Main E20 Fault Code: Causes, Fixes & Repair Costs
What does the Main E20 fault code mean?
The E20 fault code on a Main boiler signals a problem with the central heating NTC (Negative Temperature Coefficient) thermistor — the temperature sensor that monitors water temperature in the boiler's flow pipe. When the boiler's control board receives readings from this sensor that are outside the expected range, or receives no signal at all, it triggers the E20 code and locks the boiler out as a precaution. Main boilers share their control architecture with Baxi and Potterton models (all part of the same group), so the E20 code and its causes are essentially identical across all three brands. The fault itself is not immediately dangerous, but without reliable temperature data the boiler cannot regulate heat safely, which is why it shuts down until the issue is resolved.
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 is a small resistive sensor clipped onto the flow pipework inside the boiler. Over time it can degrade, give drifting resistance readings, or fail completely. Because the boiler checks sensor resistance on every startup, even a slightly out-of-range reading is enough to lock it out with an E20. This is by far the most common cause and is usually a straightforward repair for an engineer.
- Loose, corroded, or damaged wiring Common
The wiring harness that connects the thermistor to the PCB can develop loose plugs, corroded terminals, or chafed insulation — especially on older boilers or those in damp locations. A poor connection can produce intermittent or absent signals that the PCB interprets as a sensor fault, even if the thermistor itself is perfectly healthy.
- Sludge or limescale insulating the sensor Sometimes
In hard-water areas or systems without adequate inhibitor, limescale and magnetite sludge can coat the section of pipework where the thermistor sits. This acts as insulation, causing the sensor to read lower temperatures than are actually present and sending the PCB out-of-range data. Boilers without a magnetic filter or those that have never had a powerflush are most vulnerable.
- Faulty PCB (printed circuit board) Rare
If the thermistor and all its wiring test within specification but the E20 fault persists, the PCB itself may have failed to process the sensor signal correctly. PCB faults are relatively uncommon but do occur on older units or after electrical surges. Diagnosis requires an engineer with specialist test equipment.
How to fix it
- Reset the boiler once or twice DIY safe
Locate the reset button or selector switch on your Main boiler (consult your user manual — on many models you turn the dial to the 'R' position and hold for 5 seconds). Attempt a reset no more than two or three times. If the E20 code returns immediately or after a short time, a reset alone will not fix the fault — the boiler is detecting a genuine sensor issue every time it runs its startup checks.
- Check that your gas supply is working DIY safe
Make sure other gas appliances in your home (hob, gas fire) are working normally. If they are, the boiler's gas supply is fine. If nothing is working, contact your gas supplier — this is a supply issue rather than a boiler fault.
- Check and top up system pressure if low DIY safe
A low-pressure warning alongside E20 is a separate issue but worth checking. The pressure gauge on your boiler should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, top up via the filling loop following your boiler's user manual. Note that low pressure alone will not cause E20, so if pressure is fine, move on.
- Contact a Gas Safe registered engineer to test the thermistor Gas Safe engineer
An engineer will use a multimeter to measure the thermistor's resistance and compare it against Main's published specifications for that sensor at various temperatures. A reading outside the expected range confirms the sensor has failed and needs replacing. This is a relatively quick job and the most common resolution for E20.
- Engineer to inspect wiring and connections Gas Safe engineer
If the thermistor tests within specification, the engineer will check the wiring harness from the sensor to the PCB — looking for loose plugs, corroded pins, or damaged insulation. Repairing or replacing the harness is generally straightforward and inexpensive.
- Engineer to assess and treat for sludge or limescale Gas Safe engineer
If sludge or scale is suspected, the engineer may recommend a powerflush or chemical descale of the system, followed by the addition of a quality inhibitor and fitting of a magnetic filter if one is not already present. This protects the replacement thermistor and other components from the same fate.
- Engineer to assess the PCB if all other components are healthy Gas Safe engineer
Should the thermistor, wiring, and system condition all check out, the engineer will test the PCB. If it is found to be faulty, replacement is required. PCB replacement is the most expensive outcome for this fault — see the FAQ below for typical costs.
Parts you may need
- Central heating NTC thermistor (flow sensor) · from £15
- Wiring harness / thermistor connector · from £25
- Magnetic system filter (e.g. Adey MagnaClean) · from £75
- PCB (printed circuit 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 £100–£280, depending on the underlying cause.
Frequently asked questions
Can I fix the Main E20 fault myself?
The safe homeowner steps are limited to resetting the boiler (up to two or three times) and checking your gas supply and system pressure. Beyond that, diagnosing and replacing the thermistor, repairing wiring, or touching the PCB all require a Gas Safe registered engineer. Attempting internal repairs yourself risks electrical hazards, could invalidate any remaining warranty, and may create an unsafe installation.
How much does it cost to fix an E20 fault on a Main boiler?
Most people with an E20 fault pay between £100 and £280 in the UK. A wiring repair at the lower end might cost £100–£150 including call-out, while a thermistor replacement with labour typically comes in at £130–£250. If the PCB is at fault — which is uncommon — expect to pay £300–£500 or more for parts and labour. It is always worth checking whether your boiler is still under its manufacturer's warranty before booking a repair, as PCB and sensor replacements are often covered.
Why does my Main boiler keep coming back with E20 after a reset?
Resetting the boiler clears the lockout temporarily, but the boiler runs sensor checks every time it fires up. If the thermistor is genuinely out of specification, or wiring to it is damaged, the fault will return within minutes of a reset. Repeated resets without investigating the root cause will not resolve E20 — you need a Gas Safe engineer to test the sensor and associated wiring.
Is the E20 fault the same on Baxi and Potterton boilers?
Yes. Main is part of the Baxi Heating group, and Main, Baxi, and Potterton boilers share the same underlying control systems and fault-code definitions. An E20 on any of these brands points to the same central heating NTC thermistor circuit, and the diagnostic approach and repair costs are essentially identical across all three.