Worcester Bosch E5 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch E5 fault code mean?
The E5 fault code on a Worcester Bosch boiler points to a problem with the flow temperature sensor — the component that monitors how hot the water is as it circulates through your heating system. When this sensor detects that temperatures are drifting outside safe operating limits, or when it sends readings the PCB can't make sense of, the boiler locks out to protect itself from overheating damage. The most common sub-code you'll see alongside E5 is E5 218, which specifically indicates the flow temperature has climbed too high — often because of poor water circulation, a struggling pump, or air trapped in the system. You may also encounter E5 332, which typically points more directly to a sensor signal fault or wiring issue rather than a genuine over-temperature condition. In either case, the boiler won't restart until the underlying fault has been resolved.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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
- Poor circulation or pump failure Common
If the central heating pump isn't moving water efficiently, heat can build up rapidly in the heat exchanger and trigger the E5 218 sub-code. A pump that's seized, running slowly, or set to the wrong speed is one of the most common reasons this code appears, especially on older boilers or those that haven't been serviced recently.
- Faulty or degraded NTC flow temperature sensor Common
The flow sensor is an NTC thermistor — its electrical resistance changes with temperature, and the PCB uses those readings to regulate the boiler. Over time the sensor can degrade, develop internal cracks, or simply fail, causing it to send inaccurate or erratic signals that the boiler interprets as a fault condition.
- Damaged wiring or loose sensor connections Common
The sensor relies on a wiring harness to communicate with the PCB. Corrosion, moisture ingress, chafed insulation, or a connector that has worked loose can all produce false readings — the same symptoms as a physically failed sensor. This is often discovered only once a Gas Safe engineer tests the circuit with a multimeter.
- Air locks or low system pressure reducing water flow Sometimes
Trapped air in the system or pressure that has dropped below around 1 bar can restrict water movement enough to cause localised overheating near the heat exchanger. This is one of the few contributing factors a homeowner can partly address themselves by bleeding radiators and checking the pressure gauge.
- Limescale or sludge causing flow restriction Sometimes
In hard-water areas, limescale can coat the inside of the heat exchanger and restrict water flow, causing the boiler to run hotter than it should. Sludge — a mix of corroded metal particles and debris — can have a similar effect. Both issues point to the value of regular servicing and, where appropriate, a system flush.
- Faulty PCB or incorrect code plug Rare
If the PCB itself is misreading signals, or if the code plug (the small chip that tells the PCB what boiler model it's controlling) is incorrect or damaged, the boiler may log E5 faults even when the sensor is perfectly healthy. This is a less common cause but worth investigating if replacing the sensor doesn't clear the fault.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of your boiler — it should sit between 1 and 1.5 bar when the system is cold. If it's below 1 bar, top it up via the filling loop until it reaches around 1.2 bar, then attempt a reset. Low pressure won't always trigger E5 on its own, but it can contribute to restricted circulation.
- Bleed your radiators DIY safe
Air trapped in radiators reduces water flow around the system. Turn the heating on briefly, feel each radiator for cold spots (particularly at the top), then turn the system off and bleed any affected radiators with a bleed key. Re-check the pressure afterwards and top up if needed.
- Make sure all radiator valves are fully open DIY safe
Partially closed thermostatic or lockshield valves can restrict circulation enough to cause the boiler to overheat. Walk around the property and confirm all valves are in the open position, then try running the heating again.
- Reset the boiler — once or twice only DIY safe
With the gas supply confirmed on, press and hold the reset button for around 3–5 seconds (the exact method varies slightly by model — check your manual). If the boiler fires up and runs normally, monitor it closely. Do not reset more than two or three times; repeated resets on an E5 fault can mask an overheating condition and risk damage to the heat exchanger.
- If the fault returns, turn the boiler off and call a Gas Safe engineer Gas Safe engineer
A persistent E5 code requires professional diagnosis. A registered engineer will use a multimeter to test the NTC thermistor's resistance across a range of temperatures, inspect the wiring harness for damage or corrosion, assess pump operation and flow rates, and check for limescale or sludge restriction. They'll also verify the code plug is correct for your boiler model. Do not open the boiler casing or attempt sensor or pump work yourself — this is unsafe and will likely void your warranty.
Parts you may need
- NTC flow temperature thermistor (sensor) · from £25
- Sensor wiring harness / connector · from £20
- Central heating circulation pump · from £90
- Code plug · from £30
- PCB (printed circuit board) · 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–£350, depending on the underlying cause.
Frequently asked questions
What is the difference between E5 218 and E5 332 on a Worcester Bosch boiler?
E5 218 means the boiler's flow temperature has actually exceeded its safe upper limit — the water genuinely got too hot, usually because of a circulation problem such as a weak pump, airlocks, or a blocked heat exchanger. E5 332 tends to point more directly to a sensor or signal fault — the temperature reading being sent to the PCB is implausible or out of range, even if the water itself isn't necessarily overheating. In practice both sub-codes require a Gas Safe engineer to properly distinguish between them using electrical tests.
Is it safe to keep resetting a boiler showing E5?
No — you should reset it once or twice to see whether it's a temporary glitch, but if the fault comes back you should stop resetting and turn the boiler off. The E5 code can indicate genuine overheating, and repeatedly bypassing the lockout could allow temperatures to climb to damaging levels, potentially harming the heat exchanger. Switch the boiler off and book an engineer promptly, especially in cold weather.
How much does it cost to fix a Worcester Bosch E5 fault?
Most people pay somewhere between £120 and £350 all-in, covering the engineer's call-out, diagnosis, and the most common repairs such as a sensor replacement or a system bleed and flush. If the circulation pump needs replacing, expect to add roughly £150–£250 to those figures. A PCB replacement is less common and typically costs £250–£450 including labour. Heat exchanger replacements are rare but expensive (£500+); at that point it's worth discussing whether a new boiler is a better investment, particularly if the boiler is over ten years old.
Can I replace the flow temperature sensor myself?
We strongly advise against it. While the E5 fault code itself doesn't involve gas directly, working inside the boiler casing to access the sensor carries real risks — including electric shock, incorrect reassembly, and inadvertently disturbing gas or flue connections. It will almost certainly void your warranty. Sensor replacement also requires electrical testing to confirm the diagnosis first, and a Gas Safe engineer can do both in a single visit for a relatively modest cost.