Baxi E20 fault codewhat it means and how to fix it
Central heating flow thermistor open or short circuit. Baxi list E20 as a faulty component on every generation, with no householder reset; a cheap sensor swap for an engineer.
Take care
Get a Gas Safe registered engineer to look at your Baxi E20
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What E20 means
E20 is the first entry in the Baxi table on every three-digit display: "Central Heating NTC Fault" on the selector-switch Duo-tec HE and Platinum, "Heating flow NTC fault" on the 600, 800 and Compact, and "Central heating NTC sensor fault — short circuit/open circuit" on Baxi's professional page. The thermistor clipped to the flow pipe leaving the heat exchanger is how the board knows the water temperature, and when it reads as a broken wire or a dead short the board will not run the burner on a temperature it cannot see.
Baxi are consistent about what the householder can do: nothing. The Duo-tec HE A manual says "E20, E28 & E50 indicate faulty components", the Platinum user guide adds "you should make a note of the displayed error code and contact your installer or service engineer", and on the digital-display boilers E20 is shown with the spanner symbol, which "cannot be reset by the user". Baxi's fault-finding gives the engineer the test: "Temperature sensors faulty. Cold resistance approximately 10 kΩ at 25 °C (CH sensor); resistance reduces with increase in temp. Replace sensor."
On the 600 and 800 the flow and return sensors are the same part, 7207471. The one thing worth knowing before the engineer arrives is that a sensor which fails intermittently — a corroded plug, a wire chafed where it passes the casing — produces an E20 that comes and goes with vibration or temperature, and that is still a sensor-circuit fault, not a board fault. The board itself is the last suspect on Baxi's flow, reached only after the sensor and its wiring have been proven good.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Flow thermistor failed | Open or short circuit. Baxi's test is about 10 kΩ at 25 °C, falling as the sensor warms; a reading of zero, infinity or something far from the curve condemns it. Part 7207471 on the 600 and 800. | Common |
| Plug or wiring | A corroded two-pin connector or a lead chafed against the case gives an intermittent open circuit. The sensor tests fine on the bench and E20 keeps returning. | Common |
| Sensor dislodged | On boilers where the sensor clips to the pipe, one that has fallen off reads the air. This more often shows as E125 or E154 than E20, but a sensor hanging by its lead can short against the chassis. | Occasional |
| PCB input | Baxi's last step. The board's sensor input fails and reports open circuit on a good sensor. Rare, and ruled out by testing the sensor at the board connector. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£150
Time on site
30 minutes
Flow sensor (Baxi 7207471 on the 600 and 800) fitted £80–£130. Connector repair within a call-out. PCB if genuinely at fault £250–£400.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Baxi / Potterton / Main NTC temperature sensor (flow, return or DHW)
7207471£18–£45Authorised Baxi parts distributor
Price checked 28 September 2026
Baxi / Potterton / Main main PCB
£150–£320Authorised Baxi parts distributor
Price checked 28 September 2026
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Related Baxi fault codes
- E.04.01Get it checkedFlow temperature sensor short circuit (E.04.01) or disconnected (E.04.02) on the Baxi 200/400 Combi and Combi 2. A sensor or its plug; Baxi's remedy is check the connection, then the sensor.Related: temperature sensors
- E154Get it checkedFlow and return sensor temperature test failed — the two readings do not make sense together. Usually a sensor or its plug, sometimes poor circulation. Engineer diagnosis.Related: temperature sensors
- E40Get it checkedReturn thermistor open or short circuit on the digital-display Baxis. Spanner symbol, no householder reset, and the same inexpensive sensor as the flow side.Related: temperature sensors
- E50Usually simple to fixHot water thermistor fault on the selector-switch Baxis, shown as E then 50. Heating usually carries on; hot water is unreliable until the sensor is changed. Digital-display combis show E321 instead.Related: temperature sensors
- E321Usually simple to fixDomestic hot water thermistor fault on the digital-display Baxis (shown on some as 163). Heating normally continues; tap temperature control is lost until the sensor is replaced.Related: temperature sensors
- E133Get it checkedBaxi's ignition lockout: "the gas supply has been interrupted, ignition has failed or the flame has not been detected". Check the gas is on, reset once. If it returns, stop.Related: same brand
Own a Baxi boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.