Potterton E50 Fault Code: Causes, Fixes & Repair Costs
What does the Potterton E50 fault code mean?
The E50 code on a Potterton boiler points to a problem with the domestic hot water (DHW) NTC thermistor — the temperature sensor that monitors the heat of your hot water supply. Think of it as the boiler's thermometer for the hot water circuit: it sends resistance readings to the control board that change in proportion to the water temperature. When those readings drift outside the acceptable range — whether because the sensor has failed, its wiring is compromised, or water circulation isn't delivering heat properly — the control board logs E50 and typically suspends the hot water function as a precaution. Central heating may continue to work normally, but taps will run cold until the fault is 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
- Failed or out-of-range DHW NTC sensor Common
The NTC sensor is a small resistive component screwed into the left-hand manifold, sitting directly in the flow of mains hot water. Over time the resistive element drifts or fails outright, sending readings the control board cannot accept. This is by far the most frequent trigger for E50 and usually requires the sensor to be replaced along with its sealing washer.
- Loose or damaged sensor wiring Sometimes
The sensor connects to the PCB via a green lead and a plug-in connector. Vibration, heat cycling, or simply age can loosen the connector or cause a wire to chafe against the boiler casing. A poor connection produces erratic resistance readings that the board interprets the same way it would a dead sensor.
- Poor primary water circulation Sometimes
A jammed or sluggish pump, or a significant air pocket trapped in the system, can prevent heated water from reaching the sensor consistently. The board sees unstable temperature readings and raises E50. Blocked heat exchangers or low system pressure can contribute to the same effect.
How to fix it
- Reset the boiler DIY safe
Locate the reset button or turn the control knob to the RESET position and hold for 3–5 seconds. A one-off reset sometimes clears a transient sensor glitch. If E50 returns after one or two resets, do not keep resetting — a persistent fault needs investigation.
- Check system pressure DIY safe
Look at the pressure gauge on the boiler front. It should read between 1 and 1.5 bar when the system is cold. If it has dropped below 1 bar, top it up via the filling loop until it reaches 1.2–1.5 bar, then re-attempt a reset. Low pressure can contribute to poor circulation and indirectly trigger sensor faults.
- Bleed radiators to remove trapped air DIY safe
Use a radiator bleed key to release any trapped air from the radiators, starting with the ones furthest from the boiler. After bleeding, recheck the system pressure and top up again if needed. Removing air pockets helps restore proper circulation past the DHW sensor.
- Inspect sensor wiring connections — engineer required Gas Safe engineer
A Gas Safe registered engineer will remove the boiler casing and check the green lead and connector at the DHW NTC sensor for signs of corrosion, chafing, or a loose fit. They will also test continuity in the wiring back to the PCB. This step must not be attempted by a homeowner.
- Test and replace the DHW NTC sensor — engineer required Gas Safe engineer
Using a multimeter, the engineer measures the resistance of the sensor across the temperature range and compares it against the manufacturer's specification. If the readings are outside tolerance, the sensor is replaced. The procedure involves isolating the boiler electrically and turning off the mains water supply, opening a hot tap to relieve pressure, then unscrewing the sensor from its pocket with a small adjustable spanner. A new sensor is fitted with a fresh sealing washer and hand-tightened carefully before final tightening — over-torquing can snap the sensor body. Once the lead is reconnected, the water and power are restored and the boiler is tested.
- Call a Gas Safe registered engineer if E50 persists Gas Safe engineer
If the fault reappears after a reset and the straightforward checks above have been done, book a Gas Safe registered engineer to diagnose and repair the fault. Attempting to open the boiler casing or work on internal components yourself is unsafe and potentially unlawful under the Gas Safety (Installation and Use) Regulations 1998.
Parts you may need
- DHW NTC thermistor sensor · from £15
- Sensor sealing washer · from £3
- Wiring harness connector (DHW circuit) · from £12
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 £80–£160, depending on the underlying cause.
Frequently asked questions
Will my heating still work when E50 is showing?
In most cases, yes. The E50 fault targets the domestic hot water circuit specifically, so the central heating side of the boiler will often continue to function normally. Your radiators should still come on, but hot water from taps and showers will be affected until the sensor fault is resolved.
Is the E50 repair expensive?
No — it is one of the more affordable Potterton repairs. The DHW NTC sensor itself costs under £20, and the job typically takes an engineer around an hour. Expect to pay roughly £80–£160 including parts and labour at standard rates. Out-of-hours or emergency call-outs will add to that. If the fault turns out to be a PCB problem rather than the sensor itself, costs can be considerably higher — mention this to your engineer before authorising any work beyond the initial diagnosis.
Can I replace the DHW sensor myself?
No. Although the sensor is a relatively simple component, accessing it requires removing the boiler casing, which is illegal for anyone not on the Gas Safe Register under UK gas safety legislation. The job also involves working with the mains water connection inside the boiler, and an incorrectly fitted or over-tightened sensor can snap and cause a water leak. Always use a Gas Safe registered engineer.
How can I prevent the E50 fault from coming back?
An annual boiler service is the most effective preventive measure. During a service, an engineer will inspect the sensor and its wiring, check system pressure and circulation, and clean components before minor wear becomes a fault. Keeping the system inhibitor topped up and ensuring radiators are bled regularly also reduces the stress on sensors and pump components.