Overview
E20 on a Potterton Assure combi or system boiler means the boiler control has detected a fault with the central heating NTC thermistor (temperature sensor). The NTC thermistor measures water temperature in the heating circuit and reports that value to the boiler PCB. If the sensor gives an out-of-range reading, is open-circuit, shorted, or its wiring/connector is faulty, the boiler flags E20 and may stop heating or go into a safe mode to prevent incorrect operation. Common causes are a failed thermistor, a loose/corroded connector or broken wiring, or less commonly a PCB/input fault. Sometimes transient electrical glitches or a simple communication fault clear with a reset. Severity is generally moderate: the boiler will usually remain safe but you can lose central heating until the sensor/wiring is fixed. Because this involves internal sensors, wiring and a gas appliance control, basic checks and a reset are reasonable for a homeowner, but accessing internal components, testing sensor resistance or replacing parts should be done by a qualified Gas Safe engineer.
Possible Cause: Central heating negative temperature coefficient (NTC) fault
Troubleshooting Steps
Safety precautions:
1. Treat any gas appliance with caution. If you smell gas, leave the property immediately and call the emergency gas number. Do not attempt repairs.
2. Before touching the boiler, switch the electrical supply off at the fused spur or isolation switch. Do not open the boiler casing unless you are a qualified engineer. Gas and electrical safety regulations require Gas Safe registered engineers to carry out internal repairs.
3. Record the error code, the boiler model and serial number, and take a photo of the display for the engineer if needed.
Initial checks a homeowner can do (safe, no casing opening):
1. Reset the boiler: press and hold the reset button for 3–5 seconds (follow your manual). If E20 clears and stays cleared while heating runs, monitor for recurrence.
2. Check central heating demand: make sure the room thermostat or programmer is calling for heat and that any wireless/stat batteries are OK.
3. Check system pressure on the boiler gauge. If pressure is very low (below about 0.8–1.0 bar) top up to the normal level (about 1.0–1.5 bar) following manufacturer instructions and then reset the boiler. Low pressure can cause related faults or prevent normal running.
4. Inspect any visible external wiring and connectors at room stats, junction boxes and programmer for obvious damage or loose plugs. Do not open the boiler to inspect internal wiring.
5. Note whether E20 is constant or intermittent, and whether any recent events (power cut, freezing weather, plumbing work) preceded the fault.
If the simple checks do not fix it (what an engineer will do):
1. Engineer safety and isolation: qualified engineer isolates electrical and gas supplies and opens the casing using correct PPE.
2. Visual inspection: check the central heating NTC thermistor location (flow or return pocket or heat exchanger), inspect the sensor, wiring loom and connector for corrosion, damage, chafing or loose pins. Clean and reseat connectors if needed.
3. Electrical test: measure the thermistor resistance at ambient temperature with a multimeter and compare to the manufacturer’s specification (thermistor values vary by model; refer to the boiler manual or Potterton data). Check for open circuit (infinite resistance) or short (very low resistance). Also check for intermittent wiring faults by gently flexing leads while monitoring.
4. Functional test: if present, heat the circuit and watch resistance change with temperature to confirm the NTC is responding. If the thermistor reads correctly but the PCB still flags E20, test the wiring continuity back to the PCB and inspect the PCB connector for damage.
5. Replace faulty part: if the thermistor or wiring is confirmed faulty, replace the sensor or repair the wiring using the correct replacement part and secure connectors. After replacement, reassemble, restore supplies and run the boiler to confirm correct temperature reporting and that E20 does not return.
6. Further faults: if wiring and sensor check out but the error persists, the PCB input stage may be faulty and will likely require a qualified engineer to test and replace the board if necessary.
When to call a professional:
If a reset does not clear E20, if you are asked to open the boiler casing, test internal wiring, or replace the thermistor, stop and call a Gas Safe registered engineer. Any repair involving gas controls, internal wiring or PCB work must be carried out by a qualified professional. Provide the engineer with the error code, model/serial and any observations (intermittent/constant, recent events) to speed diagnosis.
Notes:
Do not attempt gas valve, burner or PCB repairs yourself. Avoid guessing replacement parts: use the correct Potterton-approved thermistor for your model. Document the fault and any steps you took to assist the engineer.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Assure Combi and system.