An E20 on a Baxi boiler points to a fault with the central heating primary flow temperature sensor — not low pressure, despite what some older web pages claim. Here's what it really means and what to do.
On Baxi combi and system boilers, the E20 fault code indicates a problem with the central heating primary flow temperature sensor — a component often called the flow NTC thermistor. The boiler has either lost a sensible signal from this sensor or is reading a temperature that's outside the range it expects, so it shuts the burner down and shows the code.
The primary flow sensor sits on the pipe carrying hot water out of the heat exchanger and into your central heating circuit (the "flow"). It's a small thermistor whose electrical resistance changes with temperature, letting the boiler's control board read exactly how hot the water leaving the boiler is.
The board relies on that reading constantly: it uses it to modulate the burner up and down so you get a steady flow temperature, and crucially to spot dangerous overheating. If the sensor reads wildly wrong, goes open-circuit, or short-circuits, the board can no longer trust it — so it errs on the side of safety and locks out with an E20 rather than firing blind.
Because this is a sealed-circuit and electronics fault rather than something on the water side, there is very little a homeowner can safely do — see below.
The only homeowner-safe step for an E20 is a single front-panel reset. Press and hold the reset button (often marked with a reset symbol or "R") for a few seconds, then give the boiler a minute to attempt to refire. Occasionally a one-off glitch will clear and not come back.
Don't be tempted to top up the pressure, bleed radiators, or open the casing chasing an E20 — none of those address a temperature-sensor fault, and anything behind the panel is engineer-only. (Pressure and bleeding are worth knowing for other codes — see our guides on low boiler pressure and bleeding a radiator.)
A Gas Safe registered engineer will diagnose the E20 properly rather than just swapping parts. Typically they will:
If you don't have cover and pay for the repair yourself, the figures below are indicative ranges for 2026 to help you budget. Actual prices vary by region, the engineer and exactly what's wrong, so always get a quote first.
| Job | What's involved | Indicative cost |
|---|---|---|
| Diagnostic call-out | Engineer attends and identifies the fault | £70–£120 |
| Wiring / connector repair | Reseat or repair a loose or corroded connector | £80–£150 |
| Flow temperature sensor replacement | New NTC sensor fitted and tested | £120–£250 |
| Control board (PCB) replacement | Rarer — only if the board is at fault | £350–£600 |
A single sensor swap is one of the cheaper boiler repairs, but a PCB replacement can cost more than a year of cover. That trade-off is why many people weigh up a monthly plan — our guides on the best boiler cover and cheap boiler cover explain how to compare what you actually get for the price, and is boiler cover worth it? walks through the maths.
No. This is a persistent myth from some older web pages. On Baxi boilers E20 is a central heating primary flow temperature sensor (NTC) fault. If your pressure is a healthy 1–1.5 bar when cold and you still see E20, topping up won't fix it.
There's very little safe DIY for an E20. You can try a single front-panel reset in case it's a one-off glitch. Beyond that, the sensor and its wiring sit inside the boiler on the sealed heating circuit, so a Gas Safe registered engineer needs to test and replace it. Don't open the casing.
The lockout is the boiler protecting itself because it can't trust the temperature reading, so it's a safety feature working as intended. It isn't an immediate danger like a gas smell, but you'll likely have no reliable heating or hot water until it's repaired. If you ever smell gas, leave it alone and call the National Gas Emergency line on 0800 111 999.
A code that returns after a reset means the underlying cause is still there — usually a failed flow sensor or a loose or corroded connector to it, and occasionally a control board issue. One reset to clear a glitch is fine; repeated resets won't cure a hardware fault, so book an engineer.
Most boiler breakdown plans are designed to cover exactly this kind of fault — you call the provider and they send an approved engineer rather than you paying a one-off bill. Always check the policy excess and any exclusions before you buy; our comparison shows cover levels side by side.
If your Baxi keeps throwing fault codes, a monthly plan can mean an approved engineer instead of a surprise bill. Compare indicative prices and cover levels across our selected panel.
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