E99 Boiler Fault Code: What It Means on Every Brand

The E99 fault code appears on 2 boiler brands — Biasi and Keston — 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 2 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 E99 means	Severity	DIY?
BBiasi	The E99 fault code on Biasi boilers indicates that the PCB (printed circuit board) — the electronic control unit that governs every function of the boiler — is reporting that it is not… Full guide →	Lockout — engineer likely	No
KKeston	The E99 code on a Keston boiler signals a communication breakdown between the front fascia (display panel) and the internal control box (main PCB). Full guide →	Lockout — engineer likely	No

E99 brand by brand

Biasi E99

The E99 fault code on Biasi boilers indicates that the PCB (printed circuit board) — the electronic control unit that governs every function of the boiler — is reporting that it is not configured or has lost its programmed settings. In practical terms, the boiler's 'brain' either does not recognise a valid configuration, has had its settings wiped (for example after a power disruption or board replacement), or the board itself has developed a hardware fault. Until the PCB is correctly configured or replaced, the boiler will not operate. This fault appears across several Biasi models including the Rinnova combi and Rinnova system boiler ranges. Because the fix involves accessing internal electronics and, in most cases, entering a protected engineer parameter mode, this is strictly an engineer-only fault — it is not resolvable by a homeowner.

Causes, fixes & costs for Biasi E99 → · All Biasi codes

Keston E99

The E99 code on a Keston boiler signals a communication breakdown between the front fascia (display panel) and the internal control box (main PCB). The two components are no longer exchanging data correctly, causing the boiler to lock out and refuse to operate until the fault is resolved. Keston's own documentation describes E99 as carrying the same definition as E41 — a fascia-to-control-box communication fault — and the manufacturer's direct guidance is to check the wiring and cables connecting these two components. Because Keston boilers use a twin-flue, modulating design that many engineers find unfamiliar, diagnosis can take longer than on mainstream boilers. The fault will remain active until manually reset; simply switching the boiler off and on will not clear it.

Causes, fixes & costs for Keston E99 → · All Keston codes

Frequently asked questions

Is the E99 fault code the same on every boiler?

Broadly, yes — on Biasi and Keston the E99 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 E99 fault myself?

Not usually. On every brand listed, E99 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 E99 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.