Boiler fault codes

Viessmann boiler faults: fault codes, causes and parts

All 285 fault codes for Viessmann boilers, plus the faults that show no code. Type your code or choose your symptom.

In brief

A fault on a Viessmann boiler usually means that the appliance has locked out or is reporting a fault code. Note the code on the display and look it up below: for each code you will find its meaning, checks you can safely do yourself and the parts that are often checked or replaced. Frequently searched: A9 (internal circulation pump blocked), F4 (no flame signal), 0C (mains voltage too low). No code? Check the water pressure (1.5 to 2 bar when cold), the power supply and the gas tap, and reset once. If the fault comes back, call in a certified gas engineer. If you smell gas, call your national gas emergency number or 112 immediately.

Most searched Viessmann faults

These are the codes that visitors to this site search for most often.

A9Internal circulation pump blocked F4No flame signal.0CMains voltage too low 18Open circuit of outdoor temperature sensor D3Communication error with fan unit F2Temperature limiter has been activated 89Internal circulation pump blocked F3Flame signal already present at burner start.

See the 25 most searched fault codes of 2026, with the figures behind them →

Viessmann boiler faults without a code on the display

Not every fault shows a code. These are the symptoms we come across most often with Viessmann appliances, with what you can check yourself before calling someone out.

No hot water, but the heating works

The boiler heats the radiators but not the tap water. Check that the hot water function is switched on in the display (on some appliances it is called DHW mode or summer mode) and that the hot water valve under the boiler is open. If the water stays cold, this usually points to a fouled or leaking DHW heat exchanger, a faulty diverter valve or a DHW sensor giving incorrect readings. View the matching parts → Read the full explanation: no hot water, six causes →

Water pressure keeps dropping

Top up to 1.5–2 bar when cold and bleed the radiators. If the pressure drops again within a few days, the expansion vessel has often lost its pre-charge or there is a small leak in the system. A pressure relief valve that keeps dripping gives the same symptom. View the matching parts → Read the full explanation: water pressure keeps dropping – expansion vessel or leak? →

Boiler shuts down and locks out

The appliance shuts down and only starts again after a reset. Reset once; if the fault comes straight back, leave the reset button alone. Repeatedly resetting a locked-out boiler is not without risk: the safety device trips for a reason. View the matching parts → Read also: is the boiler cutting out because of a sensor or overheating? →

Radiators stay cold

Check that the thermostat is calling for heat, that the radiator valves are open and that the system has enough pressure. If everything stays cold while the boiler is firing, the circulation pump is often not running or the diverter valve is stuck in the hot water position. View the matching parts → Read the full explanation: radiators stay cold →

Ticking, humming or boiling noise

A banging noise at start-up usually points to air or limescale in the heat exchanger. A humming pump may have a worn bearing. A rattling fan is often rubbing or dirty. View the matching parts → Read also: replacing the fan – what to rule out first →

Water under the boiler

Check where it is coming from: the condensate drain (siphon blocked or cracked), the pressure relief valve (pressure too high or faulty expansion vessel) or a fitting. If in doubt, switch the boiler off and close the water supply. View the matching parts → Read the full explanation: water under the boiler – condensate or a leak? →

Viessmann boiler fault codes 1 to 150 of 285

The code list is split over 2 pages. Looking for a specific code? Use the search at the top of this page: it searches all 285 codes in one go.

Code	Cause
0A	CO monitor (if present) has been activated. CO concentration too high.
0A	Gas pressure monitor (if present) has been activated. Gas pressure too low.
0C	Mains voltage too low
10	Short circuit of outdoor temperature sensor
100	PlusBus voltage error
102	Communication module error
103	Internal communication error of control unit
104	External fault message active
13	Open circuit of outdoor temperature sensor
14	Short circuit of outdoor temperature sensor
142	Communication limitation on CAN bus. INR
160	CAN bus communication error
161	Data storage access error of central electronics module HBMU
163	Checksum error accessing data memory of central electronics module HBMU
18	Open circuit of outdoor temperature sensor
182	Short circuit of outlet temperature sensor (if present)
183	Open circuit of outlet temperature sensor (if present)
184	Short circuit of flow temperature sensor/safety temperature limiter
185	Open circuit of flow temperature sensor/safety temperature limiter
19	Configuration error
20	Short circuit of system flow temperature sensor
21	Sensor outside tolerance range
22	Sensor outside tolerance range
23	Incorrect commissioning
24	DHW circulation pump out of operation
25	Cylinder charging pump out of operation
26	DHW circulation pump and cylinder charging pump out of operation
27	Incorrect commissioning
28	Open circuit of system flow temperature sensor
2A	Solar circuit flow and return swapped.
30	Short circuit of boiler temperature sensor
345	Temperature monitor has been activated.
346	Ionisation current calibration error
348	Gas modulation valve
349	Air mass flow in fan unit is not correctly recognised.
350, 351	Ionisation current not in the valid range
352	CO limit value of combustion exceeded
353	Insufficient gas supply, burner output reduced
354	Gas modulation valve tolerance not in the valid range
355	Analogue signal reference check: flame signal already present at burner start.
357	Insufficient gas supply
359	No ignition spark present
361	Flame signal is not present or too low at burner start.
365	Feedback of gas valve relay contact not plausible (relay contact is welded)
366, 367	Power supply of the gas valve does not switch off.
369	Flame loss immediately after flame formation (during the safety time)
370	Gas valve or modulation valve does not close.
372	Repeated flame loss during calibration
373	Heat take-off too low during calibration Temperature monitor has been switched off.
377	Ionisation current calibration follow-up: stabilisation conditions for recalibration not reached
378	Flame loss in the stabilisation or operating phase
379	Flame signal not present or too low
38	Open circuit of boiler temperature sensor
380	Flame loss immediately after flame formation (during the safety time)
381	Flame loss in the operating phase
382	Error counter has exceeded limit value.
383, 384	Possible dirt in the gas line
385	Short circuit signal 1 ionisation current Central electronics module HBMU faulty
386	Central electronics module HBMU defective
387	Earth short of ionisation current. Central electronics module HBMU defective.
388	Central electronics module HBMU defective
395	Earth short of IO electrode, central electronics module HBMU defective
396	Central electronics module HBMU defective
399	Earth short of IO electrode, central electronics module HBMU defective
40	Short circuit of flow temperature sensor of heating circuit 2 (with mixing valve)
400	Central electronics module HBMU defective
401	Earth short of IO electrode, central electronics module HBMU defective
402	Central electronics module HBMU defective
403	Earth short of ionisation current, central electronics module HBMU defective
404	Central electronics module HBMU defective
405	Earth short of ionisation current, central electronics module HBMU defective
406, 408, 410	Central electronics module HBMU defective
416	Flue gas temperature sensor not positioned correctly
417, 418	Central electronics module HBMU defective
425	Time synchronisation failed
44	Short circuit of flow temperature sensor of heating circuit 3 (with mixing valve)
446	Deviation of flow temperature sensor/safety temperature limiter of heat generator
447, 448	Deviation of ionisation voltage/ionisation current signal
449, 450, 451, 452	Error in temporary monitoring of program sequence
453	Synchronisation error sequence
454	Wrong parameter set of central electronics module HMU
455, 456	Error in monitoring of program sequence
457	Fan runs stiffly or is blocked.
463	Dirty combustion air, flue gas recirculation
464	Ionisation current too low during calibration. Difference from previous value not plausible.
467	Gas supply insufficient during calibration. Contaminated or undersized gas line.
468	Ionisation current too high during calibration
471	System pressure sensor not available, open circuit or short circuit
474	Error in temporary monitoring of program sequence
48	Open circuit of flow temperature sensor of heating circuit 2 (with mixing valve)
49	Assignment of the mixing valve extension for heating circuit 2 set incorrectly
49	Open circuit of flue gas temperature sensor
4C	Open circuit of flow temperature sensor of heating circuit 3 (with mixing valve)
4D	Assignment of the mixing valve extension for heating circuit 3 set incorrectly
50	Short circuit of cylinder temperature sensor or comfort sensor
50	Short circuit of flue gas temperature sensor
50	Short circuit of cylinder temperature sensor
51	Short circuit of outlet temperature sensor
51	Short circuit of outlet temperature sensor
517	Open circuit of PlusBus cable, wrong appliance address set, remote control defective
52	Short circuit of flow rate sensor
527, 528	Wrong parameter set central electronics module HBMU
540	Condensate accumulation in the heat cell
544	Open circuit in flow temperature sensor of heating circuit 2 with mixing valve Setting incorrect at commissioning
545	Short circuit of flow temperature sensor of heating circuit 2 with mixing valve
57	Open circuit of room temperature sensor
58	Short circuit of room temperature sensor
58	Open circuit of cylinder temperature sensor or comfort sensor
58	Open circuit of cylinder temperature sensor
59	Open circuit of outlet temperature sensor
59	Open circuit of outlet temperature sensor
59	Power supply undervoltage
5A	Open circuit of flow rate sensor
62	Safety temperature limiter has been activated
63	Flue gas temperature limiter has been activated
67	Ionisation current not in the valid range
68	Flame signal already present at burner start
69	Ionisation current not in the valid range
7	Open circuit of cylinder temperature sensor
70	Gas pressure monitor prevents burner start.
70	Internal fault of central electronics module HBMU
71	Mains voltage too low
71	Fan speed too low
73	Feedback input of internal extension H1 prevents burner start
73	Internal communication error
73	Feedback input of internal extension H1 prevents burner start.
738	OpenTherm remote control connected but not configured
74	Earth short of temperature limiter
74	System pressure too low
75	Earth short of internal extension H1/H2
76	Earth short of gas pressure monitor
77	Data memory of central electronics module HBMU
8	Short circuit of cylinder temperature sensor
80	Insufficient gas supply
81	No ignition spark present
82	Flame signal is not present or too low at burner start
82	Flame signal is not present or too low at burner start.
83	Flame signal already present at burner start.
83	Flame signal already present at burner start
84	Appliance-internal CO limit value exceeded
85	Gas supply insufficient during calibration
89	Internal circulation pump blocked
9	Gas shortage at gas pressure switch VPS or safety circuit not closed. Read out the error in the fault memory; burner starts automatically according to time programme. Burner starts automatically after 2 min, after 5 min and then after 1 h.
90	Short circuit of temperature sensor /
91	Short circuit of temperature sensor aÖ
91	Communication error of electronics module DIO
92	Short circuit of collector temperature sensor
92	Communication error of electronics module ADIO
93	Short circuit of collector return temperature sensor
94	Short circuit of cylinder temperature sensor

Order parts for your Viessmann boiler

Viessmann parts are available from Instalpunt; shipping to EU countries from €12.95 (€16.95 for more distant countries), usually 3–5 working days.

View Viessmann parts →

Frequently asked questions about Viessmann faults

What does a fault on a Viessmann boiler mean?

The appliance is reporting that it has shut itself down or cannot continue. The code on the display points to the group of causes: ignition, water pressure, a sensor, the fan or communication with the thermostat. The table above shows, for each code, what Viessmann means by it.

How do I reset my Viessmann boiler?

The reset button is on the control panel of your Viessmann boiler; on most appliances you hold it down for two to five seconds until the display responds. Reset once. If the same fault comes straight back, resetting will not solve it and a part or a setting needs attention. Always consult the manual for your model: the position of the button and how long to press it differ per series.

Can I fix a Viessmann fault myself?

You can top up the water pressure, bleed the system, replace filters and clean a siphon yourself. Anything behind the sealed section of a gas appliance is a different matter: work on the gas valve, burner, heat exchanger and flue must be carried out by a certified gas engineer.

How much does it cost to fix a fault?

That depends on the cause and whether a part is needed. With us you can see the part straight away with its price and delivery time, so you know where you stand in advance. If you call in an engineer, always ask about the call-out charge and hourly rate before they come.

Where can I find the right part for my Viessmann?

Check the rating plate for the exact type and use it to search our catalogue, or search directly by the manufacturer number of the faulty part. Each part lists the appliances it fits.