E.00-.04 Boiler Fault Code: What It Means on Every Brand

The E.00-.04 fault code appears on 2 boiler brands — Baxi and Remeha — 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 E.00-.04 means	Severity	DIY?
BBaxi	Baxi boilers display fault codes in two parts — the prefix and a suffix — and E.00–.04 and E.04–.00 are two entirely different faults that happen to share similar-looking digits. Full guide →	Lockout — engineer likely	No
RRemehashown as E.00.04	The E.00.04 fault code on a Remeha boiler indicates a problem with the return water temperature sensor — sometimes described as 'TReturn Open'. Full guide →	Lockout — engineer likely	No

E.00-.04 brand by brand

Baxi E.00-.04

Baxi boilers display fault codes in two parts — the prefix and a suffix — and **E.00–.04** and **E.04–.00** are two entirely different faults that happen to share similar-looking digits. Read both parts of the code on your display carefully before attempting any diagnosis. **E.00–.04** signals a permanent lockout caused by an open-circuit on the return temperature sensor. The boiler's control board is receiving no signal from the NTC thermistor that monitors the temperature of water returning from your heating system — either because the sensor has failed, its wiring has broken, or a connector has come loose. The boiler shuts down completely and will not restart until the fault is resolved and the boiler is manually reset. **E.04–.00** signals a permanent lockout caused by the boiler failing to detect its gas valve. The PCB has lost communication with the gas valve — this could be a wiring issue, a problem with the air/gas (venturi) assembly, or a failed valve. Because gas is involved, this fault is treated as a safety-critical lockout. Both codes are classed as **Permanent Faults** on Baxi boilers, meaning the appliance will remain off until the underlying problem is fixed — do not attempt to keep resetting the boiler as a workaround. Other faults in the E.04 series worth knowing: **E.04–.01** and **E.04–.02** relate to the flow temperature sensor (short-circuit and open-circuit respectively); **E.04–.03** indicates critical flow temperature, often due to poor circulation; **E.04–.04** and **E.04–.05** point to the flue temperature sensor; and **E.04–.08** means the safety thermostat has opened, usually because of a circulation failure.

Causes, fixes & costs for Baxi E.00-.04 → · All Baxi codes

Remeha E.00.04

The E.00.04 fault code on a Remeha boiler indicates a problem with the return water temperature sensor — sometimes described as 'TReturn Open'. This sensor sits on the return pipework and monitors the temperature of heating water coming back into the boiler. When E.00.04 appears, the boiler's control system has either received no signal from that sensor, or the reading is so far outside the expected range that it has been rejected entirely. In either case, the boiler locks out as a safety measure until the fault is resolved. The root cause is almost always a wiring or sensor issue rather than a deeper boiler problem, making this one of the more straightforward fault codes to diagnose — though the investigation and repair must be carried out by a qualified engineer.

Causes, fixes & costs for Remeha E.00.04 → · All Remeha codes

Frequently asked questions

Is the E.00-.04 fault code the same on every boiler?

Broadly, yes — on Baxi and Remeha the E.00-.04 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 E.00-.04 fault myself?

Not usually. On every brand listed, E.00-.04 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 E.00-.04 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.