Baxi E20 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E20 fault code mean?
The E20 fault code on a Baxi boiler indicates that the central heating NTC thermistor has reported a problem. The thermistor is a small temperature-sensing resistor fitted to the boiler's flow or return pipework; rather than measuring temperature directly, it sends resistance values to the PCB, which interprets them as water temperature readings. When those resistance readings fall outside the expected range — or stop arriving altogether — the boiler locks out and displays E20 as a safety precaution. Until the underlying sensor or wiring issue is resolved, the boiler will not fire for heating or hot water.
General guidance only — not a substitute for professional advice. 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
- Faulty NTC thermistor Common
The NTC thermistor degrades over time due to repeated heat cycling, moisture ingress, or simple component wear. When its resistance drifts outside the manufacturer's specified range, the PCB can no longer trust the readings and triggers the E20 lockout. This is by far the most frequent cause of this fault code.
- Loose or damaged wiring connections Sometimes
The wiring harness linking the thermistor to the PCB can develop loose terminals from vibration, or corroded and broken wires — particularly in older properties or boilers installed in damp cupboards. Even a partial break in continuity is enough to produce erratic resistance readings and trigger E20.
- Faulty PCB Rare
If the control board itself cannot correctly interpret the resistance signal from an otherwise healthy thermistor, it may incorrectly flag an E20 fault. PCB failure can result from power surges, age-related component burnout, or moisture damage to the board's circuits. This is an uncommon cause but should be considered once the thermistor and wiring have been confirmed good.
How to fix it
- Attempt a single boiler reset DIY safe
On most Baxi models, turn the selector dial to the reset (R) position and hold it for approximately 5 seconds, then release. The display should revert to the current temperature reading. Only attempt this once or twice — if the E20 fault reappears promptly, further resets will not resolve the underlying problem and you should move on to calling an engineer.
- Check for any other obvious issues (gas supply, pressure) DIY safe
While waiting for an engineer, confirm your gas supply is on at the meter and that other gas appliances in the property are working normally. Check the boiler's pressure gauge — it should sit between 1 and 1.5 bar. If it is below 1 bar, top it up via the filling loop. Neither of these actions will fix an E20 fault, but ruling them out avoids an unnecessary call-out charge.
- Have a Gas Safe engineer test the thermistor with a multimeter Gas Safe engineer
An engineer will disconnect the thermistor and measure its resistance at a known temperature, comparing the reading against Baxi's specification chart. A reading significantly outside tolerance confirms the sensor has failed and needs replacing. This is not a DIY task — the boiler must be isolated, and the component is internal to the appliance.
- Replace the NTC thermistor if confirmed faulty Gas Safe engineer
Thermistor replacement is a relatively straightforward job for a competent engineer and is the fix in the majority of E20 cases. The engineer will fit a manufacturer-approved replacement sensor, re-check all connections, and carry out a functional test to confirm the fault has cleared.
- Inspect and repair the wiring harness if the thermistor tests good Gas Safe engineer
If the thermistor resistance is within specification, the engineer will trace the wiring from the sensor back to the PCB, looking for corroded terminals, chafed insulation, or broken conductors. Damaged sections of the harness will be repaired or the loom replaced as necessary.
- Consider PCB replacement if all other components check out Gas Safe engineer
Should the thermistor and all associated wiring prove to be in good condition but the fault persists, the PCB itself is likely misreading the signal. PCB replacement is a more involved and costlier repair; the engineer will confirm this diagnosis before proceeding and will carry out the required safety checks afterwards.
- Call a Gas Safe registered engineer if the fault has not already been resolved Gas Safe engineer
If the boiler remains in lockout after your single reset attempt, contact a Gas Safe registered engineer to carry out a proper diagnosis. You can verify an engineer's registration at gassaferegister.co.uk. Do not continue attempting resets or attempt to access internal components yourself.
Parts you may need
- Baxi NTC thermistor (flow or return) · from £20
- Wiring harness / sensor loom · from £35
- Baxi PCB (control board) · from £180
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–£300, depending on the underlying cause.
Frequently asked questions
Can I fix the Baxi E20 fault myself?
The only DIY step you can safely take is a single boiler reset. Testing and replacing the NTC thermistor, repairing wiring, or swapping the PCB are all tasks for a Gas Safe registered engineer. Working on internal boiler components without the correct qualifications is illegal under UK gas safety regulations and will void your warranty.
How much does it cost to fix a Baxi E20 fault?
For the most common repair — NTC thermistor replacement including labour — expect to pay roughly £120–£250. If the wiring harness needs attention, costs are similar. Most people with an E20 fault end up paying in that range. A PCB replacement is considerably more expensive, typically £300–£500 or more depending on the Baxi model; however, this is only needed in a minority of cases and your engineer should confirm the diagnosis before agreeing to that repair.
My Baxi boiler keeps coming back to E20 after I reset it — what does that mean?
A fault that returns repeatedly after resetting points to an underlying component failure rather than a one-off glitch. Most likely the NTC thermistor has degraded and is giving consistently out-of-range readings. Continuing to reset the boiler will not resolve the problem and could mask a developing issue — arrange for a Gas Safe engineer to diagnose and repair it properly.
Will the Baxi E20 fault affect my hot water as well as heating?
Yes. Because the E20 fault causes a full boiler lockout, both central heating and domestic hot water are typically unavailable until the fault is cleared. If your Baxi has a separate hot water cylinder or thermal store, you may retain some stored hot water temporarily, but the boiler itself will not fire until the thermistor issue is resolved.