Overview
E50 on a Potterton Assure combi or system boiler indicates a fault with the domestic hot water (DHW) NTC temperature sensor used with a tank/cylinder. The code is commonly reported as DHW NTC sensor (tank boiler) (s.c.) or (o.c.), meaning the control electronics see the sensor circuit as either short-circuited (s.c.) or open-circuited (o.c.). In simple terms the boiler cannot read a valid temperature from the hot-water sensor so it flags the fault and may lock out or inhibit hot-water production to protect the system. This fault is usually caused by a failed sensor, a broken or corroded wiring/plug connection, water ingress around the sensor or connector, or less commonly a PCB/connector fault in the boiler. Severity is moderate: it generally prevents reliable domestic hot water and can cause boiler lockouts or service mode, but it is not normally an immediate safety hazard like a gas leak—however the boiler should not be repeatedly reset and operated if it’s showing sensor faults. Some initial checks and a reset are suitable for a competent homeowner, but diagnosing and replacing cylinder immersion sensors, repairing internal wiring or doing anything involving the gas circuit or PCB should be done by a Gas Safe registered engineer.
Possible Cause: Domestic hot water (DHW) negative temperature coefficient (NTC) sensor (tank boiler) (s.c) or DHW NTC sensor (tank boiler) (o.c.) Fault
Troubleshooting Steps
Safety precautions:
- Turn the boiler off at the electrical isolator before removing covers or accessing internal wiring. Allow components to cool.
- Do not attempt to work on gas valves, gas pipework or the burner assembly — call a Gas Safe engineer for any gas-side or sealed-component work.
- If you are not comfortable working with basic electrical checks, do not proceed; call a qualified heating engineer.
Initial homeowner checks you can do:
1) Note the exact error code and record when it appeared. Take a photo of the display for the engineer if needed.
2) Try a simple reset: switch the boiler off at the isolator, wait 30 seconds, and switch back on. Some transient sensor errors clear on reboot.
3) Check system pressure on the boiler gauge and ensure it is in the normal range for your system (acting on pressure below recommended levels may cause other faults).
4) Look for visible signs of water leaks, corrosion, damp or damaged cable runs between the boiler and the cylinder/tank. Check any accessible connector at the cylinder top for moisture or broken pins.
If the E50 persists after reset and visual checks, further diagnostic steps (competent DIYer only):
5) Locate the DHW NTC sensor wiring: this will usually run from the cylinder/immersion pocket back to a connector on the boiler. Refer to the boiler manual for sensor location before opening any covers.
6) Isolate mains power to the boiler. Remove the boiler casing only if you are confident and the boiler is isolated. Visually inspect the sensor connector at the boiler PCB for loose pins, corrosion or damaged insulation.
7) If you can safely access the sensor connector, disconnect the sensor lead from the control board and check for obvious damage to the wire and connector.
8) Using a multimeter set to resistance (ohms), measure the sensor leads with the sensor disconnected: a working NTC will show a finite resistance that changes when the sensor temperature is changed (warm the sensor slightly with your hand or warm water and watch for a change). If the meter reads infinite (open circuit) or very low near zero (short), the sensor is faulty. If you are unsure how to use a multimeter, stop and call an engineer.
9) If the sensor resistance is OK but the boiler still reports E50, check continuity of the wiring from the sensor connector back to the control board (with power isolated). If wiring is broken or shorted, repair or replacement of the cable may fix it — but permanent repairs inside the boiler or wiring conduit are best left to an engineer.
10) If you determine the sensor is faulty and are competent with plumbing tasks: replacing an immersion sensor in a hot-water cylinder often requires isolating and draining the cylinder or using an accessible sensor pocket. This can be involved and may require a plumber or heating engineer. After replacement, reconnect the sensor, restore power and test. If the fault clears, monitor the system for normal hot-water function.
When to call a professional:
- Call a Gas Safe registered heating engineer if the error persists after the basic checks, if you find wiring damage, if the sensor requires replacement in a sealed cylinder, or if there are associated faults (PCB errors, repeated lockouts, or gas/combustion warnings).
- Do not attempt gas-side or PCB repairs yourself. Provide the engineer with the error code (E50), any observations (short/open), photos and results of any multimeter checks you made.
Additional notes:
- Intermittent E50 can be caused by a loose connector or moisture; document occurrences and conditions (e.g., after heavy rain, during cold weather) to help diagnosis.
- Avoid repeatedly resetting the boiler to force operation; persistent sensor faults should be fixed to prevent damage to the hot water cylinder or the boiler controls.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Assure Combi and system.