E20 is a heating (NTC) thermistor fault on the Baxi-platform brands — Baxi, Potterton, Main, Biasi and Andrews — a cheap sensor swap. Alpha is the exception, using E20 for ignition or flame-detection failure. Check your brand: the two diagnoses lead to very different visits.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
E20 brand by brand
The Alpha E20 fault code most commonly indicates a flame detection or ignition failure — the boiler attempted to light the burner but its control board could not confirm a stable flame was present, so it locked out as a safety precaution to prevent unburned gas from accumulating. A smaller number of sources, and some specific Alpha model variants, associate E20 with excessively high system water pressure instead. Both scenarios are covered on this page. If you are unsure which applies to your boiler, check the display alongside your model manual, or describe the symptom to a Gas Safe engineer — a boiler that clicks repeatedly but won't fire points to the flame-detection meaning, while a boiler that simply stops with no ignition attempt alongside a high pressure gauge reading points to the pressure meaning.
Causes, fixes & costs for Alpha E20 → · All Alpha codes
The E20 fault code on Andrews boilers (including Andrews Water Heaters commercial units, which share the Baxi Group control platform) indicates that the boiler's control board has detected a problem with the central heating NTC thermistor — the temperature sensor that monitors water temperature in the heating circuit. NTC stands for Negative Temperature Coefficient: as water temperature rises, the thermistor's electrical resistance falls, and the PCB uses these resistance changes to track heating circuit temperature. When the boiler starts up or is running, it continuously checks that the thermistor's readings fall within an expected range. If the signal is absent, out of range, or erratic, the boiler locks out and displays E20 to prevent it from operating unsafely. The boiler will not recover on its own — the underlying sensor or wiring issue must be investigated and resolved by a Gas Safe registered engineer before normal operation can resume.
Causes, fixes & costs for Andrews E20 → · All Andrews codes
The E20 fault code on a Baxi boiler indicates that the central heating NTC thermistor has reported a problem. The thermistor is a small temperature-sensing resistor fitted to the boiler's flow or return pipework; rather than measuring temperature directly, it sends resistance values to the PCB, which interprets them as water temperature readings. When those resistance readings fall outside the expected range — or stop arriving altogether — the boiler locks out and displays E20 as a safety precaution. Until the underlying sensor or wiring issue is resolved, the boiler will not fire for heating or hot water.
Causes, fixes & costs for Baxi E20 → · All Baxi codes
The E20 fault code appears on certain boilers — most commonly Baxi models — to indicate that the primary NTC thermistor (the main flow temperature sensor) is not communicating properly with the PCB. The thermistor continuously measures the water temperature in the heating circuit and feeds that data to the control board. When the PCB receives a reading that is out of range, implausible, or simply absent, it has no safe way to regulate burner output, so it shuts the boiler down and displays E20. The result is a complete loss of both central heating and hot water until the underlying cause is resolved. Important note for Biasi owners: Biasi boilers use an 'ER' prefix fault system (for example ER 01, ER 06, ER 07) rather than an E-number system. If your boiler display shows 'ER 06' or 'ER 07', those are the Biasi equivalents for a heating circuit temperature sensor fault — the diagnostic approach and likely causes are very similar to E20 and the information on this page is still relevant. If your boiler is definitely displaying 'E20', you most likely have a Baxi boiler rather than a Biasi.
Causes, fixes & costs for Biasi E20 → · All Biasi codes
The E20 fault code on a Main boiler signals a problem with the central heating NTC (Negative Temperature Coefficient) thermistor — the temperature sensor that monitors water temperature in the boiler's flow pipe. When the boiler's control board receives readings from this sensor that are outside the expected range, or receives no signal at all, it triggers the E20 code and locks the boiler out as a precaution. Main boilers share their control architecture with Baxi and Potterton models (all part of the same group), so the E20 code and its causes are essentially identical across all three brands. The fault itself is not immediately dangerous, but without reliable temperature data the boiler cannot regulate heat safely, which is why it shuts down until the issue is resolved.
Causes, fixes & costs for Main E20 → · All Main codes
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.
Causes, fixes & costs for Potterton E20 → · All Potterton codes
Frequently asked questions
Is the E20 fault code the same on every boiler?
Broadly, yes — on Alpha, Andrews, Baxi, Biasi, Main and Potterton the E20 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.
Can I fix a E20 fault myself?
Not usually. On every brand listed, E20 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.
Who should I call about a E20 fault?
For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.
General guidance only — not a substitute for professional advice, and not certified by a registered engineer.
Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.
