Viessmann E3 Fault Code
The E3 fault means the flow NTC temperature sensor is reading outside its expected range. In most cases this is a sensor or wiring fault rather than a genuine overheating problem — but it always requires a Gas Safe engineer to confirm.
What is the Viessmann E3 fault code?
The Viessmann E3 fault is triggered when the PCB receives a temperature reading from the flow NTC (negative temperature coefficient) sensor that falls outside the expected operating range. The PCB relies on the flow NTC to monitor boiler output temperature and modulate burner power — if the reading is implausible, the boiler locks out as a precaution. The fault can be caused by a failed sensor, wiring fault, or genuine high temperature condition.
What causes the E3 fault?
The thermistor resistance drifts outside specification as it ages. The most common cause of E3 on Vitodens boilers over 8 years old.
The connector between the NTC sensor and the PCB can corrode or become loose, causing intermittent or incorrect readings without the sensor itself failing.
In South London's hard water, calcium deposits build up around the sensor housing, creating an insulating layer that causes artificially high temperature readings.
Rarely, the PCB's NTC input circuit fails and generates false E3 readings even with a functional sensor. Diagnosed by elimination.
If the flow temperature is genuinely high — due to a pump failure, blocked system, or limescale on the heat exchanger — the NTC may be reporting correctly. E3 in this case accompanies a real circulation problem.
What you can safely check yourself
If E3 appears occasionally rather than consistently, it may be an intermittent connector fault. Note whether it appears immediately on startup or after running — tell the engineer.
Low pressure can cause poor circulation and localised high temperatures. Repressurise to 1.2 bar if below 1 bar.
An immediate E3 on startup points to sensor or wiring fault. A delayed E3 after running may indicate actual overtemperature. This distinction helps the engineer narrow the diagnosis.
Do not reset the boiler more than once if the fault code returns immediately. Repeated resets without diagnosis can mask a worsening fault.
What needs a Gas Safe engineer
- Measure NTC sensor resistance at cold and operating temperature
- Check NTC wiring harness and connectors for corrosion or looseness
- Replace flow NTC sensor if resistance is out of specification
- Check PCB NTC input circuit if sensor and wiring test correctly
- Inspect for limescale accumulation around the sensor housing
- Check pump and circulation if overtemperature is suspected as the root cause
NTC faults linked to limescale around the sensor housing are more frequent in South London. Thames Water — supplying Wandsworth, Merton, Lewisham, Lambeth, Southwark, Greenwich, and Bromley — deposits calcium that can insulate the sensor over time. Annual servicing includes NTC resistance testing and sensor housing inspection to catch this early.
How much does a E3 repair cost?
Costs below are UK averages. Reion provides a fixed quote after the £80 diagnostic visit — no obligation to proceed.
How to reset a Viessmann boiler showing E3
Check system pressure and repressurise to 1.2 bar if below 1 bar.
Hold the reset button for 3 seconds.
Note whether E3 appears immediately or after the boiler has been running.
If E3 returns within the same heating cycle, call a Gas Safe engineer.
Flow NTC sensor replacement is one of the lower-cost Viessmann repairs — typically £90–£180 all in. Almost always worth doing on any Vitodens under 14 years old.
Common questions about the Viessmann E3 fault
What does E3 mean on a Viessmann boiler?
Is a Viessmann E3 fault expensive to fix?
How long does a Viessmann E3 repair take in London?
Can limescale cause a Viessmann E3 fault?
Could Viessmann E3 mean my boiler is actually overheating?
Other Viessmann fault codes
Need a Viessmann engineer in London?
Reion is Gas Safe registered (919881), based in South London, and available 24/7. Fixed price after the diagnostic visit — no hidden charges.
