Quick answer: The 1021 fault code (often shown as 1021 B) means your Worcester Bosch boiler has a problem with the hot water temperature sensor, either the sensor itself has failed or its plug isn’t connected properly. It’s most common on Greenstar 8000 models. Your heating will often keep working, but hot water becomes unreliable or unavailable. It’s a straightforward repair, but it’s an inside-the-casing job, so it needs a Gas Safe registered engineer.
Written by the engineering team at Edinburgh Boiler Company, Scotland’s number one boiler installer. The Greenstar 8000 is one of the most common boilers we fit across Edinburgh and the Lothians, so 1021 is a code we know well.
What does 1021 mean on a Worcester Bosch 8000 boiler?
The Worcester Bosch 1021 error code means the boiler can’t get a valid reading from the domestic hot water (DHW) temperature sensor. That sensor is an NTC thermistor, a small component whose electrical resistance changes as it warms up. The control board reads that resistance and converts it into a temperature.
When the board sees a reading that’s impossible, either an open circuit suggesting nothing is connected, or a short circuit, or a value wildly outside the plausible range, it flags 1021. It has no way to control hot water temperature safely without a trustworthy reading, so it stops trying.
The B suffix you’ll often see alongside it is Worcester’s sub-code, narrowing the fault down within the DHW sensor circuit.
Will my heating still work with a 1021 fault?
Usually yes, at least partially. Because the fault sits in the hot water circuit rather than the heating circuit, many boilers will continue to run central heating while refusing to produce hot water, or producing it erratically.
That’s genuinely useful in a Scottish winter. It means a 1021 fault isn’t the emergency that a total lockout would be. Book the repair properly rather than paying a premium for an out-of-hours visit, and you’ll still have heating in the meantime.
| Symptom | What it points to |
|---|---|
| Heating fine, no hot water at all | Sensor open circuit or disconnected |
| Hot water swings from scalding to cold | Sensor giving erratic readings |
| Hot water stuck lukewarm | Sensor reading high, so the boiler under-fires |
| Both heating and hot water affected | Something beyond the DHW sensor, likely PCB or wiring |
Make a note of which of those matches your boiler. It’s genuinely helpful to the engineer.
What causes a 1021 fault?
| Cause | How common | What’s happening |
|---|---|---|
| Loose or disconnected sensor plug | Very common | The connector has worked loose, often after previous service work |
| Failed NTC thermistor | Common | The sensor itself has drifted out of range or failed open |
| Corroded connector pins | Common | Moisture in the boiler enclosure has attacked the contacts |
| Damaged wiring between sensor and PCB | Occasional | A chafed or pinched loom, sometimes from a previous repair |
| Debris or scale around the sensor pocket | Less common in Scotland | Deposits insulate the sensor and skew readings |
| PCB fault | Less common | The board’s input circuit has failed rather than the sensor |
The connector deserves the top spot. A surprising share of 1021 faults are nothing more than a plug that wasn’t fully seated, and they very often appear within weeks of the boiler last being worked on. If your boiler was serviced or repaired recently and 1021 has since appeared, mention that when you book. It usually saves time and sometimes saves a part.
Does hard water cause 1021 faults in Scotland?
Rarely, and this is where a lot of the advice online doesn’t apply here. Most guides on this fault blame limescale, because they’re written with English hard-water regions in mind. Scottish Water supplies soft water to the large majority of the country, so scale build-up around a sensor pocket is much less of a factor in Edinburgh, Glasgow, Aberdeen or the Highlands than it would be in Kent or Cambridgeshire.
What we do see instead is corrosion. Scotland’s soft, slightly acidic upland water is perfectly capable of corroding components, and combined with damp boiler locations, it attacks connector pins and sensor terminals. So when a Scottish 1021 fault turns out not to be a loose plug, corrosion is a more likely explanation than scale.
There are pockets of harder water in parts of the east and around some groundwater sources, so it isn’t universal. But as a general rule, if someone diagnoses your Scottish 1021 fault as limescale without checking anything else, ask more questions.
What can I safely do myself?
Two things.
1. Reset the boiler once. Hold the reset button for three to five seconds. A loose connection will sometimes make contact again on restart. If it does, book a service anyway, because it will come back.
2. Note the symptoms carefully. Is there no hot water at all, or is the temperature erratic? Does it fail at the start of a run or partway through? Is heating affected too? Write it down. This is the single most useful thing you can hand an engineer, and it shortens the diagnostic time you’re paying for.
What you cannot do is open the casing. The DHW sensor sits inside the boiler among live electrical connections and gas components, and accessing it without Gas Safe registration is both dangerous and illegal.
When should I call a Gas Safe engineer?
Whenever 1021 shows and doesn’t clear with a single reset, which is most of the time. There’s no external check that resolves this fault, so waiting doesn’t help.
Book sooner rather than later if:
- Hot water is swinging between very hot and cold, which is a scald risk, particularly with children or older people in the house.
- Heating is affected as well as hot water, which suggests the fault is broader than the sensor.
- Other fault codes are appearing alongside it.
If hot water is running dangerously hot, turn the hot water temperature setting down and be careful at the taps until it’s fixed.
What will the engineer do?
- Read the fault history and sub-code from the boiler.
- Check the DHW sensor plug is properly seated and the pins are clean.
- Test the sensor’s resistance against the manufacturer’s temperature-resistance table, which shows definitively whether the thermistor is good.
- Check continuity through the wiring loom back to the PCB.
- Inspect the sensor pocket for deposits or debris.
- Replace the sensor, repair the connection, or escalate to the PCB depending on what the tests show.
- Verify hot water operation and temperature stability before leaving.
That resistance test in step three is what separates a proper diagnosis from a guess. A sensor either matches the table or it doesn’t, and there’s no ambiguity about it.
How much does it cost to fix a 1021 fault in Scotland?
Most 1021 repairs land between £110 and £250, which makes this one of the more affordable Worcester fault codes. Guide prices for Scotland in 2026, parts fitted.
| Fix | Guide cost | Time on site | Notes |
|---|---|---|---|
| Diagnostic visit | £70–£130 | 45–60 mins | Usually deducted from the repair |
| Reseating or cleaning the sensor plug | £70–£130 | 30–45 mins | Often resolved within the diagnostic visit |
| DHW NTC sensor replacement | £140–£240 | 1–1.5 hours | The most common paid outcome |
| Wiring loom repair or replacement | £150–£280 | 1–2 hours | If the sensor tests good |
| Sensor pocket clean | £110–£180 | 1 hour | Rarely needed in soft-water Scotland |
| PCB replacement | £350–£650 | 2–3 hours | Least likely cause, always ruled out last |
Because the Greenstar 8000 is a current range, parts are readily available and there’s no premium for scarcity. That’s a real advantage compared with fault codes on older discontinued models.
Will my Worcester guarantee cover it?
Almost certainly, if your servicing is current. The Greenstar 8000 range carries guarantees up to 10 or 12 years depending on model and installation, when fitted by a Worcester Bosch accredited installer and registered correctly. A DHW sensor is unambiguously a boiler component, so it’s exactly what that guarantee is for.
Given that many Greenstar 8000 boilers are still relatively young, a large share of the 1021 faults we see are covered work. Check your registration and your service history before you pay for anything. The requirement is usually an annual service by a registered engineer, and a missed year can invalidate the cover.
How do I stop 1021 coming back?
- Book the annual service. It keeps the guarantee valid, which on a boiler of this age is worth far more than the service costs.
- Use an engineer who reseats and checks connections as part of the service rather than just doing a combustion test.
- Improve ventilation if the boiler is in a damp cupboard, garage or outhouse. Reducing humidity around the boiler slows the corrosion that causes connector faults.
- Mention any hot water temperature oddities at your service, even minor ones. Erratic temperatures are the early warning for a sensor that’s drifting.
- Keep your registration paperwork somewhere you can find it.
What’s the Difference Between the Worcester 1021 and EA 229 Codes?
1021 is a hot water sensor fault specific to the Greenstar 8000 range. It’s a different issue from the Worcester EA 229 fault, which points to a flame or ignition problem rather than a faulty sensor reading.
The bottom line
1021 is a hot water sensor fault, most often on the Greenstar 8000, and it’s one of the less painful codes to receive. Your heating usually keeps running, the parts are affordable and available, and if your boiler is registered with an in-date service history it’s very likely covered under guarantee. Try one reset, note your symptoms, then book a Gas Safe registered engineer.
Need it diagnosed? Edinburgh Boiler Company covers Edinburgh, the Lothians and central Scotland with Gas Safe registered, Worcester-accredited engineers and clear written pricing before any work starts.
This guide is general advice. Refer to your specific Worcester Bosch model’s user manual for reset instructions, and never remove the boiler casing. Gas work must be carried out by a Gas Safe registered engineer.
