Overview
E20 on a Potterton Promax (Combi, System or Store) indicates a fault with the central heating thermistor (NTC) sensor. The thermistor is a temperature sensor fitted to the central heating flow or primary circuit that tells the boiler’s control board the water temperature so the boiler can modulate correctly. When the control detects an out-of-range value, no signal, or a short/open circuit from that sensor it will flag E20. This error is commonly caused by a failed sensor, a damaged or disconnected wiring/connector, corrosion or water ingress at the connector, or an intermittent wiring fault. Severity ranges from inconvenient (radiators not heating or boiler locking out on central heating demand) to important because running the boiler without a reliable flow temperature reading can lead to unsafe operation or repeated lockouts. Often domestic hot water operation may be unaffected if that circuit uses a separate sensor, but that depends on your specific model and piping. Some basic checks and a power reset are reasonable for a homeowner, but diagnosing deeper—testing sensors, opening the boiler casing, replacing the thermistor or repairing wiring—requires electrical and gas-appliance experience. For safety and compliance, any internal boiler repairs or sensor replacements should be carried out by a qualified, Gas Safe registered engineer. If the fault persists after simple checks, call a professional.
Possible Cause: Central heating thermistor sensor fault
Troubleshooting Steps
Safety precautions:
- If you smell gas, evacuate and call the emergency gas number immediately—do not attempt any checks.
- Turn the boiler off using the on/off switch, and isolate mains power at the fuse spur before doing any visual inspections. Do not touch or probe live electrical parts.
- If you are not comfortable with basic electrical checks or opening the outer casing, stop and call a Gas Safe engineer. Working on gas appliances or internal electrical components without qualifications is dangerous and may invalidate warranties.
Initial homeowner checks you can do (no tools or minimal tools):
1. Note when E20 appears and what else is happening (radiators cold, boiler locking out, DHW working or not). Record any other codes shown.
2. Try a simple reset: switch the boiler off, wait 30 seconds, switch back on. Some transient sensor faults clear with a reboot.
3. Check system pressure on the boiler gauge. If pressure is very low (<1.0 bar), repressurise to around 1.0–1.5 bar following the manufacturer instructions. Some sensor-related faults can occur alongside pressure/circulation issues.
4. Visually inspect any accessible wiring and connectors that are external to the sealed casing (e.g., room thermostat connections, external sensors). Look for loose plugs, disconnected leads, damage, corrosion, or signs of water. Tighten or reseat accessible connectors only with power isolated.
5. Check for frozen or blocked condensate pipe or obvious signs of water ingress around the boiler which might affect sensors or wiring.
Specific diagnostic checks (requires basic electrical tools and confidence; isolate mains power first):
1. Locate the central heating thermistor: consult the boiler manual to find the sensor location (often clipped to the flow pipe or on the primary flow). Only open panels if this is within user-access allowance in the manual and mains power is isolated.
2. With power isolated, carefully disconnect the thermistor connector from the wiring loom. Inspect the connector for corrosion, bent pins or moisture. Clean and dry if needed and reconnect securely.
3. If you have a multimeter and the manual lists expected resistance, measure the thermistor resistance at room temperature with the connector unplugged from the PCB. Many NTC sensors used in boilers are around a nominal value at 25°C (check your manual for the exact specification). Apply gentle heat (warm cloth or your hand) and watch for a change in resistance: the value should change smoothly as temperature changes. If the resistance is open-circuit, fixed (no change) or wildly out of spec, the thermistor is faulty.
4. If you cannot access the sensor or measure resistance, visually follow the sensor wiring to the PCB and check for damaged cable, chafing, rodent damage or loose screw terminals. Repairing wiring inside the boiler should only be done by a qualified engineer.
5. After any reseating/inspection, restore power and see if E20 clears. If the fault was an intermittent connector contact or moisture, the error may not return.
6. If testing shows the thermistor is faulty (open circuit or no response to temperature) the sensor needs replacing. Replacing the thermistor involves isolating power, removing the old probe and fitting the correct replacement part for your model. This is best done by a Gas Safe engineer to ensure correct part, location and commissioning.
When to call a professional:
- If the E20 code returns after a reset and basic checks, or if you identify a damaged sensor or wiring, call a qualified, Gas Safe registered heating engineer.
- Any work that requires opening the sealed boiler casing, electrical repair, or replacement of internal sensors should be performed by a professional.
- If other error codes appear alongside E20 (overheat, circulation faults, PCB errors), do not attempt further DIY—book an engineer.
Final note: E20 is usually a sensor or wiring issue and often straightforward for an engineer to diagnose and repair, but because it involves temperature feedback used to control combustion and safety limits, professional intervention is the correct, safe course if simple homeowner checks do not clear the error.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Promax Combi, System, and Store.