Andrews E18 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews E18 fault code mean?
The E18 code on Andrews boilers and water heaters covers two closely related situations depending on the model. On most Andrews boilers sharing a common control platform, E18 signals a temperature sensor error — specifically, the boiler's electronics have detected an unusually rapid temperature change on the NTC (Negative Temperature Coefficient) sensor, typically exceeding 30°C in a very short period. This suggests the sensor itself is faulty or cracked, or that something is causing abnormal temperature swings in the system (such as poor water flow or an airlock). On certain Andrews commercial water heaters — notably the CWH30/200 range and models using the Andrews CWH Master Board — E18 instead points to an internal RAM fault on the control board, meaning the board's own memory or processing has encountered an error. In either case, the boiler will lock out for safety and require attention before it will fire again.
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 or cracked NTC temperature sensor Common
The NTC sensor monitors water temperature and feeds that data to the control board. If the sensor has cracked, degraded, or failed internally, it can send wildly inaccurate readings — including a sudden apparent temperature jump of more than 30°C — which triggers E18. This is the single most frequent cause across Andrews boiler ranges.
- Loose, corroded, or damaged sensor wiring Common
Even if the sensor itself is sound, the cable connecting it to the PCB can suffer from loose terminals, corrosion, or physical damage. A poor connection produces erratic resistance readings that mimic a faulty sensor and can generate the E18 lockout without any sensor replacement being needed.
- Restricted water flow causing genuine temperature spikes Sometimes
If the circulating pump is failing, an airlock has developed, or there is a blockage reducing flow through the heat exchanger, water can overheat locally and cause a real temperature spike rather than a sensor reading error. The boiler registers this as an abnormal temperature change and locks out with E18.
- Internal RAM or PCB fault (CWH Master Board models) Sometimes
On Andrews commercial water heaters fitted with the CWH Master Board, E18 is specifically a control error linked to the board's internal RAM. If the fault reappears immediately after a reset and sensor checks are clear, the PCB itself may be failing. Parts for specialist commercial Andrews units can be harder to source than those for mainstream residential brands.
- Intermittent electrical interference or supply issue Rare
Voltage fluctuations or interference on the electrical supply can occasionally cause the control board to log a spurious E18. This is rare but worth noting if the fault appears once and does not return after a single reset.
How to fix it
- Reset the boiler and observe whether the fault clears DIY safe
Locate the reset button on the boiler's control panel (consult your user manual if needed) and press it. Hold for the duration indicated in your manual — usually around three seconds. If E18 clears and the boiler fires normally, monitor it closely over the next few hours. If E18 returns, do not continue resetting repeatedly; two or three attempts is the sensible limit before calling for professional help.
- Check that the gas supply and other utilities are functioning normally DIY safe
Confirm that other gas appliances in the property are working, that your gas meter is not in emergency shut-off mode, and that the electrical supply to the boiler is live. Rule out any external supply issue before assuming a boiler fault.
- Contact a Gas Safe registered engineer to inspect the NTC sensor and its wiring Gas Safe engineer
A qualified engineer will use a multimeter to measure the resistance of the NTC sensor against the manufacturer's expected values at a known water temperature. They will also inspect the sensor cable for loose terminals, corrosion, or damage at the connector ends. If the sensor is out of specification or physically cracked, it should be replaced and a reset performed. This is not a task for a homeowner — accessing the sensor requires removing the boiler casing and working near live components.
- Have the engineer investigate water flow and pump operation Gas Safe engineer
If the sensor and its wiring check out as healthy, the engineer should verify that the circulation pump is running correctly and that there are no airlocks or blockages reducing flow through the heat exchanger. A pump that is running slowly or intermittently can cause localised overheating that genuinely triggers E18 rather than a sensor error.
- Request a PCB assessment if the fault persists after sensor and flow checks Gas Safe engineer
On CWH Master Board models especially, a recurring E18 that survives sensor replacement may indicate an internal RAM fault on the control board. PCB diagnosis and replacement must only be carried out by a qualified engineer. Be aware that Andrews commercial PCBs can be specialist items and may need to be ordered — factor this into your timeline. Always get a written, fixed-price quote before authorising PCB replacement, and verify the engineer's Gas Safe registration at gassaferegister.co.uk.
Parts you may need
- NTC flow temperature sensor · from £35
- NTC return temperature sensor · from £35
- Sensor wiring harness / connector loom · from £25
- Circulation pump · from £90
- Andrews CWH Master Board PCB · 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 £120–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Andrews E18 fault myself?
The only safe steps for a homeowner are pressing the reset button (up to two or three times) and checking that the gas supply is on. Everything beyond that — removing the boiler casing, testing the NTC sensor, inspecting wiring, or replacing any component — must be carried out by a Gas Safe registered engineer. Working on a boiler without registration is illegal under UK gas safety regulations and could invalidate your home insurance.
How much does it cost to fix an Andrews E18 fault in the UK?
Most people with an E18 fault pay somewhere between £120 and £350 all in. A Gas Safe engineer's call-out and diagnostic typically runs £60–£100, and an NTC sensor replacement including labour usually adds up to £100–£200 total. If the control board (PCB) turns out to be the problem, expect to pay £250–£400 or more for parts and labour. Andrews is a specialist commercial brand, so parts may need to be ordered rather than collected from a local merchant, which can add time and occasionally cost. Prices in London and the South East tend to run 15–25% higher than the national average.
Why does my Andrews boiler keep showing E18 after a reset?
A fault that clears on reset but keeps returning usually means the underlying issue has not been resolved — the reset simply clears the lockout temporarily. The most likely culprits are an NTC sensor that is drifting in and out of an acceptable resistance range, an intermittent wiring connection at the sensor plug, or a circulation problem causing real temperature spikes. A Gas Safe engineer can identify which of these is responsible by measuring the sensor and checking flow while the boiler is running.
Is E18 the same fault on all Andrews models?
Not quite. On most Andrews boilers sharing a common electronics platform, E18 means a temperature sensor error — the boiler has detected a temperature change on the NTC sensor that exceeds 30°C, suggesting the sensor is cracked or faulty. However, on Andrews commercial water heaters using the CWH Master Board (such as the CWH30/200), E18 is specifically listed as a Control Error indicating an internal RAM fault on the board itself. If your boiler is a commercial Andrews unit and the sensor tests healthy, the control board should be investigated next.