H.03-.00 Boiler Fault Code: What It Means on Every Brand

The H.03-.00 fault code appears on 2 brands — Baxi and Remeha — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.

Up to: Lockout — engineer likely 2 brands use this code Meaning differs by brand

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 H.03-.00 means	Severity	DIY?
BBaxi	The H.03-.00 code is a blocking fault on Baxi boilers, meaning the boiler pauses to protect itself but may restart automatically once the underlying issue resolves — unlike a permanent… Full guide →	Engineer may be needed	No
RRemehashown as H.03.00	The H.03.00 code indicates a Safety Parameter Error on your Remeha boiler. Full guide →	Lockout — engineer likely	No

H.03-.00 brand by brand

Baxi H.03-.00

The H.03-.00 code is a blocking fault on Baxi boilers, meaning the boiler pauses to protect itself but may restart automatically once the underlying issue resolves — unlike a permanent lockout (E-code), which requires a manual reset before the boiler will fire again. This particular code points to a communication problem with the flue NTC (Negative Temperature Coefficient) temperature sensor. The flue NTC sensor sits within the boiler's exhaust flue and continuously monitors the temperature of the outgoing combustion gases. When the boiler's control board loses reliable communication with this sensor — whether because the sensor has failed electrically, its wiring has come loose, or there is a corroded connection somewhere in the circuit — it triggers H.03-.00 and temporarily suspends normal operation to prevent unsafe running conditions. In some Baxi variants the same code can also relate to missing boiler identification data for the safety device, so always cross-reference with your specific model's documentation if the descriptions below don't quite match your situation.

Causes, fixes & costs for Baxi H.03-.00 → · All Baxi codes

Remeha H.03.00

The H.03.00 code indicates a Safety Parameter Error on your Remeha boiler. Specifically, the control system has detected that the internal safety parameters at levels 2, 3, and 4 are either missing or have become invalid. In Remeha's fault classification system, an 'H' prefix means 'Blocking' — the boiler has shut itself down as a precautionary measure but this is not a permanent lockout. It will remain blocked until the underlying issue is resolved. This fault sits within the H.03.xx group, which covers faults in the communication and safety layer between the Control Unit (CU-GH) and the Gas Valve Controller (GVC). Related codes in this group include H.03.01 (no valid data received from the CU to the GVC), H.03.02 (flame loss detected internally), H.03.05 (internal blocking condition), and H.03.17 (periodic safety check in progress). H.03.00 specifically points to the safety configuration data held within the Control Unit being corrupt, incomplete, or mismatched — a serious internal electronics fault that almost always requires a Gas Safe registered engineer.

Causes, fixes & costs for Remeha H.03.00 → · All Remeha codes

Frequently asked questions

Is the H.03-.00 fault code the same on every boiler?

No. H.03-.00 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a H.03-.00 fault myself?

Not usually. On every brand listed, H.03-.00 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 H.03-.00 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.