Remeha E.00.04 Fault Code: Causes, Fixes & Repair Costs
What does the Remeha E.00.04 fault code mean?
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.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Return sensor displaced or poorly secured Common
The return sensor is a small clip-type device that clamps onto the return LTHW pipework. If it has worked loose, been knocked out of position, or was never seated correctly, it will struggle to read an accurate temperature. This is the first thing an engineer will check, as it is a quick and cheap fix if that turns out to be the issue.
- Damaged or disconnected sensor wiring Common
The sensor connects to the boiler's PCB via a thin cable. If that cable has been pinched, chafed against pipework, or had its plug partially pull out of the board connector, the signal will be lost or corrupted. Insulation covering the pipe and sensor can hide damage, so a thorough inspection of the full cable run is needed.
- Faulty return temperature sensor Sometimes
If the wiring and connections all check out, the sensor itself may have developed an internal fault. Sensors can fail due to age, moisture ingress, or thermal stress. A resistance check with a multimeter will confirm whether the sensor is reading within its correct range; if not, replacement is straightforward once diagnosed.
- PCB connector or board fault Rare
In a smaller number of cases, the problem lies not with the sensor or its cable but with the connector block or input circuit on the PCB itself. A corroded or cracked connector can break the signal path even when everything else looks fine. PCB faults are less common but should be considered if sensor and wiring checks come back clear.
How to fix it
- Do not attempt to open the boiler casing or handle any wiring yourself Gas Safe engineer
The return sensor and its wiring are internal components connected to the boiler's control board. Touching these without the correct training and tools risks electric shock and could invalidate your warranty or home insurance. Leave all internal inspection to a Gas Safe registered engineer.
- Check that your gas supply is on and other gas appliances are working DIY safe
Before calling an engineer, quickly confirm that your gas cooker or gas fire is working normally. If you have no gas supply at all, contact your gas supplier rather than a boiler engineer — the fault may be unrelated to the sensor.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on the boiler as described in your user manual. Do this no more than once or twice. If E.00.04 returns immediately or after a short time, the underlying fault is still present and resetting further will not help.
- Contact a Gas Safe registered engineer to inspect the sensor and its wiring Gas Safe engineer
A qualified engineer will start by confirming the return sensor is correctly clipped to the return pipework and fully secure. They will then trace the cable back to the PCB, checking for physical damage, loose plugs, or corroded terminals along the route.
- Engineer to test sensor resistance with a multimeter Gas Safe engineer
Using a multimeter, the engineer will measure the electrical resistance across the sensor and compare it against Remeha's specified values for the expected water temperature. A reading that falls outside the acceptable range confirms a failed sensor that needs replacing.
- Replace the return temperature sensor if found to be faulty Gas Safe engineer
If the sensor has failed, the engineer will fit a compatible replacement, re-route and secure the cable tidily, and reconnect it to the PCB. The boiler can then be restarted and monitored to confirm the fault code has cleared and return temperatures are being read correctly.
- Investigate PCB if fault persists after sensor replacement Gas Safe engineer
If a new sensor and clean wiring connections do not clear E.00.04, the fault lies deeper in the control board. The engineer will assess whether the PCB connector or input channel is damaged and advise on repair or board replacement as appropriate.
Parts you may need
- Return water temperature sensor (NTC type) · from £25
- Sensor wiring loom / cable assembly · from £35
- PCB connector block · from £20
- Replacement PCB (if required) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£280, depending on the underlying cause.
Frequently asked questions
Can I fix Remeha E.00.04 myself?
Not in any meaningful way, unfortunately. The checks that matter — inspecting the sensor clip, testing cable resistance, and verifying PCB connections — all require access to the boiler's internals and a multimeter. You can safely try a single reset to see if the fault clears on its own, but if E.00.04 comes back you will need a Gas Safe registered engineer to diagnose and repair it.
How much will it cost to fix E.00.04 on my Remeha boiler?
For the most common scenario — a loose sensor or short cable run needing repair plus a sensor replacement — expect to pay somewhere in the region of £100–£200 all-in, including parts, labour, and VAT. If the wiring loom needs replacing as well, you might reach £250–£280. A PCB fault is less likely but would push costs higher; if you are quoted for a board replacement, it is worth getting a second opinion and comparing the repair cost against the boiler's age and remaining service life.
Why does E.00.04 say 'TReturn Open' — what does 'open' mean here?
In sensor terminology, 'open' means the electrical circuit is broken — no current is flowing between the sensor and the control board. This is the opposite of a 'short circuit' where wires touch each other unintentionally. An open circuit usually points to a disconnected plug, a broken wire, or a sensor whose internal element has failed, rather than a more serious boiler problem.
Will my Remeha boiler work at all while showing E.00.04?
No — E.00.04 is a lockout fault, meaning the boiler shuts down and will not fire until the fault is resolved and the boiler is successfully reset. This is a deliberate safety feature: without a reliable return temperature reading, the boiler cannot manage heat exchange safely. You will have no central heating or hot water until the sensor issue is fixed.