Boiler
E02
The E02 error code on a Viessmann boiler indicates a fault with the boiler’s temperature sensor — specifically, the flow or return temperature sensor is sending a reading that falls outside the expected range, causing the boiler to lock out as a safety precaution. It’s a moderately serious fault: your boiler will stop working until the issue is resolved, but it doesn’t mean your boiler is beyond repair. In this guide, you’ll learn exactly what the E02 code means, what’s most likely causing it, and the safe steps you can take before calling a Gas Safe registered engineer.
What does this error mean?
The E02 error code on Viessmann boilers — including popular models in the Vitodens 050-W, 100-W, and 200-W ranges — signals a problem with one of the boiler’s temperature sensors, most commonly the NTC (Negative Temperature Coefficient) flow temperature sensor or return temperature sensor. These small sensors continuously monitor the temperature of the water circulating through your boiler. When the boiler’s control board receives a reading that is implausible — either far too high, far too low, or a completely absent signal — it triggers the E02 lockout to protect the heat exchanger and other internal components from damage. In plain terms: your boiler’s thermometer is either broken, disconnected, or sending back nonsense data, and the boiler has shut itself down rather than risk overheating or running incorrectly. You’ll typically see the E02 code displayed on the boiler’s digital panel and the unit will have stopped producing heat and hot water.
Est. repair cost
£100–£200 for an NTC sensor replacement (parts + labour); £300–£550 if the PCB is at fault. Diagnostic call-out alone typically costs £60–£100.
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Check the boiler pressure gauge: Look at the pressure gauge on the front of your Viessmann boiler. It should read between 1 and 1.5 bar when the system is cold. If it’s below 1 bar, repressurise the system using the filling loop (usually a silver braided hose under the boiler) until the needle sits around 1.2 bar. Low pressure alone can sometimes cause sensor misreadings and trigger E02.
Step 2 — Perform a manual reset: Once pressure is correct (or if it was already fine), attempt a boiler reset. On most Viessmann Vitodens models, press and hold the reset button (often marked with a flame symbol or labelled ‘Reset’) for 3–5 seconds. Wait 5 minutes to see if the boiler fires up normally. This clears temporary lockouts and is always worth trying first.
Step 3 — Check for any visible error history: If your Viessmann boiler has a digital display (such as on the Vitodens 100-W or 200-W), navigate to the fault history menu if accessible. Multiple E02 entries suggest a persistent sensor fault rather than a one-off blip — make a note of this to share with your engineer.
Step 4 — Bleed your radiators to remove air: If the boiler reset didn’t work, trapped air in the system may be the culprit. Turn off the boiler and, using a radiator bleed key (available from any DIY store for around £1–£2), open the bleed valve on each radiator slightly until water (not air) trickles out, then close it. Start upstairs and work down. Re-check the boiler pressure afterwards and top up if needed.
Step 5 — Check your boiler’s manual for E02 specifics: Viessmann produces several Vitodens model variants and the exact sensor referenced by E02 can vary slightly. Download your model’s manual from the Viessmann UK website (viessmann.co.uk) using your model number (found on the data plate inside the boiler casing) and confirm what E02 refers to for your specific unit.
Step 6 — Do NOT attempt further DIY beyond this point: If the error persists after repressurising, bleeding radiators, and resetting, the fault almost certainly lies with a physical component inside the boiler — the NTC sensor, its wiring, or the PCB. Opening the boiler casing and working on internal gas or electrical components is dangerous and illegal without the correct Gas Safe registration. Stop here and call a qualified Gas Safe registered engineer.
📞 When to call a professional
You should call a Gas Safe registered engineer if the E02 error returns after a reset, if your boiler pressure was already correct and bleeding the radiators made no difference, or if you see the error appearing repeatedly. This is a fault that almost always requires professional diagnosis and parts replacement — do not attempt to open the boiler casing, disconnect wiring, or replace the sensor yourself. When you call, tell the engineer: ‘My Viessmann [model name, e.g. Vitodens 100-W] is showing an E02 error code. I’ve checked the pressure, bled the radiators, and reset the boiler, but the fault keeps returning.’ This saves time and helps them arrive with the right parts. You can find a Gas Safe registered engineer at gassaferegister.co.uk — always check the register before booking, and ask the engineer to show you their Gas Safe ID card on arrival.
⚖️ Repair or replace?
For most Viessmann boiler owners, repair is the right call — provided the boiler is under 10 years old and has been reasonably well maintained. NTC temperature sensors for Viessmann Vitodens models are widely available from UK boiler parts suppliers and typically cost £15–£40 for the part alone. Labour from a Gas Safe engineer to diagnose and replace a sensor usually brings the total to £100–£200. If the fault turns out to be a PCB failure, costs rise significantly — a Viessmann PCB can cost £200–£400 for the part, making total repair bills of £300–£550 realistic. At that point, if your boiler is over 10 years old, replacing it starts to make financial sense. A new Viessmann Vitodens 050-W (the entry-level combi replacement) typically costs £1,500–£2,500 supply and install from a Viessmann-approved installer. Viessmann boilers have a strong reputation for longevity and parts availability in the UK, so a well-maintained unit under a decade old is generally worth repairing. If your boiler is over 12 years old and has had multiple faults, use this repair as a trigger to get replacement quotes — energy efficiency gains from a new A-rated boiler can also reduce your gas bills by £100–£300 per year.