Viessmann B1 error (primary temperature sensor fault)

The B1 error on a Viessmann boiler indicates a primary temperature sensor fault, usually located at the heat exchanger outlet. This sensor is critical — it provides the reference for combustion regulation and overheating protection. Its failure puts the boiler into safety lockout.

Symptoms

• Boiler stopped, won’t restart
• No heating, no DHW (both can be affected)
• Sometimes falls back to « default values » mode before permanent lockout
• Can appear after a power outage or storm

Affected Viessmann models

• Vitodens 100, 111-W, 200-W, 222-W
• Vitodens 300, 300-W
• Vitopend 100, 200
• Configurations with extended sensors (Vitotronic 200)

Common causes

• Failed primary sensor — NTC ageing (50%)
• Oxidised connector — humid environment or nearby leak
• Cable pinched or severed — after an intervention or property move
• Post-storm surge — electronic component damaged
• Control board sensor-side fault — deeper diagnostic needed

B1 vs F3 vs F5 — what’s the difference?

B1, F3, F5 may look similar but concern different sensors:
B1 — Primary temperature sensor (on the boiler side, near burner)
F3 — Safety thermostat sensor (overheating limiter)
F5 — Flow temperature sensor (on the heating-circuit side)

Professional diagnosis identifies the exact sensor so the right part is replaced.

Checks and repair

1. Does the fault appear cold or only after some runtime?
2. Did it appear after a recent intervention (service, descaling, replacement)?
3. Check visible wiring condition in the boiler compartment
4. Multi-meter test the sensor if accessible — typical 10 kΩ at 20°C

Intervention costs

• Diagnosis + primary sensor test: on request
• Primary sensor replacement: on request
• Full wiring re-run: on request
• Deep diagnosis if board suspected: on request

Frequently asked questions

Can I still use the boiler with B1?

No — it’s an active safety preventing restart. The primary sensor is critical for overheating protection.

How long does a Viessmann primary sensor last?

10–15 years typically. Boilers installed before 2010 are often approaching end-of-life for this sensor.

Is a boiler surge protector worth it?

Yes, strongly recommended in Luxembourg, especially in storm-prone zones. A boiler surge protector costs on request and extends electronic component life by several years.

Need an English-speaking Viessmann specialist in Luxembourg?

Weber & Fils is certified on all Viessmann boilers and heat pumps installed in Luxembourg. Their technicians speak English fluently and stock the most common parts.

For quotes and emergency call-outs: weberetfils.lu/en or call +352 20 60 22 22 — 24/7.

↑ Back to the complete boiler error code guide