E321 Boiler Fault Code: What It Means on Every Brand

The E321 fault code appears on 3 boiler brands — Baxi, Main and Potterton — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 3 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What E321 means	Severity	DIY?
BBaxi	The E321 code appears on Baxi boilers when the control board detects a problem with the hot water NTC (Negative Temperature Coefficient) thermistor — the sensor responsible for… Full guide →	Lockout — engineer likely	No
MMain	The E321 code on a Main boiler indicates that the boiler's control system has detected a problem with the hot water NTC (Negative Temperature Coefficient) thermistor — the sensor… Full guide →	Lockout — engineer likely	No
PPotterton	The E321 code appears on Potterton boilers when the control board detects an issue with the heating NTC (negative temperature coefficient) thermistor — the sensor responsible for… Full guide →	Lockout — engineer likely	No

E321 brand by brand

Baxi E321

The E321 code appears on Baxi boilers when the control board detects a problem with the hot water NTC (Negative Temperature Coefficient) thermistor — the sensor responsible for continuously measuring the temperature of the domestic hot water. Without a reliable reading from this sensor, the boiler cannot regulate hot water output safely, so it shuts down and locks out until the fault is properly resolved. In plain terms, the boiler's internal thermometer for hot water has either failed, is sending implausible readings, or has a wiring problem between the sensor and the PCB.

Causes, fixes & costs for Baxi E321 → · All Baxi codes

Main E321

The E321 code on a Main boiler indicates that the boiler's control system has detected a problem with the hot water NTC (Negative Temperature Coefficient) thermistor — the sensor responsible for monitoring hot water temperature. When this sensor feeds back readings that fall outside acceptable limits, or stops responding altogether, the boiler cannot safely regulate hot water output and shuts down as a precaution. The fault applies specifically to system and store boilers that serve a hot water cylinder. Because Main, Baxi, and Potterton all share the same BDR Thermea boiler platform and fault code system, E321 carries the same meaning across all three brands. The boiler will remain locked out until the root cause is identified and resolved.

Causes, fixes & costs for Main E321 → · All Main codes

Potterton E321

The E321 code appears on Potterton boilers when the control board detects an issue with the heating NTC (negative temperature coefficient) thermistor — the sensor responsible for monitoring the temperature of the water flowing through the boiler. On store and system boiler models, this can also relate to the hot water cylinder temperature sensing circuit. When the readings from this sensor fall outside the expected range — whether too high, too low, or erratic — the boiler locks out as a precaution to prevent potential component damage. In some Potterton models, E321 can additionally point to a fault with the Boiler Chip Card (BCC) or the PCB (printed circuit board), both of which govern how the boiler interprets and responds to sensor data. Poor water circulation caused by an air lock, sludge blockage, or low system pressure can also produce abnormal temperature readings and trigger this code.

Causes, fixes & costs for Potterton E321 → · All Potterton codes

Frequently asked questions

Is the E321 fault code the same on every boiler?

Broadly, yes — on Baxi, Main and Potterton the E321 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 E321 fault myself?

Not usually. On every brand listed, E321 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 E321 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.