Worcester Bosch E5 218 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch E5 218 fault code mean?
The E5 218 fault code indicates that the boiler's flow temperature has exceeded safe operating limits — specifically, the flow NTC thermistor is detecting water temperatures above 105°C. When this threshold is breached, the boiler locks out as a built-in safety precaution to prevent damage or danger. The root cause is usually a problem with water circulation (so heat builds up instead of being carried away) or a fault with the flow temperature sensor itself. Until the underlying issue is resolved the boiler will remain locked out and will not restart permanently on a reset alone.
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 water circulation — pump or airlock issue Common
If the circulating pump is running slowly, seized, or an airlock is blocking flow, heat cannot be carried away from the heat exchanger efficiently. Water temperature then rises rapidly until the flow thermistor triggers the E5 218 lockout. This is the single most common root cause of this code and should be investigated first.
- Closed or restricted radiator valves or isolation valves Common
If thermostatic radiator valves (TRVs) are all closed, or if a service valve somewhere in the circuit has been left shut, flow is severely restricted. The boiler essentially heats water with nowhere to go, causing rapid temperature rise. Check that all TRVs are open and that no isolation valves on the heating circuit have been accidentally closed.
- Limescale or sludge build-up restricting flow Common
In hard-water areas, limescale can coat the inside of the heat exchanger; in older systems, black iron oxide sludge accumulates in pipes and radiators. Either problem reduces the volume of water circulating, meaning heat is not transferred away quickly enough. The boiler overheats and triggers the fault.
- Faulty or degraded flow NTC thermistor Sometimes
The flow NTC thermistor is a small resistance-based temperature sensor. Over time it can drift out of calibration, develop an internal fault, or become coated in scale that insulates it from the water temperature. When this happens it may report a falsely high temperature even when actual flow temperature is normal, triggering a spurious E5 218.
- Wiring fault to the flow thermistor Sometimes
Damaged, corroded, or loose wiring between the flow NTC thermistor and the PCB can cause the PCB to receive an incorrect signal — either reading too high or interpreting an open circuit as an extreme temperature. Vibration over time or poor previous repair work are common contributors.
- PCB fault or incorrect code plug Rare
If the PCB is failing or the boiler's code plug (the small component that tells the PCB which boiler variant it is controlling) is wrong or damaged, temperature signals can be misread and persistent or apparently random E5 218 faults can result. This is the least common cause and is usually only considered after the sensor, wiring, and circulation have been ruled out.
How to fix it
- Check system pressure and top up if needed DIY safe
Look at the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, use the filling loop to top up slowly until it reaches about 1.2 bar. Low pressure alone is unlikely to cause the E5 218, but it can contribute to poor circulation. Do not overfill above 2 bar.
- Check that all radiator valves and TRVs are open DIY safe
Walk around the property and ensure every thermostatic radiator valve (TRV) is turned to at least position 3 or 4, and that any lockshield valves are open. With all TRVs closed the boiler has almost nowhere to push water and can overheat very quickly. If your boiler has a bypass valve, make sure it is not blocked.
- Bleed the radiators to remove any airlocks DIY safe
Turn off the boiler and allow the system to cool slightly, then use a radiator bleed key to release any trapped air from each radiator in the system, starting with the ground floor. Air pockets reduce the volume of water in circulation and can cause uneven heating and hot spots. Once all radiators are bled, re-check system pressure and top up if it has dropped.
- Attempt a boiler reset (no more than twice) DIY safe
Press and hold the reset button for approximately 5 seconds until the boiler attempts to restart. If it fires up and runs normally, monitor it over the next hour. If the E5 218 returns quickly, do not keep resetting — repeated resets bypass safety protections and risk causing further damage. Two resets is the sensible limit if the fault keeps returning.
- Arrange a Gas Safe registered engineer to inspect the circulation pump Gas Safe engineer
If the homeowner checks above have not resolved the fault, the most likely culprit is the circulation pump. An engineer will check whether the pump is running at the correct speed, is partially seized, or has an internal fault. A pump replacement is a straightforward job for an engineer and resolves the fault in the majority of cases.
- Engineer to test the flow NTC thermistor with a multimeter Gas Safe engineer
A Gas Safe engineer can disconnect the flow thermistor and measure its resistance across a range of temperatures to verify it matches the manufacturer's expected values. A sensor that reads incorrectly at ambient temperature or shows an open circuit will need replacing. The part itself is inexpensive; the labour is the main cost.
- Engineer to inspect wiring and connections to the thermistor Gas Safe engineer
The engineer should check the wiring loom between the flow NTC thermistor and the PCB for signs of chafing, corrosion, loose terminals, or heat damage. A damaged wire is a quick fix; if the issue traces back to the PCB itself, a board repair or replacement may be required.
- Consider a powerflush if sludge or scale is found Gas Safe engineer
If the engineer finds evidence of heavy sludge in the system — dark or cloudy water, cold spots on radiators, or restricted flow — a powerflush may be recommended. This uses specialist equipment to force a cleaning agent through the whole system at high velocity, removing debris that restricts circulation. A magnetic system filter fitted afterwards helps prevent recurrence.
- Call a Gas Safe registered engineer if any doubt remains Gas Safe engineer
Any persistent E5 218 fault that does not clear after the basic homeowner checks above must be diagnosed by a Gas Safe registered engineer. Do not continue operating or repeatedly resetting a boiler that is locking out on a high-temperature fault — the safety system is there for a reason.
Parts you may need
- Flow NTC thermistor (temperature sensor) · from £18
- Central heating circulation pump (e.g. Grundfos UPS2 or equivalent) · from £85
- System magnetic filter (e.g. Fernox TF1 or Adey MagnaClean) · from £55
- Boiler code plug · from £25
- 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 £150–£420, depending on the underlying cause.
Frequently asked questions
Is it safe to keep resetting the boiler when I see the E5 218 code?
No — resetting once or twice to see whether the fault clears is reasonable, but repeatedly resetting a boiler that is locking out on a high-temperature fault is not safe. The lockout exists to prevent the water from reaching dangerous temperatures. Bypassing it repeatedly could cause damage to the heat exchanger or, in a worst-case scenario, create an unsafe situation. If the fault returns after two resets, switch the boiler off and call a Gas Safe engineer.
How much does it typically cost to fix an E5 218 fault?
For most households the repair falls between £150 and £420 including parts and labour. Replacing a flow NTC thermistor is at the lower end — the part costs around £15–25 and the job is relatively quick. Replacing the circulation pump typically costs £200–£380 including parts and labour. If a powerflush is needed it usually adds £300–£500 on top. In rare cases where the PCB is at fault, the cost can exceed £500 — if your boiler is over 10 years old it may be worth comparing that figure against a new installation quote.
Why does my Worcester Bosch boiler keep showing E5 218 even after it has been repaired?
A recurring E5 218 after repair usually points to one of three things: sludge or scale that has not been fully cleared from the system (restricting circulation again over time), a failing PCB that is misreading thermistor signals, or an incorrect or damaged code plug. Ask your engineer to fit a magnetic system filter if one is not already installed, and if the problem persists after a second repair visit, request that the PCB and code plug are checked.
Could low boiler pressure cause an E5 218 fault?
Low pressure on its own rarely triggers an E5 218 directly, but it can contribute to poor circulation — less water in the system means less capacity to carry heat away from the burner. Always check that pressure is between 1.0 and 1.5 bar as part of your initial checks. If the pressure is correct and all radiator valves are open yet the fault persists, the cause is more likely the pump or sensor.