Potterton E40 fault codewhat it means and how to fix it
Central heating return NTC fault on the Potterton Assure and Promax Ultra. The return sensor is open or short. Shown with a spanner; engineer.
Take care
Get a Gas Safe registered engineer to look at your Potterton E40
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What E40 means
The digital-display Pottertons carry a sensor the rotary-switch ranges never had: a return NTC on the water coming back from the radiators, which lets the board compare flow and return (the 154 test) and modulate more finely. Code 40 is that sensor failing open or short — "Central heating return NTC fault" in the Assure and Promax Ultra tables, in the group the installation manual introduces as "followed by 20, 28, 40, 160 or 321 indicates possible faulty components". It is displayed with the spanner symbol, which the Assure user guide explains: "The spanner symbol appears it cannot be reset by the user. Please contact your Installer, Annual Service Provider or Baxi Customer Support."
The fault-finding chart takes "20, 28, 40 or 321 flashing" to section A, the sensor checks. Practically this is E20's twin. The two heating sensors are supplied as one spare on the Assure — "Flow sensor and return sensor" 7207471 — and the same lead and connector faults apply. There is nothing for the owner to do but report it, and no reset to try.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Connector or lead to the return sensor | Loose plug or damaged lead reads as open circuit. First check. | Common |
| Return thermistor out of range | Tested against the flow sensor and the 10 kΩ at 25 °C figure; replaced as the pair. | Common |
| PCB input | Rare; after a new sensor still shows 40. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£50–£120
Time on site
30 minutes
Sensor pair and fitting £60–£110.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
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Baxi / Potterton / Main NTC temperature sensor (flow, return or DHW)
7207471£18–£45Authorised Baxi parts distributor
Price checked 28 September 2026
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Related Potterton fault codes
- E20Get it checkedCentral heating NTC fault on every Potterton code display — the flow temperature sensor is open or short circuit. The user guide says note the code and call; no reset is offered.Related: temperature sensors
- E50Get it checkedHot water NTC fault on rotary-switch Potterton combis — Promax Combi, Titanium, Gold Combi, Store, Heatmax. The domestic hot water sensor is open or short. Note the code and call.Related: temperature sensors
- E154Get it checkedFlow/return sensor temperature test on the Potterton Assure and Promax Ultra: the two heating sensors disagree with each other or with physics. Usually circulation; sometimes a sensor.Related: temperature sensors
- E321Get it checkedHot water NTC fault on the Potterton Assure Combi and Promax Ultra — the digital-display successor to E50. Shown with a spanner; hot water stops, engineer needed.Related: temperature sensors
- E119Get it checkedThe sealed system on a rotary-switch Potterton — Promax, Titanium, Gold, Heatmax — has dropped below half a bar. The window shows E1 and then 19. Fill to 1–1.5 bar and it clears itself.Related: same brand
- E133Get it checkedGas supply interrupted, ignition failed or no flame detected — every Potterton generation. Confirm the gas supply, one reset (5 s on the knob, 1–3 s on the button), and call if it returns.Related: same brand
Own a Potterton boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.