Worcester Bosch E5 Fault Code
The E5 fault means the flow thermistor is returning an out-of-range reading. The boiler cannot safely monitor its own temperature and locks out. It is often a straightforward sensor replacement.
What is the Worcester Bosch E5 fault code?
The Worcester Bosch E5 fault is triggered when the PCB receives a reading from the flow thermistor (the temperature sensor on the primary flow pipe) that falls outside the expected range. This could be a genuine over-temperature reading, a short circuit (reading too high), an open circuit (reading too low or open), or a wiring fault between the sensor and the PCB. Because the boiler relies on the flow thermistor to control modulation, it cannot operate safely without an accurate reading and locks out.
What causes the E5 fault?
The thermistor resistance drifts out of specification as it ages. This is the most common cause of E5 — the sensor itself has failed and needs replacing.
The connector between the thermistor and the PCB can corrode or become loose. A poor connection causes intermittent or incorrect resistance readings.
In hard water areas, scale can build up around the thermistor housing, creating an insulating layer that causes the sensor to read higher temperatures than are actually present.
Rarely, the PCB's thermistor input circuit fails and generates false E5 readings even when the sensor and wiring are intact. Diagnosed by elimination.
If the system is actually overheating (due to limescale, pump failure, or blocked heat exchanger), the thermistor may be reporting correctly. In this case E5 accompanies an actual temperature problem that needs addressing.
What you can safely check yourself
Press and hold the reset button for 3 seconds. If E5 only appears occasionally rather than consistently, it may be an intermittent sensor contact fault. Monitor over the next few heating cycles.
Low pressure can cause the pump to run poorly, leading to localised overheating that triggers a genuine E5. Check the pressure gauge and repressurise to 1.2 bar if below 1 bar.
Does E5 appear immediately on startup, or only after the boiler has been running? An immediate E5 suggests a sensor or wiring fault. A delayed E5 may suggest actual overheating — tell the engineer when you call.
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 thermistor resistance at various temperatures using a multimeter
- Check thermistor wiring harness for corrosion or loose connections
- Replace flow thermistor if resistance is out of specification
- If resistance is correct: check PCB thermistor input circuit
- Inspect for limescale accumulation around the sensor housing
- If overheating is confirmed: investigate pump, heat exchanger, and flow restrictions
Thermistor faults linked to limescale accumulation are more frequent in South London than in low-hardness areas. Thames Water — which supplies Wandsworth, Merton, Lewisham, Lambeth, Southwark, Greenwich, and Bromley — deposits calcium inside the boiler, including around sensor housings. Annual servicing includes thermistor resistance testing, which catches drift before it triggers an E5 lockout.
How much does a E5 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 Worcester Bosch boiler showing E5
Check system pressure and repressurise to 1.2 bar if below 1 bar.
Press and hold the reset button for 3 seconds.
Note whether E5 appears immediately or after the boiler has been running for several minutes.
If E5 returns within the same heating cycle, call a Gas Safe engineer.
Tell the engineer whether the fault is immediate or delayed — it helps narrow the diagnosis.
Flow thermistor replacement is one of the lower-cost boiler repairs — typically £80–£160 all in. It is almost always worth doing on any boiler under 14 years old. If the PCB is the cause, the repair-vs-replace calculation depends on the boiler's overall condition and age.
Common questions about the Worcester Bosch E5 fault
What does E5 mean on a Worcester Bosch boiler?
Is a Worcester Bosch E5 fault expensive to fix?
How long does an E5 repair take in London?
Can limescale cause an E5 fault on a Worcester Bosch boiler?
Could an E5 fault mean my boiler is actually overheating?
Other Worcester Bosch fault codes
Need a Worcester Bosch 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.
