Keston E33 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E33 fault code mean?
The E33 code points to a problem with the domestic hot water (DHW) thermistor circuit — the sensor responsible for monitoring the temperature of your hot water. The precise nature of the fault differs slightly between models: on the Keston C36 Combi, E33 indicates an open circuit (a broken or disconnected connection to the DHW sensor), whereas on the Keston Qudos 28s, E33 signals a closed or short circuit (wires that are touching or bridged when they shouldn't be). In both cases, the boiler's control board is receiving an abnormal signal — or no signal at all — from the DHW thermistor, so it shuts down hot water production as a precaution. The boiler will not resume normal operation until the fault is cleared and the reset button is pressed; simply switching the boiler off and back on will not clear it.
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
- Loose or disconnected wiring connector Common
Vibration from normal boiler operation can gradually work a wiring connector free from the DHW thermistor. This is the most frequent trigger — the sensor itself may be perfectly healthy, but the break in the circuit produces an open-circuit reading on the C36 or causes abnormal readings on other models.
- Damaged or corroded wiring Common
Wires leading to the thermistor can fray, corrode, or develop an internal break over time, particularly in environments with condensation. On the Qudos 28s, damaged insulation can cause two wires to contact each other, producing the short-circuit condition E33 reports.
- Failed DHW thermistor Sometimes
The thermistor itself can degrade or burn out, sending a resistance reading outside the expected range to the control board. This is a relatively inexpensive component to replace, and failure becomes more likely as the boiler ages.
- PCB (control board) fault Rare
If the wiring and thermistor both test correctly, the PCB may be misinterpreting the thermistor signal. A faulty PCB is the least likely root cause but becomes the default suspect once all other components have been ruled out.
How to fix it
- Press the boiler reset button DIY safe
Locate the reset button on your Keston boiler's fascia and hold it for the duration specified in your user guide (typically around three seconds). If the E33 code clears and the boiler fires normally, monitor it over the next few hours. If the code returns, do not keep repeatedly resetting — two or three attempts is the safe limit before calling an engineer.
- Check the wiring connections to the DHW thermistor Gas Safe engineer
A Gas Safe registered engineer will open the boiler casing and visually inspect the connector and wiring to the DHW sensor. On the C36, the focus is on ensuring the connector is firmly seated; on the Qudos 28s, the engineer will also check for any signs of short-circuiting such as chafed insulation or bridged wires. Loose connectors are often simply reseated; damaged wiring will be repaired or replaced.
- Test and replace the DHW thermistor if necessary Gas Safe engineer
If the wiring is intact and properly connected, the engineer will use a multimeter to measure the thermistor's resistance and compare it against the expected values for your model. A reading outside the acceptable range confirms the sensor has failed, and a replacement thermistor will be fitted. This is one of the more affordable boiler component replacements.
- Investigate the PCB if the fault persists Gas Safe engineer
Should both the wiring and the thermistor pass testing, the engineer will assess the control board. PCB diagnosis and replacement is specialist work — the engineer will confirm whether the board is at fault before committing to a replacement, as it is the most costly outcome for this fault code.
- Call a Gas Safe registered engineer Gas Safe engineer
If resetting the boiler does not resolve E33, or the code returns shortly afterwards, contact a Gas Safe registered engineer. All internal boiler work — including wiring inspection, sensor testing, and PCB assessment — must be carried out by a registered professional. You can verify an engineer's credentials at GasSafeRegister.co.uk.
Parts you may need
- DHW thermistor (NTC sensor) · from £25
- Thermistor wiring harness · from £35
- PCB (control board) · from £220
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
Why does E33 mean something different on the C36 versus the Qudos 28s?
Keston use the same fault code across different product families, but the specific circuit condition it describes varies by model. On the C36 Combi, E33 means the DHW sensor circuit is open — essentially a break in the connection. On the Qudos 28s, it means the circuit is closed or shorted — wires making contact where they shouldn't. The underlying component involved (the DHW thermistor) is the same; it's the nature of the electrical fault that differs. Always check your specific model's documentation if you're unsure which applies to you.
Will I still have central heating while E33 is showing?
In most cases, yes — E33 relates specifically to the domestic hot water thermistor, so space heating may continue to function normally. However, if the boiler has locked out entirely, both heating and hot water will be unavailable until the fault is resolved and the boiler is reset.
Can I replace the DHW thermistor myself to save money?
No — even though the thermistor itself is an inexpensive part, the work involves opening the boiler casing and handling electrical components, which must only be done by a Gas Safe registered engineer. Attempting this yourself could void your warranty, invalidate your home insurance, and create a safety risk.
What if the repair costs more than expected — could it be the PCB?
If the engineer finds that the thermistor and wiring are both in good condition, the fault may lie with the PCB (printed circuit board). PCB replacement is the costliest outcome for E33, with the part alone typically running from around £180 to £300 or more depending on the model, plus labour. Your engineer should confirm the PCB is genuinely at fault before replacing it, as it is worth getting a second opinion if your boiler is older and a new PCB would bring the total repair close to the cost of a new boiler.