Potterton E20 Fault Code: Causes, Fixes & Repair Costs
What does the Potterton E20 fault code mean?
The E20 code on a Potterton boiler signals that the control board has detected an unexpected or out-of-range reading from the central heating flow thermistor — the temperature sensor that monitors the water temperature leaving the boiler through the flow pipe. The thermistor works by changing its electrical resistance as water temperature rises or falls; the boiler's PCB expects this resistance to stay within a defined window. If the signal falls outside that window at start-up or during operation, the boiler shuts down as a precaution. You may notice cold radiators, the boiler firing briefly then cutting out, or repeated lockouts before the code becomes permanent. Note that fault-code behaviour can differ slightly across the Promax, Assure, and Gold or Titanium ranges, so always cross-reference with your specific model's manual if something doesn't match.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty central heating NTC thermistor Common
The flow thermistor is the most frequently blamed component. Over time the thermistor element can drift out of calibration or fail outright, producing a resistance reading that no longer corresponds to the actual water temperature. Engineers typically expect around 12 kΩ at 20 °C; a reading significantly above or below that range points to a failed sensor.
- Loose, corroded, or damaged wiring and connectors Common
Boilers vibrate continuously during operation, which can gradually work thermistor plug connections loose or cause wire insulation to chafe against metal edges. A poor connection produces intermittent or erratic resistance signals that the PCB interprets as a sensor fault, even if the thermistor itself is perfectly healthy.
- Thermistor not seated correctly on the pipe Sometimes
If the thermistor has been knocked or not refitted properly after a previous service, it may not be in full contact with the flow pipe or pocket. Without good thermal contact it cannot sense the true water temperature, leading to readings the PCB rejects.
- Sludge or scale buildup insulating the sensor Sometimes
In systems that have not been regularly flushed or treated with inhibitor, magnetite sludge or limescale can coat the thermistor pocket, effectively insulating the sensor from the water temperature it is supposed to measure. This is more common in hard-water areas or older, unprotected systems.
- Water-damaged or failing PCB Rare
If the main circuit board has been exposed to moisture — from a leaking pump, a weeping heat exchanger, or condensation — it can misinterpret perfectly normal thermistor signals as out-of-range, triggering an E20 even when the sensor wiring is sound. PCB faults tend to produce erratic or intermittent codes before becoming permanent.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button or dial on your Potterton boiler (position varies by model — check your user guide) and perform a single reset. If the boiler fires up and runs normally, monitor it over the next hour. Should the E20 return, do not keep resetting it — repeated resets will not repair a failed sensor or a loose connection and may mask a developing problem.
- Check your system pressure DIY safe
Glance at the pressure gauge on the boiler fascia. It should read between 1.0 and 1.5 bar when the system is cold. Very low pressure can sometimes accompany or worsen sensor-related shutdowns. If pressure is below 1 bar, top it up via the filling loop following the instructions in your user manual, then try one further reset.
- Do not remove the boiler casing or touch internal components Gas Safe engineer
All further investigation — testing thermistor resistance, inspecting wiring looms, checking PCB connectors, and assessing for moisture damage — must be carried out by a Gas Safe registered engineer. Removing the casing and probing electrical components is unsafe for an unqualified person and could invalidate your warranty or home insurance.
- Book a Gas Safe registered engineer to diagnose the fault Gas Safe engineer
An engineer will carry out a structured electrical diagnosis of the temperature-sensing circuit. This includes measuring the thermistor's resistance (looking for approximately 12 kΩ at 20 °C), inspecting the wiring harness for chafing or corrosion, checking plug and socket integrity, and assessing whether moisture has reached the PCB. Once the root cause is confirmed they can replace the thermistor, repair the wiring, or — if the PCB is at fault — fit a replacement board and carry out the required safety checks before returning the boiler to service.
Parts you may need
- Central heating NTC thermistor (flow sensor) · from £25
- Thermistor wiring harness / connector · from £20
- Potterton PCB (model-specific) · from £180
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£220, depending on the underlying cause.
Frequently asked questions
Can I fix the Potterton E20 fault myself?
The only DIY steps that are safe and appropriate are a single boiler reset and, if pressure is low, topping up via the filling loop. Everything inside the boiler casing — testing the thermistor, reseating connectors, inspecting wiring, or replacing the PCB — requires a Gas Safe registered engineer. Attempting internal repairs yourself is unsafe and may void your warranty.
How much does it typically cost to repair an E20 fault?
When the thermistor itself is to blame, most homeowners pay somewhere between £100 and £220 including parts and labour — the sensor itself is inexpensive and the work is straightforward. If the wiring needs repair the cost is similar. A PCB replacement is considerably more expensive, often £350–£500 depending on the exact Potterton model, and if the boiler is old and several other components are approaching end of life, your engineer may recommend a new boiler rather than a costly board replacement.
Why does my Potterton boiler keep showing E20 intermittently and then clearing itself?
An intermittent E20 that clears on its own is a classic sign of a loose or corroded wiring connection rather than a completely failed thermistor. Boiler vibrations can cause a marginal connection to make and break, sending erratic signals to the PCB. The fault will almost certainly become permanent over time, so it is worth getting an engineer to inspect the thermistor wiring and connectors sooner rather than later, before you lose heating entirely.
Could a lack of annual servicing cause the E20 code?
Indirectly, yes. Regular servicing gives an engineer the chance to spot loose connections, check sensor readings, and assess sludge levels before they cause a shutdown. In systems without adequate inhibitor, sludge can build up around the thermistor pocket and prevent accurate temperature sensing. An annual service — and a system flush if sludge is found — reduces the risk of sensor-related faults like E20 developing in the first place.