Worcester Bosch 1021 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch 1021 fault code mean?
Fault code 1021 indicates that the boiler's hot water (cylinder charging) temperature sensor — a small NTC thermistor — is either defective or has lost its electrical connection to the PCB. The thermistor works by changing its electrical resistance in response to temperature shifts; the PCB monitors this resistance continuously to control heating output. When the PCB receives a reading that falls outside expected parameters — because the sensor is faulty, its wiring is damaged, or its plug has come loose — the boiler locks out and displays 1021 to prevent uncontrolled overheating. This code is most commonly seen on the Worcester Bosch Greenstar 8000 series, and has also been reported on the Greenstar 4000 range.
General guidance only — not a substitute for professional advice. 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 sensor plug Common
The electrical plug connecting the hot water NTC sensor to the boiler's wiring harness can work loose over time due to vibration or previous servicing. A poor connection produces erratic resistance readings that the PCB interprets as a sensor fault. This is the single most common root cause and is often fixed by a simple reconnection.
- Faulty or failed NTC thermistor Common
The thermistor itself can fail — through water ingress, heat cycling fatigue, or general component ageing — causing it to report an out-of-range resistance value. The boiler treats this as a sensor error and locks out. A straightforward sensor replacement typically resolves this.
- Damaged wiring or corroded terminals Sometimes
The cable run between the sensor and the PCB can suffer from chafing, pinching, or terminal corrosion, particularly in older installations or where condensation collects. Damaged insulation can cause intermittent short circuits that mimic a sensor failure.
- Incorrectly positioned sensor Rare
If a sensor has been disturbed during a service and not refitted correctly — for example, not fully seated in its pocket or mounted too far from the intended measurement point — it will produce inaccurate readings and trigger the fault.
How to fix it
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working normally. If there is no gas supply, contact your gas supplier rather than investigating the boiler further.
- Reset the boiler once or twice DIY safe
Locate the reset button on your Worcester Bosch boiler (usually a flame symbol on the control panel), press and hold it for around 3 seconds, then allow the boiler to restart. If the fault clears and stays clear, monitor the boiler. If code 1021 returns within a short period, do not keep resetting — repeated resets without a fix will not resolve an underlying sensor fault.
- Do not attempt to open the boiler casing yourself Gas Safe engineer
Diagnosing and repairing a sensor fault requires removing the boiler casing and working on internal electrical components. This must only be done by a Gas Safe registered engineer. Attempting this yourself risks electric shock, gas escape, and invalidates your boiler warranty.
- Engineer inspection — check sensor plug and wiring Gas Safe engineer
A Gas Safe engineer will open the casing, locate the hot water NTC sensor and its plug, and inspect the connector for signs of looseness, corrosion, or damaged terminals. A loose plug is often reseated and secured, resolving the fault without needing a new part.
- Engineer testing — measure sensor resistance Gas Safe engineer
Using a multimeter, the engineer checks the thermistor's resistance at a known temperature and compares it against the manufacturer's specification. A reading outside the expected range confirms the sensor has failed and needs replacing.
- Engineer repair — replace the NTC sensor if required Gas Safe engineer
If the sensor has failed, the engineer will fit a replacement NTC thermistor, reconnect all wiring securely, and retest the system through a full hot water demand cycle to confirm the fault has cleared.
- Call a Gas Safe registered engineer if the fault persists or you are unsure Gas Safe engineer
If code 1021 reappears after reset, or you have not already done so, book a Gas Safe registered engineer to carry out the inspection and repair. You can verify an engineer's registration at gassaferegister.co.uk.
Parts you may need
- Worcester Bosch hot water NTC thermistor sensor · from £25
- Sensor wiring harness / loom · from £35
- Sensor connector plug and terminals · from £12
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–£250, depending on the underlying cause.
Frequently asked questions
Can I fix Worcester Bosch fault code 1021 myself?
No. Although the underlying problem — a loose sensor plug or failed thermistor — sounds straightforward, resolving it requires opening the boiler casing and working on internal electrical components. This must be carried out by a Gas Safe registered engineer. The only steps a homeowner should take are checking the gas supply is on and attempting a single reset.
How much does it cost to fix fault code 1021 in the UK?
For a straightforward sensor reconnection or replacement, most homeowners pay between £100 and £250 all-in (parts and labour). The NTC sensor itself typically costs £15–£40; the rest is labour at roughly £60–£100 per hour. If the fault turns out to involve a damaged wiring loom or — in rare cases — a PCB issue, costs can rise above this range, but sensor faults are usually among the more affordable boiler repairs to resolve.
Why does fault code 1021 appear mainly on the Greenstar 8000?
The Greenstar 8000 uses a dedicated hot water cylinder charging sensor as part of its heating circuit design, making the 1021 sensor fault specific to models configured this way. The Greenstar 4000 can also display this code, but it is reported less frequently. Older Greenstar i or CDi models use a different fault-coding system and would not show code 1021.
Will an annual boiler service help prevent fault code 1021?
Yes. During a service, an engineer checks all sensor connections and inspects wiring for early signs of corrosion or wear. Catching a loose sensor plug or deteriorating cable at a service — which typically costs £80–£120 — is far cheaper than an unplanned breakdown call-out, especially outside standard working hours when emergency rates apply.