A08 Boiler Fault Code: What It Means on Every Brand

The A08 fault code appears on 3 boiler brands — Andrews, Ferroli and Vokera — 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 A08 means	Severity	DIY?
AAndrews	The A08 code indicates a problem with the return NTC thermistor — the temperature sensor that monitors the water coming back into the boiler after it has circulated around the heating… Full guide →	Lockout — engineer likely	No
FFerroli	The A08 code means your Ferroli boiler has detected overheating at the heat exchanger and has locked itself out as a safety precaution. Full guide →	Lockout — engineer likely	No
VVokera	The A08 fault code on Vokera boilers signals a problem with the return NTC thermistor — the sensor that monitors the temperature of water flowing back into the boiler to be reheated. Full guide →	Lockout — engineer likely	No

A08 brand by brand

Andrews A08

The A08 code indicates a problem with the return NTC thermistor — the temperature sensor that monitors the water coming back into the boiler after it has circulated around the heating system. The boiler's PCB continuously compares readings from the flow and return thermistors; if the temperature differential falls outside acceptable limits, or if the return sensor sends a signal that is clearly out of range, the boiler treats this as a safety risk and locks out. Note: although A08 is sometimes labelled as an 'Andrews' code, it is formally associated with Vokera boilers, which share engineering roots and fault-code conventions with certain Andrews commercial heating products. If your display shows A08, this page applies to your boiler.

Causes, fixes & costs for Andrews A08 → · All Andrews codes

Ferroli A08

The A08 code means your Ferroli boiler has detected overheating at the heat exchanger and has locked itself out as a safety precaution. Ferroli uses 'A' prefix codes to indicate a hard lockout — the boiler will not attempt to restart on its own and must be manually reset once the underlying problem is resolved. The trigger is the overtemperature protection circuit intervening to prevent damage to the heat exchanger, which can happen when the heating sensor has failed, water is not circulating properly around the system, or air is trapped in the circuit. You may also see this fault appear as F08 in its blocking (non-lockout) form before it escalates to A08.

Causes, fixes & costs for Ferroli A08 → · All Ferroli codes

Vokera A08

The A08 fault code on Vokera boilers signals a problem with the return NTC thermistor — the sensor that monitors the temperature of water flowing back into the boiler to be reheated. The code can appear for three related reasons: the return thermistor has developed a fault and is sending inaccurate readings, it is registering an excessively high temperature on the return side, or — most commonly — the temperature differential between the flow and return pipes is inverted. In a correctly functioning system, water leaves the heat exchanger via the flow pipe at its hottest, travels around the radiators and pipework losing heat along the way, and returns cooler via the return pipe. If the boiler's control board detects that the return is actually hotter than the flow, it treats this as an unsafe condition and locks out, displaying A08 to protect the system from overheating or further damage.

Causes, fixes & costs for Vokera A08 → · All Vokera codes

Frequently asked questions

Is the A08 fault code the same on every boiler?

Broadly, yes — on Andrews, Ferroli and Vokera the A08 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 A08 fault myself?

Not usually. On every brand listed, A08 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 A08 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.