Worcester Bosch E2 233 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch E2 233 fault code mean?
The E2 fault code signals that your boiler has detected a problem with the central heating (CH) flow NTC temperature sensor — the component responsible for measuring the temperature of water leaving the boiler. Because an inaccurate temperature reading could allow the boiler to overheat, it locks itself out and displays the flashing E2 code alongside the three-digit cause code 233 (visible by pressing the return/spanner button). The boiler will not fire until the fault is cleared and a manual reset is performed. Note: if your display shows 9U 233 rather than E2 233, this points to a different issue — a code plug fault within the Heat Control Module (HCM) — most commonly seen on CDi Compact models. That variant requires the HCM code plug to be reprogrammed or replaced by a qualified engineer; it is covered in the FAQ below.
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
- Failed or worn-out NTC flow sensor Common
The NTC sensor is a small thermistor that changes its electrical resistance as temperature rises or falls. Over time the thermistor element can burn out or drift out of range, producing a reading the PCB cannot trust. This is the single most frequent cause of an E2 233 lockout.
- Loose or disconnected wiring connector Common
The plug-in connector at the sensor terminal can work loose due to vibration, thermal cycling, or during previous servicing work. A disconnected connector produces a signal identical to a failed sensor, triggering the same lockout.
- Damaged or corroded wiring harness Sometimes
The low-voltage signal cables running between the NTC sensor and the PCB can chafe, corrode, or develop moisture ingress at the connector pins. Even partial damage can distort the resistance reading enough to cause a fault.
- Moisture contamination at the sensor Sometimes
A small leak nearby — from a compression fitting, the pump seal, or the heat exchanger — can allow water to track along the sensor body and enter the connector, causing a short circuit or erratic resistance readings.
- PCB misreading the sensor signal Rare
In a small number of cases the sensor and wiring test as perfectly healthy, pointing the fault toward the PCB itself interpreting the signal incorrectly. This is typically only confirmed after the sensor and wiring have been ruled out.
How to fix it
- Check your gas supply is on DIY safe
Confirm that other gas appliances in your home (hob, fire) are working normally. If there is no gas supply, contact your gas supplier before proceeding. A complete loss of gas will cause multiple faults simultaneously and is unrelated to the NTC sensor.
- Attempt a single boiler reset DIY safe
Press and hold the reset button for approximately three seconds until the fault code clears and the boiler attempts to restart. On most Greenstar models the reset button is marked with a flame symbol or labelled 'Reset'. If the E2 233 code returns within a short time, do not reset again — repeated resets will not repair the underlying fault and can mask important diagnostic information.
- Check around the boiler for visible water leaks DIY safe
Look underneath the boiler casing for drips or damp patches near pipework connections. If you find a leak, turn the boiler off and call an engineer. Moisture reaching the sensor connector is a known trigger for this fault and any leak must be fixed before the sensor work is carried out.
- Do not open the boiler casing or handle internal components Gas Safe engineer
The NTC sensor, its wiring, and all internal connections sit inside the boiler casing. Only a Gas Safe registered engineer should open the appliance, test the sensor resistance with a multimeter, inspect the wiring harness, and determine whether the sensor needs reseating or replacing.
- Have a Gas Safe engineer test the NTC sensor and wiring Gas Safe engineer
The engineer will disconnect the sensor connector and measure its resistance at ambient temperature, then cross-reference with the manufacturer's resistance-temperature table to confirm failure. They will also inspect the connector pins for corrosion or moisture damage and check the cable run to the PCB.
- Sensor replacement or connector repair Gas Safe engineer
If the sensor has failed, the engineer will fit a new CH flow NTC sensor (a straightforward part swap on most Greenstar models). If the connector or wiring is at fault, they will repair or replace the relevant section of the harness. Any moisture source identified earlier must also be addressed at this stage.
- PCB investigation if sensor and wiring are both healthy Gas Safe engineer
Should the sensor and wiring test within specification, the engineer will assess whether the PCB is processing the signal correctly. PCB replacement or repair is a more involved and costly job and is confirmed only after other causes have been eliminated.
- Reset and commission the boiler Gas Safe engineer
Once the repair is complete, the engineer will reset the boiler, verify it fires and reaches the correct flow temperature, and confirm the E2 233 code does not return. They should also check system pressure (typically 1–1.5 bar when cold) before leaving.
Parts you may need
- CH flow NTC temperature sensor · from £32
- Sensor wiring harness / connector loom · from £45
- Heat Control Module (HCM) with code plug — CDi Compact · from £120
- PCB (printed circuit board) · from £280
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–£250, depending on the underlying cause.
Frequently asked questions
Can I fix Worcester Bosch E2 233 myself?
No. While you can safely attempt a single reset and check for obvious water leaks around the boiler, the actual fault — a failed or disconnected NTC temperature sensor — sits inside the boiler casing. Accessing and testing internal components requires a Gas Safe registered engineer. Opening the casing yourself is unsafe, can invalidate your warranty, and may be illegal if it involves gas-carrying parts.
What is the difference between E2 233 and 9U 233 on a Worcester Bosch boiler?
They are two distinct faults. E2 233 is a central heating flow NTC sensor fault: the boiler cannot read the water temperature correctly and locks out. 9U 233 is a Heat Control Module (HCM) error related to the code plug — a small device inside the HCM that stores configuration data specific to your boiler model. The 9U 233 code appears most often on CDi Compact models. If your display shows 9U 233, an engineer will need to inspect the HCM, check the code plug is correctly fitted and programmed, and replace the module if necessary. Check your display carefully: the letter before the number matters.
How much does it cost to repair Worcester Bosch E2 233 in the UK?
For a straightforward NTC sensor replacement during standard hours, most homeowners pay £120–£200 with an independent Gas Safe engineer — this covers the call-out, the part (typically £15–£50), and around an hour of labour. If the fault turns out to involve the PCB, costs can rise significantly, often £300–£500 or more including parts and labour, as the PCB alone can cost around £200–£300. If your boiler is over 10–12 years old and repair costs are climbing, it may be worth getting a new boiler quote alongside the repair estimate.
Why does my Worcester Bosch keep showing E2 233 after a reset?
Resetting only clears the lockout — it does not repair whatever caused it. If E2 233 returns quickly after each reset, the NTC sensor has almost certainly failed, or there is a persistent wiring or connection fault that the boiler re-detects every time it tries to fire. Continuing to reset without fixing the cause delays diagnosis and occasionally masks secondary faults. Book an engineer to investigate rather than repeatedly resetting.
Is E2 233 a serious fault — will I lose heating and hot water?
Yes, E2 233 is a locking fault, meaning the boiler shuts down completely until it is repaired and reset. You will lose both central heating and hot water while the fault is active. In most cases the repair is straightforward (a sensor replacement), so you should not be without heat for long once an engineer is booked. In cold weather, especially with vulnerable occupants in the house, treat this as an urgent call-out.