Remeha E.00.21 Fault Code: Causes, Fixes & Repair Costs
What does the Remeha E.00.21 fault code mean?
The E.00.21 code means your Remeha boiler has detected a problem with the flue gas temperature sensor — either the sensor circuit has developed a short, or the sensor is reading a temperature beyond its upper limit. The flue gas sensor monitors the temperature of exhaust gases as they leave the boiler, and when it reports an out-of-range or shorted signal the boiler triggers a safety lockout and stops operating. You will need to manually reset the boiler to attempt a restart, but if the underlying fault remains the lockout will return.
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
- Short circuit in the sensor wiring or connectors Common
The most frequent trigger for E.00.21 is a short circuit somewhere in the wiring loom or connector block that links the flue gas temperature sensor to the boiler's control board. Vibration over time, heat cycling, or moisture ingress can cause insulation to break down and wires to touch, producing a false 'above-range' signal.
- Faulty flue gas temperature sensor Common
The NTC sensor itself can fail — particularly the sensing element inside. When the element degrades or its resistance drifts significantly below the expected value it mimics a short circuit. The Remeha part reference commonly used in the UK is S49297, an NTC 12kΩ 3% flue gas sensor, available from heating spare parts merchants.
- Flue gas recirculation or restricted flue Sometimes
If the flue terminal is partially blocked, incorrectly positioned, or if exhaust gases are being drawn back into the air inlet, the actual flue gas temperature can climb high enough to trip the sensor's upper threshold as a genuine over-temperature condition rather than a sensor fault. Staining around the flue terminal or an unusual smell indoors can be signs of this.
- Incorrectly fitted or poorly seated sensor Sometimes
A sensor that has not been properly inserted into its pocket, or whose connector is not fully engaged, can produce erratic resistance readings that the control board interprets as a short. This is more likely to occur after recent servicing or any work near the heat exchanger area.
- Control board (PCB) failure Rare
If all wiring and sensor checks come back normal, the control board may be misreading the sensor signal. A PCB fault is relatively rare but should be considered only after other causes have been ruled out by a Gas Safe engineer.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button (or follow the reset procedure in your user guide) to attempt a restart. If the boiler fires and runs normally, monitor it over the next hour. If E.00.21 returns, do not keep resetting — repeated lockouts indicate a real fault that needs professional attention.
- Check your gas supply is active DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working and that your gas meter isolation valve is fully open. A loss of gas supply can sometimes trigger secondary faults, though it is not the primary cause of E.00.21.
- Inspect the flue terminal from outside (visual only) DIY safe
From outside the property, check that the boiler's flue outlet is not visibly blocked by debris, a bird's nest, ice, or physical damage. Do not attempt to dismantle or probe the flue — simply note anything unusual and report it to the engineer.
- Engineer: inspect sensor wiring and connector block Gas Safe engineer
A Gas Safe engineer should isolate the boiler electrically, then carefully examine the wiring harness running to the flue gas temperature sensor. They should look for chafed insulation, corroded terminals, loose pins, or any sign of a short between conductors. Resistance across the connector should be checked with a multimeter.
- Engineer: test and replace the flue gas temperature sensor if faulty Gas Safe engineer
With the boiler cold, the engineer should measure the sensor's resistance and compare it against the expected NTC characteristic curve (nominally 12kΩ at 25 °C). A reading significantly below spec confirms sensor failure. Replacement with the correct NTC 12kΩ part (e.g. Remeha S49297) should restore normal operation.
- Engineer: inspect the flue system for blockage or recirculation Gas Safe engineer
If wiring and the sensor test out correctly, the engineer should carry out a full flue integrity check — including the terminal position, any joints in the flue run, and combustion analysis — to confirm that hot exhaust gases are not recirculating back to the sensor or that the flue is not restricted.
- Engineer: replace the control PCB if all other checks pass Gas Safe engineer
Should the wiring, sensor, and flue all test as satisfactory, the control board may be misinterpreting the sensor signal. The engineer can confirm this by temporarily substituting a known-good sensor; if the fault persists, a PCB replacement is the next step.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the boiler has locked out on E.00.21 more than once and you have not already done so, arrange a visit from a Gas Safe registered engineer. You can verify an engineer's registration at gassaferegister.co.uk. Do not attempt further resets or any internal boiler work yourself.
Parts you may need
- Remeha Flue Gas Temperature Sensor NTC 12kΩ 3% (S49297) · from £55
- Sensor wiring loom / harness (Remeha compatible) · from £35
- Remeha control PCB (model-specific) · 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–£320, depending on the underlying cause.
Frequently asked questions
Can I fix Remeha E.00.21 myself by just resetting the boiler?
A single reset is worth trying — occasionally a transient electrical glitch can cause a false lockout that clears on restart. However, if the boiler locks out again with E.00.21, resetting it repeatedly will not solve the underlying problem. The fault points to a sensor circuit issue or a genuine over-temperature condition, both of which need diagnosis by a Gas Safe registered engineer.
How much does it typically cost to fix Remeha fault code E.00.21 in the UK?
For most people, the repair involves either a wiring fix or a sensor replacement, which typically costs between £100 and £200 including parts and labour. If the control PCB turns out to be the cause, expect to pay £250–£320 or more. Always get a written quote before work begins. Note that emergency or out-of-hours call-outs carry a premium on top of these figures.
What is the flue gas temperature sensor on a Remeha boiler and why does it matter?
The flue gas temperature sensor is a small NTC thermistor fitted in the exhaust path of the boiler. It continuously monitors the temperature of waste gases leaving the heat exchanger. If those gases become dangerously hot — which could signal a blocked flue, poor combustion, or a scaling problem — the sensor triggers a safety lockout before any damage occurs. E.00.21 means the boiler can no longer trust the reading from this sensor (due to a short or an out-of-range value), so it shuts down as a precaution.
Could a blocked or damaged flue cause E.00.21?
Yes. If the flue is partially obstructed, or if exhaust gases are recirculating back towards the air inlet, the actual temperature at the sensor can exceed its normal operating range, causing a legitimate over-temperature lockout rather than a sensor fault. Signs to watch for include sooty or brown staining around the flue terminal, an unusual smell inside the property, or the boiler locking out repeatedly in cold or windy weather. A Gas Safe engineer can carry out a flue integrity and combustion check to rule this out.