200-W / 222-F / VITOTRONIC 300-K WITH 100 / VITOCONTROL-S	OPERATES ON OUTDOOR TEMPERATURE OF 32°F / 0°C CONSTANT MODE OUTDOOR TEMPERATURE SENSOR CABLE BROKEN
VITOTRONIC / 100 / 200 / 300 / 050/200-H / 300-K / VITOCONTROL-S, MW1 WITH VITOTRONIC 100	OPERATES ON BASIS OF 32°F / 0°C OUTDOOR TEMPERATURE OPEN CIRCUIT – OUTDOOR TEMPERATURE SENSOR
VITOTRONIC 200-H / VITOTRONIC 300-K, MW1B WITH VITOTRONIC 300 / VITOTRONIC 300-K, MW2B WITH VITOTRONIC 200	CONTROLS AS IF THE OUTDOOR TEMPERATURE LEAD BREAK – OUTDOOR TEMPERATURE SENSOR
VITOTRONIC 050/200-H	ACTIVATES AFTER 0°C / 32°F OUTDOOR TEMPERATURE CABLE BREAK – OUTDOOR TEMPERATURE SENSOR
VITODENS 100-W	BURNER IN FAULT MODE DIP SWITCH S2 IS SET TO ‘ON’

 

Fault Code A4         

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 222-F	BURNER BLOCKED – THE VENTING PROGRAM STARTED AUTOMATICALLY AIR LOCK IN THE INTERNAL CIRCULATION PUMP OR MINIMUM FLOW RATE NOT ACHIEVED

 

Fault Code 0C          

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 100-W	BURNER BLOCKED – INSUFFICIENT POWER SUPPLY
VITODENS 200	BURNER CONTROL SWITCHES TO LOCKOUT – FLAME SIGNAL STILL PRESENT AFTER SWITCHING OFF

 

Fault Code C1          

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 200	BURNER CONTROL UNIT SWITCHES TO LOCKOUT (DUE TO LARGE LINE VOLTAGE FLUCTUATIONS) DIFFERENTIAL AIR PRESSURE SENSOR / OPEN CIRCUIT

 

Fault Code B9, ED, F0, F-E5, F-EC, F-ED, F-EE, F-EF, F-FD and F-FF

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOCONTROL-S WITH VITOTRONIC 100 / VITOTRONIC 100 / 300 / VITOTRONIC 300-K WITH VITOTRONIC 300	BOILER COOLS DOWN FAULT: MAXIMUM PRESSURE LIMITER 1 2

 

Fault Code 41

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOTRONIC 300	OPERATION WITHOUT ACCOUNTING FOR RETURN WATER TEMPERATURE SHORT CIRCUIT – RETURN WATER TEMPERATURE SENSOR OF MIXING VALVE CIRCUIT 2

 

Fault Code 88, 89 and E6

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOCONTROL-S WITH VITOTRONIC 100 / VITOTRONIC 300-K WITH VITOTRONIC 100 / 200	CONTROL MODE COMMUNICATION FAULT: VITOTRONIC 100 OF BOILER 1-4

 

Fault Code AD

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 200	SETTING VIA SERVICE LEVEL NOT POSSIBLE DATA POINTS ARE NOT STORED – INTERNAL FAULT VR20

Viessmann Boiler Fault Code 19

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOTRONIC 300	ACTIVATES AFTER 0°C / 32°F OUTDOOR TEMPERATURE COMMUNICATION FAULT: WIRELESS OUTDOOR TEMPERATURE SENSOR

 

Fault Code 21

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOTRONIC 200 / 300	OPERATION WITHOUT INFLUENCE OF RETURN WATER TEMPERATURE SHORT CIRCUIT – RETURN WATER TEMPERATURE SENSOR

 

Fault Code 0b

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 200	BURNER CONTROL SWITCHES TO LOCKOUT – FAN DOES NOT IDLE AFTER AUTO RESET (INCORRECT FAN RPM) SEVERE WIND CONDITIONS IN THE VENT SYSTEM
VITODENS 100-W	BURNER BLOCKED INSUFFICIENT WATER FLOW – INCORRECT FLOW DIRECTION – DEFECTIVE FLOW SWITCH. (BLOCKAGE)

 

Fault Code 20

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOCONTROL-S WITH VITOTRONIC 100 / VITOTRONIC 300 / 300-K / 200 / 100	STAND-ALONE CONTROL UNIT WITHOUT SUPPLY TEMPERATURE SENSOR (SUPPLY TEMPERATURE MIGHT NOT BE HIGH ENOUGH) SHORT CIRCUIT – COMMON SUPPLY TEMPERATURE SENSOR
VITOCONTROL-S WITH VITOTRONIC 100 / VITOTRONIC 300-K WITH VITOTRONIC 200	VITOCONTROL-S CONTROLS IN A STAND-ALONE MANNER WITHOUT SUPPLY TEMPERATURE SENSOR (SUPPLY TEMPERATURE MIGHT NOT BE HIGH ENOUGH) COMMON SUPPLY TEMPERATURE SENSOR SHORTED OUT
VITOCROSSAL 300	REGULATES WITHOUT SUPPLY TEMPERATURE SENSOR (LOW-LOSS HEADER) SHORT CIRCUIT – SYSTEM FLOW TEMPERATURE SENSOR
VITODENS 200-W / 222-F	REGULATES WITHOUT SUPPLY TEMPERATURE SENSOR (LOW-LOSS HEADER) SHORT CIRCUIT – SUPPLY SYSTEM TEMPERATURE SENSOR
VITOTRONIC 020/200-H / 200 / 200-H / 300	MIXING VALVE CLOSES OR CLOSED SHORT CIRCUIT – SUPPLY TEMPERATURE SENSOR / MIXING VALVE CIRCUIT M1
VITORONIC 300-K WITH VITOTRONIC 100	VITOTRONIC 300-K, MW1S CONTROLS IN A STAND-ALONE MANNER WITHOUT STRAP-ON SENSOR (SUPPLY TEMPERATURE MIGHT NOT BE HIGH ENOUGH) SHORT CIRCUIT – STRAP-ON TEMPERATURE SENSOR
VITODENS 200-W	REGULATES WITHOUT SUPPLY TEMPERATURE SENSOR (LOW LOSS HEADER) SYSTEM SUPPLY TEMPERATURE SENSOR SHORTED OUT

 

Fault Code 55, 56 and 57

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOTRONIC 300-K WITH VITOTRONIC 200	CONTROL MODE MAINTENANCE OR FAULT ON VITOTRONIC 100 OF BOILER 5-8

 

Fault Code EE and EF        

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 100-W / 200 / 0 / VITOTRONIC 300	BURNER BLOCKED FAULTY AIR PRESSURE SWITCH – AIR DAMPER (FLAP) STUCK

 

Fault Code FE4        

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOCROSSAL 200	BURNER SHUTS DOWN MULTIPLE DETECTION OF UNDERVOLTAGE

 

Fault Code F-B7, F-F4, F-F2, F-F7, F-F8 and F-F9

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOCROSSAL 200	CASE A: BURNER CONTROL UNIT IN A FAULT STATE; SYSTEM COOLS DOWN; BURNER CONTROL LOCKED OUT. CASE B: TYPE OF CODING CARD CASE A: CODING CARD NOT INSERTED IN THE BURNER CONTROL UNIT; FAULTY OR INCORRECT CODING CARD. CASE B: CODING CARD DOES NOT MATCH BURNER CONTROL UNIT

Fault Code 50, 51, 58 and 59

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 200-W / 222-F	NO DHW HEATING / FAULT WITH DHW SHORT CIRCUIT/OPEN CIRCUIT – DHW TANK TEMPERATURE SENSOR OR LEAD BREAK

 

Fault Code A9, BA, BB, BC & FE0

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 200	BOILER COOLS DOWN SENSOR INPUTS ARE NOT READ CORRECTLY

 

Fault Code F-FA, F-FB and FR

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOCROSSAL 200 / 400-620 / 186-311	FAN RUNS WITHOUT DEMAND; BURNER CONTROL UNIT IN A FAULT STATE FAN NOT STOPPED; CABLE 100A FAULTY; FAN FAULTY; BURNER CONTROL UNIT FAULTY

 

Fault Code 8C, 8D, 8E and 8F

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOTRONIC 300	BURNER CONTROL UNIT IN A FAULT STATE LEAD BREAK, BOILER WATER TEMPERATURE SENSOR, BURNER CONTROL UNIT

 

Fault Code 52

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOTRONIC 300	CONTROL MODE SHORT CIRCUIT – SUPPLY TEMPERATURE SENSOR LOW-LOSS HEADER
VITODENS 100-W COMBIPLUS	BURNER BLOCKED (COMBIPLUS ONLY) FLOW SENSOR SHORTED OUT

 

Fault Code 38

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 200-W / 222-F / COMBIPLUS	BURNER BLOCKED BOILER WATER TEMPERATURE SENSOR CABLE BROKEN
VITOTRONIC 100 / 200 / 300 / VITOCONTROL-S WITH VITOTRONIC 100	BURNER IS SWITCHED ON AND OFF BY ADJUSTABLE HIGH LIMIT OPEN CIRCUIT – BOILER TEMPERATURE SENSOR
VITOTRONIC 300-K WITH VITOTRONIC 300	BURNER IS STARTED AND STOPPED VIA A TEMPERATURE CONTROLLER LEAD BREAK, BOILER WATER TEMPERATURE SENSOR
VITODENS 100-W	BURNER BLOCKED BOILER WATER TEMPERATURE SENSOR LEAD BROKEN

 

Fault Code A7

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOCONTROL-S WITH VITOTRONIC 100 /VITOTRONIC 100 / VITOTRONIC 300-K WITH VITOTRONIC 300	CONTROL MODE ISOLATION VALVE CONFIGURATION FAULT – OC:2, OC:3 OR OC:4 SET AND CODING 4E:1 NOT SET

 

Fault Code 0F

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 222-F	CONTROL MODE CODE 0F IS ONLY DISPLAYED IN THE FAULT HISTORY (SERVICE REQUIRED)
VITOTRONIC 300 / VITOTRONIC 300-K WITH VITOTRONIC 300 / VITODENS 200-W / VITOCONTROL-S WITH VITOTRONIC 100	CONTROL MODE – CONTROLLED OPERATION CODE 0F IS ONLY DISPLAYED IN THE FAULT HISTORY (SERVICE REQUIRED)
VITOTRONIC 200 /300	NORMAL OPERATION SERVICE REQUIRED
VITOCONTROL-S, MW1 WITH VITOTRONIC 100 / VITOTRONIC 300-K, MW1S WITH VITOTRONIC 100	CONTROL MODE MAINTENANCE 0F IS ONLY DISPLAYED IN THE FAULT HISTORY

 

Fault Code E1, E7, E8, E9, EA, EB, EC and FF2

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 200-W / 222-F /VITOTRONIC 300	BURNER IN FAULT MODE HEATING WATER FLOW RATE TOO LOW DURING CALIBRATION; FLOW SWITCH CAUSED SHUTDOWN

 

Fault Code 29, 41, 49, F80, F81, F83, F88 and F89

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOTRONIC 200 / 300	NORMAL OPERATION WITHOUT INFLUENCE OF RETURN WATER TEMPERATURE OPEN CIRCUIT 1 OR 2 – RETURN WATER TEMPERATURE SENSOR (OF MIXING VALVE CIRCUIT 1)

Viessmann Boiler Fault Code A3, B0, F1, E-E1 and F-E2

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOCROSSAL 300 / VITODENS 200-W / 222-F / 300	CONTROL MODE (STAYS IN FACTORY DEFAULT SETTING) FAULTY PROGRAMMING UNIT

 

Fault Code 28

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOCONTROL-S WITH VITOTRONIC 100	VITOCONTROL-S, VD2/CT3/CM2 CONTROLS IN A STAND-ALONE MANNER W/O SUPPLY TEMPERATURE SENSOR (SUPPLY TEMPERATURE MIGHT NOT BE HIGH ENOUGH) OPEN CIRCUIT – COMMON SUPPLY TEMPERATURE SENSOR
VITOCONTROL-S WITH VITOTRONIC 100 / VITOTRONIC 300-K WITH VITOTRONIC 100	VITOCONTROL-S, WB2B CONTROLS IN A STAND-ALONE MANNER W/O SUPPLY TEMPERATURE SENSOR (SUPPLY TEMPERATURE MIGHT NOT BE HIGH ENOUGH) COMMON SUPPLY TEMPERATURE SENSOR CABLE BROKEN
VITOCROSSAL 300 / VITODENS 200-W / 22-F	REGULATES WITHOUT SUPPLY TEMPERATURE SENSOR (LOW-LOSS HEADER) SYSTEM SUPPLY TEMPERATURE SENSOR CABLE BROKEN
VITOTRONIC 050/200-H	MIXING VALVE CLOSES CABLE BREAK – SUPPLY TEMPERATURE SENSOR
VITOTRONIC 050/200-H	MIXING VALVE IS CLOSED OPEN CIRCUIT – SUPPLY TEMPERATURE SENSOR OF MIXING VALVE CIRCUIT M1
VITOTRONIC 200-H / VITOTRONIC 200 / 300	MIXING VALVE IS BEING CLOSED LEAD BREAK, SUPPLY TEMPERATURE SENSOR, HEATING CIRCUIT WITH MIXING VALVE M1 (HEATING CIRCUIT 1)
VITOTRONIC 300 / 300-K WITH VITOTRONIC 300 / 200	STAND-ALONE CONTROL UNIT WITHOUT SUPPLY TEMPERATURE SENSOR (SUPPLY TEMPERATURE MIGHT NOT BE HIGH ENOUGH) LEAD BREAK, COMMON SUPPLY TEMPERATURE SENSOR
VITOTRONIC 300-K WITH VITOTRONIC 100	VITOTRONIC 300-K, MW1S CONTROLS IN A STAND-ALONE MANNER W/O STRAP-ON SENSOR (SUPPLY TEMPERATURE MIGHT NOT BE HIGH ENOUGH) OPEN CIRCUIT – STRAP-ON TEMPERATURE SENSOR

 

Fault Code B1

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOCONTROL-S WITH VITOTRONIC 100 / VITOTRONIC 100 / 200 / 300 / VITOTRONIC 300-K WITH VITOTRONIC 100	EMISSIONS TEST MODE INTERNAL ELECTRONICS FAULT
VITOCONTROL-S WITH VITOTRONIC 100 / VITODENS 200-W	REGULATES AS IF THE OUTSIDE TEMPERATURE IS 32°F (0°C) INTERNAL FAULT
VITOTRONIC 020/200-H	UNDEFINED CONTROL CHARACTERISTICS INTERNAL ELECTRONICS FAULT

 

Fault Code E4

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOCONTROL-S WITH VITOTRONIC 100 / VITOCROSSAL 300 / VITODENS 200-W / 222-F / VITOTRONIC 300	BURNER IN FAULT MODE AT BURNER START, FLAME SIGNAL IS TOO WEAK OR MISSING

 

Fault Code A0, A1, A2, AA and AC

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITODENS 200	PUMP RUNS AT MAX SPEED OPEN CIRCUIT – BUS CONNECTION WITH VARIABLE SPEED HEATING CIRCUIT PUMP

 

Fault Code 30

BOILER MODEL	SYSTEM BEHAVIOUR & DIAGNOSIS
VITOCONTROL-S WITH VITRTRONIC 100 / VITICROSSAL 300/ VITODENS 200-W / 222-F / VITOTRONIC 100 / 200 / 300	BURNER IS STARTED AND STOPPED VIA AN ADJUSTABLE HIGH LIMIT SHORT CIRCUIT – BOILER WATER TEMPERATURE SENSOR
VITOCONTROL-S WITH VITOTRONIC 100 / VITODENS 100-W (COMBIPLUS) / 200-1	BURNER BLOCKED BOILER WATER TEMPERATURE SENSOR SHORTED OUT

 

Viessmann Boiler Fault Codes – All Boilers

FAULT CODE	SYSTEM BEHAVIOUR & DIAGNOSIS
B3 and E5	CONSTANT MODE – INVALID HARDWARE RECOGNISED
FA and FB	CONTROL MODE – COMMUNICATION FAULT
A8	CONTROL MODE – COMMUNICATION FAULT – PROGRAMMING UNIT
90, 91, 92, 93, 94, 98, 99, 9A, 9B, 9C, 9E, 9F and AB	CONTROL MODE – SHORT CIRCUIT – TEMPERATURE SENSOR [5] / [6] / [7] / [10]
C9, CA and CB	BOILER DRIVES UP TO ELECTRONIC MAXIMUM LIMIT BOILER DOES NOT LOG IN AT THE CASCADE CONTROL UNIT
5A, B7 and B8	CONTROL MODE  – LEAD BREAK, LOW-LOSS HEADER SUPPLY TEMPERATURE SENSOR BURNER BLOCKED / CONTROL MODE – SENSOR CABLE BROKEN
CC and F0	BURNER IN FAULT MODE IONISATION CURRENT TOO HIGH DURING CALIBRATION – BOILER TEMPERATURE LIMITER RESPONDED (F2) – AIR OR BLOCKAGE IN SYSTEM/PUMP FAILURE
B6	CONTROL MODE OPEN CIRCUIT – KM BUS TO SOLAR CONTROL UNIT – EXTENSION AM1 / DE1-DE3 M1 – COMMUNICATION FAULT
D5	BURNER BLOCKED FLUE GAS TEMPERATURE SENSOR NOT PROPERLY POSITIONED / CHECK SENSOR EXTERNAL PUMP FITTED ON THE SYSTEM (100-W) A-3 FAULT
60, 68, 70, 78, BD, BE and BF	NO OUTPUT REDUCTION – RETURN CONTROL OPEN – BOILER WITH MAXIMUM TEMPERATURE SHORT/OPEN CIRCUIT – TEMPERATURE SENSOR [17A]
E2 and E3	BURNER BLOCKED – FAULT: SUPPLY VOLTAGE 24V
40 and 44	MIXING VALVE CLOSES HEATING CIRCUIT 2 WITH MIXING VALVE SUPPLY SHORT CIRCUIT ON TEMPERATURE SENSOR
48 and 4C	MIXING VALVE CLOSES OR CLOSED HEATING CIRCUIT 2 WITH MIXING VALVE SUPPLY TEMPERATURE SENSOR CABLE BROKEN / OPEN CIRCUIT
FC	BURNER IN FAULT MODE GAS VALVE FAULTY OR FAULTY MODULATION VALVE CONTROL / OR VENT SYSTEM BLOCKED. CHECK THE FAN, FAN CABLE AND PCB
FE	BURNER BLOCKED OR IN FAULT MODE STRONG INTERFERENCE (EMC) FIELD NEARBY OR FAULTY MAIN PCB
B2	CONTROL MODE – INTERNAL ELECTRONICS FAULT
B5	CONTROL MODE – EXTERNAL SAFETY EQUIPMENT
80, 81, 82, 83 and FD	BURNER CONTROL UNIT IN A FAULT STATE; SYSTEM COOLS DOWN; BURNER CONTROL UNIT LOCKED OUT SHORT CIRCUIT – BOILER WATER TEMPERATURE SENSOR, BURNER CONTROL UNIT
B4	NORMAL OPERATION / FAULT AROUND BURNER CONTROL UNIT INTERNAL FAULT (PCB)
C2-C8, CD-CF, D2-D8, DA-DF and E0	BOILER COOLS DOWN FIXED HIGH LIMIT HAS RESPONDED OR FUSE F2 HAS BLOWN
F5	GAS PRESSURE SWITCH SHOWS NO GAS PRESSURE / ELECTRICAL CONNECTION FAULT GAS SHUT-OFF VALVE CLOSED; GAS PRESSURE SWITCH FAULTY; MULTIPLE GAS PROBLEMS WITH GAS SUPPLY
F6, 58, F9, F-E1, F-E2 and F-F6	BURNER IN FAULT MODE / EMERGENCY OPERATION FAN NOT AT STANDSTILL / PLUG-IN CONNECTION