Potterton E321 fault codewhat it means and how to fix it
Hot 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.
Take care
Get a Gas Safe registered engineer to look at your Potterton E321
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
Is the heating still working?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Yes, only hot water is out
Consistent with the hot water sensor.
Engineer; short visit.
Nothing works
The board has locked out on the sensor fault.
Engineer.
What E321 means
On the Assure Combi and Promax Ultra the domestic hot water temperature sensor has the code 321, "Hot water NTC fault", replacing the E50 that Promax and Titanium owners knew. The Assure System table still lists 321 even though a system boiler has no hot water sensor of its own; the row is inherited from the shared platform, and on a System it should not appear. It is in the installer manual's "possible faulty components" group with 20, 28, 40 and 160, displayed with the spanner rather than the R, not on the reset list, and sent by the fault-finding chart to section A with the other sensor codes.
The Promax Ultra parts list names a separate "DHW NTC Sensor" (720789201); the Assure list does not itemise one, and the standard NTC is used. The same limescale point applies as on the older combis: a sensor and plate heat exchanger furred in a hard-water area read slowly and can be marked faulty by the board, so a descale is sometimes the fix rather than a part.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor lead or connector | Loose or corroded plug at the sensor on the hydraulic block, or at the board. | Common |
| Thermistor failed | Out of range against the 10 kΩ at 25 °C figure. Replaced. | Common |
| Limescale on the sensor or plate heat exchanger | Slow readings in hard-water areas; a descale restores them. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£50–£150
Time on site
30 minutes
Sensor £60–£110 fitted. Descale £90–£150.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Baxi / Potterton / Main NTC temperature sensor (flow, return or DHW)
7207471£18–£45Authorised Baxi parts distributor
Price checked 28 September 2026
Baxi / Potterton / Main DHW plate heat exchanger
7683965£60–£140Authorised 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
- E40Get it checkedCentral heating return NTC fault on the Potterton Assure and Promax Ultra. The return sensor is open or short. Shown with a spanner; engineer.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.