Worcester Bosch D1 240 fault codewhat it means and how to fix it
The return temperature sensor has short circuited. The boiler has lost its reading of how warm the water is when it comes back from the radiators.
Take care
Get a Gas Safe registered engineer to look at your Worcester Bosch D1 240
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 does the area around the boiler look like?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
There is damp, staining or limescale inside the case
Very likely the answer. Find and fix the drip and the sensor may well test perfectly once it has dried out.
Point out the staining. Replacing a sensor without stopping the leak means doing it again in a few months.
It started after the boiler was serviced or a part was fitted
A lead pinched or displaced when a panel went back on is a common consequence of otherwise good work.
Go back to the engineer who did the job before paying for a fresh diagnosis.
Nothing unusual — it appeared on its own
Most likely age. The sensor is an inexpensive part and this is a straightforward job.
Book a repair. There is no hazard in waiting a few days.
What D1 240 means
D1 240 is a short circuit on the return temperature sensor, and Worcester publish it for the Compact ranges: "D1 240 — return sensor error — sensor wet or damaged (the boiler will block for 300 secs before this condition)", with D1 241 as the same sensor disconnected and D1 286 as the return water genuinely too hot.
The return sensor tells the boiler how much heat the system actually took out of the water it sent round. That reading matters more than people expect: the difference between flow and return is how the appliance judges circulation, and a low return temperature is what allows a condensing boiler to condense and reach its advertised efficiency. Lose the signal and the boiler is running blind on half of that picture, so it pauses for five minutes as a blocking fault and then, if the reading has not come back, locks out.
Worcester's own wording gives the commonest cause away — wet or damaged. These sensors sit where a dripping joint above can wet them, and water across the terminals reads as a short. An engineer will often dry and retest before condemning anything. The Greenstar i ErP has no return sensor of this kind and does not publish D1; the 8000 Life lists its return sensor code, 1025, as not fitted.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Water on the sensor or its terminals | A weeping joint above drips onto the sensor body and bridges the contacts, which reads as a dead short. | Common |
| Sensor failed short | Thermistors of this type tend to fail towards zero resistance as they age, which is exactly what the board sees. | Common |
| Chafed sensor lead shorting to the case | Where the pair of wires crosses a metal edge the insulation wears through and the two conductors touch. | Occasional |
| Corrosion in the plug at the board | Deposits between adjacent pins provide a conductive path that mimics a shorted sensor. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£100–£220
Return sensor £100–£180 fitted. Drying and retesting a wet connection may fall within a call-out charge of £80–£150.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Worcester Bosch fault codes
- E9 219Stop — call an engineerThe boiler has overheated and the safety thermostat has tripped. Water is not carrying heat away from the exchanger, and it should not be run until that is fixed.Related: overheat and circulation
- E9 224Stop — call an engineerOne of the two safety thermostats has tripped: the flue overheat thermostat or the high-limit stat on the heat exchanger. Worcester's own check list includes the burner gasket for flue gas leakage.Related: overheat and circulation
- A1 229Stop — call an engineerThe boiler has detected a dry-fire condition — it is trying to heat water that is not moving. Turn it off and leave it off until an engineer has seen it.Related: overheat and circulation
- E5 218Get it checkedThe flow temperature sensor has measured water leaving the heat exchanger far hotter than it should be. The boiler shut down to protect itself, and heat is not being carried away fast enough.Related: overheat and circulation
- E2 222Get it checkedThe flow temperature sensor is giving a reading the appliance cannot trust. Usually the sensor rather than the temperature.Related: overheat and circulation
- D4 341Get it checkedThe water temperature in the boiler is climbing faster than it should, which means very little of it is moving. Worcester's checks are pressure, air, circulation and the pump, in that order.Related: overheat and circulation
Own a Worcester Bosch boiler? Save it and its codes come first next time.
Related guides
Common questions about Worcester Bosch D1 240
Is this the same sensor as E2?
No. E2 222 is the flow sensor, on the pipe leaving the boiler. D1 240 is the return sensor, on the pipe coming back. Same kind of part, opposite ends of the circuit, and an engineer needs to know which one so they arrive with the right item.
What is the difference between D1 240 and D1 241?
The direction of the failure. 240 is a short circuit, where resistance has collapsed. 241 is an interrupted circuit, where the connection is broken altogether. A short often means moisture; a break more often means a damaged lead or plug.
Is it safe to leave it?
There is no gas or combustion hazard here, and the boiler has stopped rather than run without the reading. The practical problem is simply that you have no heating or hot water until it is repaired.
What should it cost?
Among the cheaper faults. £100 to £180 for the sensor fitted, and often less if drying out a wet connection is all that turns out to be needed.
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.