Gledhill thermal store / unvented water heater shows E01 and will not heat water

Safety first: Isolate the water heater from the mains supply at the fused spur and allow the unit to cool before removing any covers. Hot water and live terminals can cause serious injury.

Steps to check and remedy:

• Power reset: Turn off power for 5 minutes, then restore. If E01 clears and does not return, monitor operation. If it returns, continue.
• Locate the flow NTC: On most Gledhill thermal stores and unvented cylinders, the flow temperature sensor (NTC) is clipped or inserted into a pocket on the primary flow pipe or cylinder shell near the top/front of the unit. Consult the wiring diagram on the inside cover.
• Inspect wiring: Check the sensor lead from the NTC to the main control PCB for damage, loose push‑fit connectors, or signs of overheating. Reseat all plugs firmly.
• Resistance test: With power still isolated, disconnect the NTC from the PCB and measure resistance with a multimeter. At around 20°C, most Gledhill NTCs read roughly 10kΩ (check the service manual for the exact curve). If the reading is open (infinite) or near 0Ω, the sensor is faulty and should be replaced.
• Check PCB input: If the NTC tests OK, inspect the PCB for burnt tracks or corrosion at the sensor input terminals. If damage is visible or the fault persists with a known‑good sensor, the PCB is likely defective.
• Replace parts as needed: Replace the flow NTC like‑for‑like, ensuring it is fully inserted into its pocket and clipped securely to the pipe or cylinder. If replacing the PCB, label all wires before removal and transfer them one by one to the new board.

When to stop: If you are not confident using a multimeter or working inside the appliance, or if the PCB appears damaged, contact a qualified Gledhill service engineer.