Worcester Bosch E2 350 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch E2 350 fault code mean?
The E2 code on a Worcester Bosch boiler points to a locking fault with the flow temperature NTC (Negative Temperature Coefficient) sensor — the component that measures the temperature of water leaving the boiler. The sub-code 350 tells you the sensor has been detected as defective, most commonly due to a short circuit or complete failure. When E2 350 appears, the boiler locks out entirely and will not fire until the underlying fault is resolved and a manual reset is performed. You'll usually see a flashing fault indicator alongside a triangle symbol on the display. Because the boiler relies on this sensor to prevent overheating, it shuts down as a safety measure rather than risk running blind on temperature.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Flow NTC sensor short circuit Common
The most frequently seen trigger for E2 350. The sensor's internal resistance drops unexpectedly — often due to age, moisture ingress, or physical damage — causing the boiler's PCB to read an impossible temperature and lock out immediately.
- Loose or damaged sensor wiring Common
The wiring loom connecting the flow NTC to the PCB can develop loose terminals, chafing, or corrosion over time. A poor connection can mimic a failed sensor, producing the same E2 350 lockout even when the sensor itself is still serviceable.
- Complete NTC sensor failure Sometimes
Like any electronic component, the flow NTC sensor can fail outright, particularly in older boilers. Once it stops producing a valid resistance reading, the boiler has no way to monitor flow temperature and will lock out to protect itself.
- Sensor incorrectly positioned or poorly seated Sometimes
If the sensor has been disturbed during previous servicing, or was not seated correctly against the pipe, it may read erratic temperatures. This can produce intermittent E2 350 faults that seem to clear on reset before returning.
- PCB fault affecting sensor input Rare
In a minority of cases, the fault lies not with the sensor itself but with the PCB circuit that processes the sensor signal. If the sensor and its wiring both test correctly, the PCB may be misreading the input — a less common but more costly diagnosis.
How to fix it
- Check your gas supply is live DIY safe
Before assuming a sensor fault, confirm other gas appliances (hob, fire) are working normally. A disrupted gas supply can sometimes trigger unrelated lockouts. If there is no gas to the property, contact your gas supplier.
- Reset the boiler once DIY safe
Press and hold the reset button for approximately three seconds until the boiler attempts to restart. Allow a full restart cycle before deciding whether the fault has cleared. If E2 350 returns after one or two resets, do not keep resetting — repeated resets without fixing the root cause can mask the problem and cause unnecessary wear.
- Note down the exact code displayed DIY safe
Write down 'E2 350' as shown on the display, along with the time and how frequently it appears. This information helps a Gas Safe engineer diagnose the fault more quickly on arrival, potentially reducing labour time and cost.
- A Gas Safe engineer should inspect the flow NTC sensor and its wiring Gas Safe engineer
The engineer will use a multimeter to measure the sensor's resistance at a known temperature and compare it against Worcester Bosch's specified resistance curve. They will also check the wiring terminals for corrosion, looseness, or damage and reseat the sensor if necessary.
- A Gas Safe engineer should replace the flow NTC sensor if faulty Gas Safe engineer
If the sensor fails its resistance test, the engineer will fit a replacement flow NTC sensor. This involves draining down or isolating the relevant part of the system, removing the old sensor, fitting the new one, and verifying correct operation before leaving. The boiler casing must be opened to access the sensor — do not attempt this yourself, as it can invalidate your warranty and is unsafe without proper training.
- A Gas Safe engineer should investigate the PCB if sensor and wiring are both sound Gas Safe engineer
When the sensor and all associated wiring check out correctly, the fault signal is likely originating from the PCB's sensor input circuit. PCB diagnosis and replacement must be carried out by a qualified engineer, as it involves working on the boiler's internal electronics.
- Call a Gas Safe registered engineer if the fault persists or keeps returning Gas Safe engineer
If E2 350 reappears after reset, or you have not already arranged a visit, contact a Gas Safe registered engineer. You can verify any engineer's credentials at gassaferegister.co.uk. Do not continue using the boiler while it is in lockout.
Parts you may need
- Flow temperature NTC sensor (Worcester Bosch compatible) · from £25
- NTC sensor wiring harness / loom · from £35
- PCB (printed circuit board) — if sensor circuit damaged · from £220
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£320, depending on the underlying cause.
Frequently asked questions
Can I fix a Worcester Bosch E2 350 fault myself?
The safe DIY actions are limited to checking your gas supply is on, attempting a single reset, and noting the fault code for the engineer. The sensor itself sits inside the boiler casing, and accessing, testing, or replacing it requires a Gas Safe registered engineer. Opening the boiler casing without proper qualifications is unsafe and can void your warranty.
How much does it cost to fix an E2 350 fault in the UK?
For most households the repair falls between £120 and £320, covering the engineer's callout, labour, and the cost of a replacement flow NTC sensor if needed. If the fault has also damaged the PCB, costs can rise considerably — PCB replacements alone often add £200–£350 in parts on top of labour. If your boiler is over 10–12 years old and has been faulting regularly, it is worth asking the engineer whether a replacement boiler would be more cost-effective in the long run.
Why does my Worcester Bosch keep showing E2 350 after I reset it?
If the fault clears on reset but returns within hours or days, it suggests the underlying component — most likely the flow NTC sensor or its wiring — is intermittently failing rather than completely dead. Intermittent faults like this often worsen over time. Repeated resetting without a proper repair is not a long-term fix and may mask a developing problem. Book a Gas Safe engineer to test and replace the sensor before it causes a full breakdown at an inconvenient time.
What is the difference between the E2 code and the 350 sub-code on Worcester Bosch boilers?
Worcester Bosch uses a two-part display system: the letter-number combination (E2 in this case) identifies the general fault category — here, a locking fault related to the flow temperature NTC sensor. The three-digit sub-code (350) gives the specific cause within that category, confirming that the sensor has been detected as defective. You may see other E2 sub-codes on different faults, so always note both parts of the code and pass them on to your engineer.