Error E50

Safety precautions:

- Do not work on gas components. If you suspect a gas issue or see flame problems, turn off the boiler and call a Gas-Safe engineer.

- Isolate electrical power to the boiler before opening the casing or touching internal connectors. Switch off at the isolator and, if present, at your consumer unit.

- Allow hot components to cool and avoid contact with live terminals. If in doubt, wait for a professional.

Initial checks a homeowner can do:

1. Note the exact error and any other codes showing. Take a photo of the display for the engineer.

2. Try a simple reset: switch the boiler off at the mains or isolator, wait 30 seconds, then switch back on. Some sensor errors clear if transient. If E50 returns, proceed.

3. Check system pressure on the gauge. Low pressure can cause other faults; pressurise to the normal operating range (refer to your manual). Do not attempt if you are unsure—follow the boiler manual instructions.

4. Run a hot tap and observe whether the boiler attempts to heat water or immediately shows E50. Note whether central heating still works (this helps isolate DHW vs CH faults).

5. Do a visual inspection (without opening the boiler casing if you are not qualified): look for obvious loose wires, burnt connectors, signs of water leakage or corrosion around pipework and external sensors.

Specific diagnostic steps (for competent DIYers with basic electrical tools):

1. If you are comfortable and safe to do so, isolate mains power and remove the boiler casing to access the sensor wiring. Only proceed if the boiler is powered down.

2. Locate the DHW sensor/thermistor wiring and its connector to the PCB or wiring loom. On Promax models the DHW probe is often on the hot water flow or in the domestic cylinder thermostat pocket (store models). Refer to your boiler manual or the label inside the casing.

3. Check the connector is fully seated and free from corrosion. Re-seat the connector and gently tug the wires to check for breaks.

4. With power still isolated, you can test the sensor with a multimeter set to measure resistance. Disconnect the sensor plug from the board and measure resistance across the two sensor wires. Gently warm the sensor (running warm water over it externally) and watch for a change: an NTC sensor’s resistance should change with temperature (resistance falls as temperature rises). If you see no change or an open circuit, the sensor is likely faulty.

5. If the sensor and wiring look OK but the fault persists, reconnect everything, restore power and run the boiler while watching for E50 or any other codes. If other codes appear, record them.

Fix steps and next actions:

- If a sensor is confirmed failed by the resistance test or obvious physical damage, replacement of the DHW probe will usually clear E50. Sensor replacement involves isolating power, draining/working around water where sensors sit, and refitting/commissioning. If you are not fully confident, do not proceed—call a Gas-Safe engineer.

- If you found a loose/dirty connector and after reseating the fault disappears and DHW works again, monitor the system. Recurring connector issues should be checked by an engineer because repeated faults can indicate underlying wiring or PCB problems.

- If the resistance test looks OK but the boiler still flags E50, the wiring to the PCB, the PCB input circuit, or a related flow switch could be at fault—these are best investigated by a qualified engineer.

When to call a professional:

- Call a Gas-Safe qualified heating engineer if the error persists after a reset and simple checks, if any internal wiring or PCB looks damaged, if you need the sensor replaced, or if you are uncomfortable performing any of the diagnostic steps above. Any work involving gas, pressure vessels, internal wiring, or commissioning must be done by a competent, registered engineer.

What to prepare for the engineer:

- Have the boiler model and serial number ready, the exact error code (E50) and any other codes, the behaviour you observed (no hot water, lockouts, whether central heating still works), and any photos you took. This helps the engineer diagnose and fix the issue quickly.

Note: avoid attempting gas or complex electrical repairs yourself. Sensor faults are commonly resolved by replacing the DHW probe or repairing a wiring connection, but correct replacement and safety checks should be completed by a Gas-Safe registered engineer.