What E50 means
E50 is a sensor fault on the hot water side of a Baxi combi. The boiler's board is no longer getting a sensible reading from the hot water temperature sensor, so it cannot control how hot the water to your taps is. The fix is normally a replacement sensor or a repaired connection, carried out by a Gas Safe registered engineer.
The sensor is an NTC thermistor, whose resistance drops as the water passing it warms up. A combi heats tap water on demand, and the board uses this reading to adjust the burner so the water arrives at the temperature you have set. Without it, the board has no way of knowing whether the water is lukewarm or scalding.
Baxi's Duo-tec Combi HE A installation and servicing instructions (UK Comp No 5122323) list E50 as "Hot Water NTC Fault" and add that it is shown on the display as E and then 50.
Which boilers show E50, and which show 321
This is the part that confuses people searching for the code. Baxi changed the numbering on its newer ranges, so the same fault has two codes depending on the age of your boiler.
| Boiler | Document | Code for a hot water sensor fault |
|---|---|---|
| Baxi Duo-tec Combi 24, 28, 33, 40 HE A | Installation and Servicing Instructions, p.52 | E50 (shown as E, then 50) |
| Potterton Promax Combi HE Plus A | User Guide, UK Comp No 5114443, section 5.3 | E50 |
| Potterton Gold Combi 24, 28, 33 | User Guide, p.8 | E50 |
| Baxi Duo-tec 2 Combi GA | Installation and Servicing Instructions 7206334-08 | 321 |
| Baxi Assure Combi 25, 30, 36 | Assure Combi User Guide, Table 9 | 321, "Hot water NTC fault" |
| Potterton Promax Ultra | Promax Ultra fault table | 321 |
E50 does not appear in the Duo-tec 2, Assure or Promax Ultra tables. If your boiler shows 321, everything on this page still applies. We did not find E50 in the Megaflo System HE IE table either, which fits with a system boiler heating a separate hot water cylinder rather than tap water directly.
Because these older displays only have two characters, it helps to be sure what you are reading. "E" followed by "50" is E50. "E1" followed by two digits is a three-digit code, which our Baxi E1 guide explains how to read.
Likely causes
Every source we checked names the same cause: a faulty hot water sensor. In practice an engineer will treat it as one of three things:
- the thermistor itself has failed, which is the most common outcome for an NTC code;
- the plug on the sensor has worked loose or corroded;
- the lead from the sensor back to the control board is damaged.
All three are inside the casing on the hot water side of the boiler, so this is not a code you can fix by topping up pressure or bleeding radiators.
What you can check before calling someone
- Confirm the code. Watch the display change and make sure you have E then 50, or 321 on a newer model.
- Test the heating side. Turn the heating up at the thermostat and note whether the radiators warm. Whatever you find is useful information for the engineer, and our guide to no hot water when the heating works covers other causes of that pattern.
- Check more than one hot tap. Note whether every hot tap is affected, and whether the code appears as soon as you open a tap.
- Check the power. Make sure the boiler's fused spur is on and the display is lit.
- Try one reset. Turn the selector to Reset and hold it for 5 seconds on a Duo-tec, Promax or Gold. If E50 comes straight back, the sensor circuit is still faulty.
Do not take the cover off to reach the sensor. It sits next to hot pipework, gas components and live wiring, and work inside a gas boiler is for a Gas Safe registered engineer only.
What the engineer will do
The engineer will isolate the boiler, open it and test the hot water sensor's resistance, checking that it changes as expected with temperature. The Duo-tec fault-finding chart gives cold resistance figures for the heating and flue sensors but not for the hot water sensor, so the engineer will work from Baxi's data for that part. They will also check the plug and the lead to the board. Once a new sensor is fitted, they should run a hot tap and confirm that the outlet temperature settles at the setting on the boiler.
Typical repair costs in the UK
| Job | Typical cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Hot water temperature sensor (NTC thermistor) replaced | Typically £100 to £200 |
| Sensor connector or wiring repair | Typically £100 to £180 |
If your Baxi or Potterton is still under warranty, contact Baxi first so the repair does not affect a claim.
What to tell the engineer when you book
A few details help the engineer arrive with the right part and avoid a second visit:
- the exact code, E50 or 321, and the model name from the front of the boiler;
- whether the radiators still heat up when the code is showing;
- whether the code appears every time a hot tap is opened or only now and then;
- how many times you have reset the boiler and whether the code came straight back.
Related Baxi sensor faults
E50 belongs to the same group as E20 for the central heating sensor and E28 for the flue sensor. If more than one of them appears, tell the engineer, because a shared connection or board fault becomes more likely than several sensors failing together. Our Baxi fault code hub lists every Baxi code we cover.