703 Boiler Fault Code: What It Means on Every Brand

The 703 fault code appears on 2 boiler brands — Ariston and Chaffoteaux — 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 703 means	Severity	DIY?
AAriston	Fault code 703 appears on certain Ariston boilers to indicate that the temperature sensor monitoring the outgoing (flow) water on heating Zone 3 has either failed or is returning a signal… Full guide →	Lockout — engineer likely	No
CChaffoteaux	Fault code 703 appears on Chaffoteaux multizone boilers and tells you that the control board has detected either an open circuit or a short circuit on the Zone 3 flow (supply) temperature… Full guide →	Lockout — engineer likely	No

703 brand by brand

Ariston 703

Fault code 703 appears on certain Ariston boilers to indicate that the temperature sensor monitoring the outgoing (flow) water on heating Zone 3 has either failed or is returning a signal outside the expected range. This sensor — an NTC thermistor — works by changing its electrical resistance in proportion to temperature: as the water heats up, the resistance drops, and the boiler's PCB uses this reading to regulate how much heat is produced. When the PCB detects that this sensor's signal is absent, erratic, or implausible, it raises the 703 fault and suspends normal operation to prevent uncontrolled heating on Zone 3. The fault points specifically at Zone 3's flow sensor rather than a general sensor failure, so it is most relevant to multi-zone Ariston system installations.

Causes, fixes & costs for Ariston 703 → · All Ariston codes

Chaffoteaux 703

Fault code 703 appears on Chaffoteaux multizone boilers and tells you that the control board has detected either an open circuit or a short circuit on the Zone 3 flow (supply) temperature probe. In plain terms, the board is trying to read the temperature of the water leaving the boiler into heating Zone 3, but the NTC thermistor sensor responsible for that reading is sending back either no signal at all or a signal that makes no sense. Codes 701 and 702 cover the same fault on Zones 1 and 2 respectively, so if you see any of those on the display, the diagnosis and remedy are essentially identical — only the affected zone differs. Until the board can get a reliable temperature reading from that probe circuit, the boiler will not operate normally.

Causes, fixes & costs for Chaffoteaux 703 → · All Chaffoteaux codes

Frequently asked questions

Is the 703 fault code the same on every boiler?

Broadly, yes — on Ariston and Chaffoteaux the 703 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 703 fault myself?

Not usually. On every brand listed, 703 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 703 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.