Baxi E.00-.05 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E.00-.05 fault code mean?
The E.00-.05 fault code on Baxi boilers indicates that the return water temperature sensor (an NTC thermistor) has been detected as short-circuited by the boiler's PCB. This is a locking fault, meaning the boiler shuts itself down completely and will not fire again until the underlying problem has been resolved and the appliance manually reset. The return thermistor sits on the pipework carrying cooled water back into the boiler from your radiators. It feeds a continuous stream of resistance-based temperature readings to the control board, allowing the boiler to regulate output and protect itself from running outside safe temperature limits. When the PCB sees a resistance value consistent with a short circuit — rather than the expected range for a functioning sensor — it concludes the data cannot be trusted and locks out for safety.
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
- Failed return NTC thermistor Common
The sensor itself has developed an internal short circuit, producing a resistance reading that falls well outside the valid operating range. This is the most frequent root cause. NTC thermistors do degrade over time, particularly in older boilers or systems where water quality is poor, causing the component to fail in a shorted state.
- Loose or poorly seated wiring connector Common
The plug connecting the return sensor to the wiring loom can work loose due to vibration, a previous service, or simply age. A partially disconnected connector can create an intermittent or permanent short-circuit signal at the PCB even when the sensor itself is undamaged.
- Damaged or corroded wiring harness Sometimes
The cables running between the return thermistor and the PCB can suffer from heat exposure, moisture ingress, or physical pinching — for example where they pass through a grommet. Damaged insulation allows conductors to touch and mimic a short circuit, triggering the E.00-.05 lockout.
- PCB fault misreading the sensor circuit Rare
In a small number of cases, the control board itself develops a fault on the input circuit it uses to read the return sensor. The sensor and wiring may both be perfectly sound, but the PCB reports a short circuit regardless. This is relatively uncommon and is usually only confirmed after the sensor and wiring have already been checked and found to be in order.
How to fix it
- Reset the boiler once or twice DIY safe
Locate the reset button on the boiler's front panel — on most Baxi models it is marked with a flame or reset symbol. Press and hold it for 3–5 seconds until the boiler attempts to restart. Allow a full ignition cycle to complete before deciding whether the fault returns. If the lockout clears and the boiler runs normally, monitor it over the next day or two. If it locks out again with the same code, a reset alone will not fix the underlying problem and you should move to the next steps. Do not reset more than two or three times in quick succession.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home — a hob or a gas fire, for example — are working normally. If nothing gas-powered is functioning, contact your gas supplier rather than continuing to fault-find the boiler. This step rules out a broader supply interruption before any engineer is called.
- Inspect the wiring connections at the return sensor (engineer task) Gas Safe engineer
A Gas Safe registered engineer will access the return thermistor, typically located on the return pipe close to where it enters the heat exchanger, and check the connector for signs of corrosion, back-out pins, or looseness. They will reseat any suspect connectors and examine the wiring loom for visible damage such as melted insulation or chafing against metalwork.
- Test the return NTC sensor with a multimeter (engineer task) Gas Safe engineer
The engineer will disconnect the sensor and measure its resistance across the terminals at a known ambient temperature. A healthy NTC thermistor produces a predictable resistance at a given temperature — typically around 10,000 ohms at 25 °C, falling as temperature rises. A short-circuited sensor will show near-zero resistance regardless of temperature, confirming it needs replacement.
- Replace the return temperature sensor if found to be faulty (engineer task) Gas Safe engineer
Genuine Baxi NTC return sensors are readily available from heating parts suppliers and are specific to the Baxi and Potterton range. The engineer will drain down the relevant section of the system or use a valve to isolate the sensor pocket, swap the component, refit the connector, and carry out a full function test to confirm the fault code has cleared and the boiler is operating correctly.
- Arrange PCB investigation if the fault persists after sensor replacement (engineer task) Gas Safe engineer
If a new sensor and sound wiring do not clear the E.00-.05 code, the engineer will turn their attention to the PCB. A faulty input circuit on the control board can produce identical symptoms. PCB repair or replacement is a more involved job and the engineer may need to source the specific board for your boiler model before the repair can be completed.
- Call a Gas Safe registered engineer DIY safe
Beyond a reset, this fault requires a qualified professional. Contact a Gas Safe registered engineer and quote the fault code E.00-.05 along with your boiler make and model so they can arrive prepared with likely replacement parts. You can verify any engineer's registration at gassaferegister.co.uk.
Parts you may need
- Baxi return NTC thermistor (e.g. part 7809939 or equivalent) · from £18
- Wiring loom / sensor harness · from £35
- Baxi PCB (control board) — if sensor and wiring are ruled out · 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 £90–£220, depending on the underlying cause.
Frequently asked questions
Can I fix Baxi E.00-.05 myself just by resetting the boiler?
A reset is always worth trying first, and in rare cases a one-off glitch will clear. However, E.00-.05 is a locking fault caused by a sensor short circuit, so if it returns after one or two resets the underlying component fault has not gone away. Repeated resets without fixing the root cause will not help and may mask a developing problem. Beyond pressing the reset button, the diagnosis and repair — checking wiring, testing the sensor with a multimeter, and replacing parts — must be carried out by a Gas Safe registered engineer.
How much does it cost to fix a Baxi E.00-.05 fault in the UK?
For most homeowners this is a straightforward repair. The NTC return sensor itself costs roughly £15–£30 for a genuine Baxi part, and the job is usually completed within an hour. Factoring in an engineer's call-out and labour, the total typically comes to somewhere between £90 and £220 depending on your location and which engineer you use. Weekend or emergency call-outs will cost more. In the uncommon situation where the fault turns out to be a failed PCB rather than the sensor, costs rise significantly — a replacement control board for a Baxi boiler usually adds £150–£300 to the bill on top of labour.
Will my Baxi warranty cover an E.00-.05 fault?
It may well do. Baxi offers a minimum two-year warranty on new boilers, extending to up to ten years when the boiler was installed by a Baxi-accredited engineer and has been serviced annually. If your boiler is within its warranty period and has been maintained as required, contact Baxi directly before booking an independent engineer, as a warranty repair will be free of charge. Have your installation and service records to hand when you call.
What is the difference between the flow sensor and the return sensor on a Baxi boiler?
Baxi boilers use two NTC thermistors to monitor water temperature. The flow sensor sits on the pipe carrying hot water out of the boiler towards your radiators, while the return sensor monitors the cooler water coming back. The PCB compares both readings to manage boiler output efficiently and to protect the heat exchanger from overheating. The E.00-.05 code relates specifically to the return sensor. A fault on the flow sensor will produce a different error code, so the code on your display is a useful guide to which component the engineer should focus on first.