Andrews E20 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews E20 fault code mean?
The E20 fault code on Andrews boilers (including Andrews Water Heaters commercial units, which share the Baxi Group control platform) indicates that the boiler's control board has detected a problem with the central heating NTC thermistor — the temperature sensor that monitors water temperature in the heating circuit. NTC stands for Negative Temperature Coefficient: as water temperature rises, the thermistor's electrical resistance falls, and the PCB uses these resistance changes to track heating circuit temperature. When the boiler starts up or is running, it continuously checks that the thermistor's readings fall within an expected range. If the signal is absent, out of range, or erratic, the boiler locks out and displays E20 to prevent it from operating unsafely. The boiler will not recover on its own — the underlying sensor or wiring issue must be investigated and resolved by a Gas Safe registered engineer before normal operation can resume.
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
- Faulty NTC thermistor Common
The NTC thermistor itself is the most frequent culprit. Over time, the sensor can drift out of calibration, develop an internal open or short circuit, or simply fail due to age and heat cycling. When this happens, the resistance readings sent to the PCB no longer match expected values for any real water temperature, so the board raises E20 and locks out the boiler.
- Loose or corroded wiring connections Common
The thin wiring harness connecting the thermistor to the PCB is exposed to heat, moisture, and vibration over thousands of operating hours. Connectors can work loose, terminals can corrode, and wires can chafe through their insulation. Any break or high-resistance joint in this circuit produces the same erratic signal as a failed sensor — and can be harder to find without systematic testing.
- Low system water pressure Sometimes
Andrews and Baxi-platform boilers require adequate system pressure — typically 1.0–1.5 bar when cold — for sensors to receive stable, consistent readings. If pressure drops below roughly 0.5 bar, the boiler may generate secondary sensor faults including E20. Check the pressure gauge on the boiler; if it reads below 1 bar, the system may simply need repressurising via the filling loop.
- Limescale build-up affecting the sensor pocket Sometimes
In hard-water areas, limescale can accumulate around the thermistor pocket or heat exchanger, creating an insulating barrier between the water and the sensor. This causes the thermistor to read temperatures that do not reflect the actual water temperature, potentially triggering E20.
- Faulty PCB (control board) Rare
If the thermistor tests within its correct resistance range and all wiring checks out, the PCB itself may be failing to interpret the sensor signal correctly. PCB faults can also be triggered by power surges or water ingress into the boiler casing. This is a less common cause but should be considered after all other possibilities are ruled out.
How to fix it
- Check the boiler's pressure gauge DIY safe
Look at the pressure gauge on the boiler front panel. It should read between 1.0 and 1.5 bar when the system is cold. If the needle is below 1 bar, low pressure may be contributing to the fault. You can top up pressure by using the filling loop — a silver or chrome braided hose usually located beneath the boiler — until the gauge reads around 1.2 bar, then close both filling loop valves firmly. If you are unsure how to locate the filling loop on your Andrews unit, consult the user guide or ask your engineer.
- Reset the boiler once DIY safe
After checking pressure, attempt a single reset using the boiler's reset button or control panel. Hold for the duration specified in your user guide (typically 3–5 seconds). If E20 returns within a short time, do not keep resetting — repeated resets will not fix a sensor or wiring fault and may mask the problem from a diagnosing engineer.
- Check your gas supply DIY safe
Confirm that other gas appliances in the property (cooker, gas fire) are working normally. If they are not, contact your gas supplier rather than an engineer, as the issue lies with the supply rather than the boiler.
- 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 ambient temperature, comparing the reading against the manufacturer's resistance–temperature curve. A reading that is open-circuit, short-circuit, or significantly out of spec confirms the thermistor has failed and needs replacement.
- Replace the NTC thermistor if confirmed faulty Gas Safe engineer
The engineer will fit a like-for-like replacement thermistor designed for the Andrews/Baxi-platform boiler. After fitting, they will clear the fault, run the boiler through a full heating cycle, and verify that temperature readings are stable before signing off the repair.
- Inspect and repair the wiring harness Gas Safe engineer
If the thermistor tests correctly, the engineer will trace the wiring from the sensor back to the PCB, checking connectors for corrosion, loose pins, and damaged insulation. Any faulty section of the harness will be repaired or replaced and connections re-seated securely.
- Assess and replace the PCB if the fault persists Gas Safe engineer
If both the thermistor and wiring check out, the PCB is the remaining suspect. A Gas Safe engineer will source the correct replacement board for the Andrews model, fit it, and carry out all required safety checks and functional tests before the boiler is returned to service.
- Call a Gas Safe registered engineer Gas Safe engineer
If you have not already done so, contact a Gas Safe registered engineer to diagnose and repair this fault. You can verify an engineer's registration at gassaferegister.co.uk. Do not attempt to access internal boiler components yourself — doing so is unsafe and will invalidate your warranty.
Parts you may need
- NTC thermistor (CH sensor) — Baxi/Andrews compatible · from £18
- Wiring harness / thermistor connector loom · from £35
- PCB (printed circuit board) — model-specific · 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 £100–£280, depending on the underlying cause.
Frequently asked questions
Can I reset my Andrews boiler to clear the E20 fault?
A single reset is worth trying after you have checked the system pressure is correct. However, E20 is caused by a sensor or wiring fault rather than a one-off glitch, so a reset alone is very unlikely to clear it for more than a few minutes. If it returns after one reset, stop resetting and call a Gas Safe engineer — repeatedly resetting a locked-out boiler will not fix the underlying problem.
How much does it cost to fix an Andrews E20 fault in the UK?
For the most common repairs — replacing the NTC thermistor or repairing a wiring connection — most homeowners pay in the region of £100 to £280 including parts and labour. If the PCB turns out to be faulty, the board itself can cost £180–£300 or more depending on the model, pushing the total bill higher. Prices also tend to be somewhat higher in London and the South East than in the rest of the UK.
Is the E20 fault dangerous?
E20 is a protective lockout rather than an immediate safety emergency — the boiler has shut itself down to avoid operating with unreliable temperature data. However, you should not attempt to bypass the lockout or keep resetting in the hope it clears. Leave the boiler off and arrange for a Gas Safe engineer to inspect it. If you smell gas at any point, do not attempt any work yourself: leave the property, open windows, and call the National Gas Emergency Service on 0800 111 999.
How can I prevent the E20 fault from coming back?
Annual servicing by a Gas Safe registered engineer is the single most effective preventative measure — an engineer will check sensor resistance values, inspect wiring connections, and clean the system before minor issues become faults. In hard-water areas, a scale reducer or inhibitor added to the heating system can slow limescale build-up around the sensor pocket. Keeping system pressure topped up to around 1.2 bar when cold also helps ensure sensors operate correctly.