What E20 means on a Baxi boiler
E20 tells you the boiler no longer trusts its central heating temperature sensor, so it has shut the burner down rather than heat water it cannot measure. In practice this is a sensor or connection fault, and the usual fix is a sensor replacement or a wiring repair by a Gas Safe registered engineer.
Baxi's fault tables word it as a "Central Heating NTC Fault". NTC stands for negative temperature coefficient. The sensor is a thermistor, a small component whose electrical resistance falls as it gets warmer. The control board reads that resistance to work out how hot the water leaving the boiler is, then uses the figure to control the burner and protect the heat exchanger. If the reading is missing or makes no sense, the board cannot run the boiler safely.
The Megaflo System HE IE installation manual (UK Comp No 5129838) puts it plainly: "E20 and E28 indicate faulty components". That separates E20 from codes such as E119 or E133, where an outside cause like low pressure or a gas supply problem is common.
Baxi boilers that use E20
| Boiler range | Document | How the code appears |
|---|---|---|
| Duo-tec Combi 24, 28, 33 and 40 HE A | Installation and Servicing Instructions, UK Comp No 5122323, p.52 | E, then 20 |
| Duo-tec 2 Combi 24, 28, 33 and 40 GA | Installation and Servicing Instructions 7206334-08, error code table p.45 | Listed as E20 |
| Assure Combi 25, 30 and 36 (and LPG) | Assure Combi User Guide, Table 9 | 20, with a symbol beside it |
| Megaflo System 15 to 32 HE IE | Installation and Servicing Instructions, UK Comp No 5129838, section 18.2 | E, then 20 |
The Potterton Gold Combi user guide uses the identical wording for E20, which is no surprise as Potterton is part of Baxi Heating UK. You can find the other Potterton codes we cover from our Baxi fault code hub and the Potterton section.
On the two-character displays, a two-digit code shows as E and then the number, so E20 appears as "E" followed by "20". If you see "E1" followed by two more digits instead, that is a different, three-digit code, which our guide to reading Baxi E1 explains.
What newer Baxi boilers show instead
The 200, 400, 600 and 800 Combi 2 ranges do not use E20. On these boilers the Combi 2/2.1 installation and service manual (7875578-02) lists E.04.01, "Flow temperature sensor short circuited", and E.04.02, "Flow temperature sensor disconnected". They describe the same part failing in two specific ways.
What usually causes E20
- A failed sensor. This is what Baxi's "faulty component" wording points to. Thermistors can fail open or short after years of heating and cooling.
- A poor connection. For its flow sensor codes, the Combi 2/2.1 manual tells engineers to check the connection between the sensor and the PCB. A loose plug or corroded terminal gives the board the same nonsense reading as a dead sensor.
- Damaged wiring. A chafed or broken lead between the sensor and the board has the same effect.
All three sit inside the casing, on or near the heating flow pipe, so none of them is a homeowner job.
What you can safely check
- Read the code carefully. Make sure it is E then 20, not part of a longer code.
- Check the power. Confirm the fused spur that feeds the boiler is switched on and that the display is lit steadily.
- Note the pressure gauge. E20 is not a pressure code, but the reading helps the engineer. The Assure user guide gives a normal range of 1 to 2 bar.
- Try one reset. On a Duo-tec, turn the selector to the Reset position and hold it for 5 seconds. On an Assure, press Reset for about one second; the guide warns that holding it for more than two seconds will not reset the boiler. If E20 returns straight away, the sensor circuit is still faulty and further resets will not help.
- Look underneath. Check for drips from the boiler or its pipes without opening anything, and mention any you see.
Do not remove the casing to look at the sensor. It sits next to hot pipework and mains-voltage parts, and only a Gas Safe registered engineer should work inside a gas boiler.
What the engineer will test
The fault-finding chart in the Duo-tec Combi HE A manual gives the engineer a reference figure: the CH sensor's cold resistance should be "approximately 10kΩ @ 25° C", and the resistance falls as the temperature rises. The engineer will isolate the boiler, measure the sensor with a meter and compare the reading with that figure. If the sensor is within range, they move on to the plug, the terminals and the lead back to the board.
A replacement sensor is a small part, and once it is fitted the engineer should run the heating and confirm that the boiler reads a sensible flow temperature and modulates normally.
Typical repair costs in the UK
| Job | Typical cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Central heating temperature sensor (NTC thermistor) replaced | Typically £100 to £200 |
| Sensor plug, terminal or wiring repair | Typically £100 to £180 |
If the boiler is still under its Baxi warranty, contact Baxi before booking anyone else, as work by a different engineer can affect a claim.
E20 and the other Baxi sensor codes
Baxi uses a small family of NTC codes, and knowing which one you have tells the engineer which part to bring. E20 covers the central heating sensor. E28 is the flue sensor and E50 is the hot water sensor on older combis, while the Assure adds 40 for the central heating return sensor. If your boiler shows more than one of these together, say so when you book, as a shared wiring or board problem becomes more likely than two sensors failing at once.