Firebird E2 Fault Code: Causes, Fixes & Repair Costs
What does the Firebird E2 fault code mean?
The E2 code on Firebird Envirogreen condensing boilers signals that the control board has detected an issue with the flue thermistor — a temperature sensor fitted in the exhaust flue that monitors the temperature of gases leaving the boiler. When the board cannot read a reliable signal from this sensor, it raises the E2 fault and typically stops the boiler from operating. Note that fault code meanings can vary across Firebird ranges: E2 refers to the flue thermistor fault on Envirogreen models, but other ranges such as the Enviromax may assign this fault to a different code (e.g. E3). Always check the manual for your specific model before proceeding.
General guidance only — not a substitute for professional advice, and not certified by an OFTEC technician. Always have your specific appliance assessed by a qualified professional. Any oil boiler work must be carried out by an OFTEC registered technician. If you smell oil or fumes, or suspect carbon monoxide, switch the boiler off, ventilate the property and call an OFTEC registered technician. If you suspect a CO leak, call 999.
Common causes
- Failed or deteriorated flue thermistor Common
The flue thermistor is exposed to high exhaust temperatures every time the boiler fires. Over time the sensor element can degrade, drift out of its correct resistance range, or fail altogether. This is the most frequent root cause of the E2 fault and usually requires straightforward sensor replacement by an OFTEC technician.
- Loose, corroded, or damaged wiring Common
The wiring loom that connects the flue thermistor to the PCB can develop loose terminals, corrosion at the plug, or chafed insulation — any of which can create an open-circuit or intermittent signal that the board interprets as a sensor fault. This is particularly common on older installations or where the boiler is in a damp utility room or outhouse.
- Scale build-up or moisture around the sensor Sometimes
Scale deposits or water ingress around the thermistor body can insulate the sensor from the flue gases it is meant to measure, causing inaccurate readings. Condensate pooling near the sensor in condensing models can accelerate this.
- Incorrectly installed or obstructed flue Sometimes
If the flue was not installed to the manufacturer's specification, or if debris or a bird nest is partly blocking it, exhaust gas temperatures and flow patterns can be abnormal. This can stress the thermistor or give the PCB an implausible reading, triggering E2 even when the sensor itself is undamaged.
- PCB fault Rare
In rare cases the control board itself is unable to process the thermistor signal correctly, showing E2 even though the sensor and wiring are sound. This is usually only confirmed once the thermistor and wiring have already been tested and found to be good.
How to fix it
- Check that oil is available and the supply valve is open DIY safe
Before assuming an electronic fault, confirm your oil tank has sufficient fuel and that the isolation valve on the supply line to the boiler is fully open. An oil starvation lockout can occasionally display secondary fault codes on some boards.
- Do not repeatedly press the burner reset button DIY safe
Unlike a straightforward burner lockout, the E2 fault is a sensor or wiring fault rather than a failed ignition attempt. Pressing the reset more than once or twice without resolving the underlying cause will not clear the code and may mask how the fault is developing. Reset once, observe whether the boiler runs normally, and if E2 returns do not reset again — call a technician.
- Note whether the fault is constant or intermittent DIY safe
Check whether E2 appears immediately on every start-up or only after the boiler has been running for a while. An intermittent fault that appears only when hot often points to a wiring issue or a sensor on its way out; one that is present from cold is more likely a failed sensor or wiring break. Write this down — it is useful information for the engineer.
- Have an OFTEC technician inspect and test the flue thermistor OFTEC technician
A registered oil boiler engineer will use a multimeter to measure the thermistor's resistance at ambient temperature and compare it against the manufacturer's specification. A reading outside tolerance confirms sensor failure. This step must be carried out by a competent technician — the sensor is inside the boiler's flue passage.
- Have the technician check the wiring loom and connections OFTEC technician
The engineer will inspect the wiring from the thermistor back to the PCB, checking for loose or corroded terminal connectors, pinched or heat-damaged cables, and continuity through the loom. Many E2 faults are resolved at this stage without any parts being needed.
- Replace the flue thermistor if found to be faulty OFTEC technician
If the sensor is outside specification, the technician will fit a compatible replacement Firebird flue thermistor, re-secure the wiring, and confirm the resistance reading is correct before firing the boiler.
- Inspect the flue installation OFTEC technician
The engineer will check that the flue terminals and pipe runs are correctly fitted to Firebird's specification, are clear of obstruction, and that no rainwater or condensate is pooling back into the sensor area.
- Call an OFTEC registered technician if E2 persists after initial checks OFTEC technician
If the fault code returns after the sensor and wiring have been addressed, the PCB may also require testing or replacement. An OFTEC registered engineer can carry out all combustion-side work safely and legally on your oil boiler. You can find registered technicians at oftec.org.
Parts you may need
- Firebird flue thermistor (NTC sensor) · from £25
- Thermistor wiring loom / harness · from £35
- Firebird PCB (control board) · from £220
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 £120–£320, depending on the underlying cause.
Frequently asked questions
Can I reset the E2 fault myself on my Firebird boiler?
You can press the reset button on the burner once to see if the boiler resumes normal operation, but E2 is a sensor fault rather than a simple ignition lockout. If the code returns after a single reset, pressing it again will not fix the underlying problem. You should call an OFTEC registered engineer to investigate before resetting further.
Is the flue thermistor a safety-critical part?
Yes. The flue thermistor monitors exhaust gas temperatures and helps the control board detect potentially dangerous conditions such as overheating in the flue. Running the boiler with a confirmed sensor fault is not advisable, and the boiler will usually prevent normal operation until the fault is cleared for this reason.
How much does it cost to fix a Firebird E2 fault in the UK?
For most homeowners, fixing an E2 fault involves a call-out, wiring inspection, and possibly a thermistor replacement. Expect to pay in the region of £120–£320 all in for a straightforward repair. If the PCB has also been damaged, replacement parts alone can add £200 or more on top of labour, but this is an uncommon outcome. Getting a written quote before authorising any work is always a good idea.
Will the E2 fault void my Firebird boiler warranty?
Firebird offers a standard two-year warranty, with extended cover of up to ten years available on certain ranges. To keep any warranty valid, the boiler must be serviced annually by an OFTEC registered engineer. Having an OFTEC technician diagnose and repair the E2 fault through official channels should preserve your warranty entitlement — always check with Firebird directly if you are uncertain about your specific cover.