What fault 1021 means
Fault 1021 means your Worcester Bosch boiler cannot get a sensible reading from the sensor that measures hot water temperature. Worcester's installation and maintenance instructions for the Greenstar 8000 Life gas fired condensing combi (document 6720883865, 2021/11 edition, section 10.1.2) list 1021 as "Cylinder charging or hot water temperature sensor faulty". The earlier 2019/02 edition of the same manual puts it more simply: "Hot water temperature sensor is defective".
If your copy is the 2019 print, you will find the code in the fault table on pages 62 to 63 with the shorter wording. In the 2021 print it sits in the table that runs from page 75 to page 79. Both describe the same fault and share the same document number, so either edition applies.
The models in those manuals are the GR8300iW C combis (30, 35, 40, 45 and 50 C). On a combi there is no cylinder to charge, so in practice the code refers to the domestic hot water temperature sensor. The combined wording appears because the same fault table is written to cover both set-ups.
What you are likely to notice
The boiler uses this sensor to control the temperature of the water coming out of your taps. With no reliable reading it cannot do that properly, so most people notice the code alongside a hot water problem, such as water that stays cold, runs too hot or changes temperature. Write down exactly what the taps are doing, as it helps the engineer.
A blocking code, not a lockout
The 8000 Life table marks 1021 with the letter B, a blocking fault. In Worcester's manuals a blocking fault stops the boiler without the usual lockout, and it usually clears by itself once the cause goes away. The Greenstar i manuals also note that some blocking faults turn into locking faults if they last long enough. So 1021 may come and go, which often points to a loose plug or a sensor on its way out rather than one that has failed completely.
Other four-digit codes on the 8000 Life
The Greenstar 8000 Life uses four-digit fault numbers, and a few near 1021 are easy to mix up.
| Code | Manual wording | Who deals with it |
|---|---|---|
| 1017 | "System pressure too low" | You first, by topping up the pressure |
| 1018 | "Maintenance interval expired" | Your engineer, at the next service |
| 1019 | "Incorrect pump type detected" | Gas Safe engineer |
| 1021 | "Cylinder charging or hot water temperature sensor faulty" | Gas Safe engineer |
1021 is also unrelated to A21, which comes from Worcester's heating controls rather than the boiler.
What causes fault 1021
The remedies in the 2021 manual give three causes, in the order an engineer checks them.
Sensor or plug not connected
The first step is to check the hot water temperature sensor is connected and its plug is properly fitted. A plug that has worked loose, perhaps after a repair nearby, can break the reading on and off.
Sensor out of range
The sensor works by changing its electrical resistance with temperature. The manual tells engineers to measure that resistance and replace the sensor if it is wrong. This is the most common outcome when the code stays on all the time.
Damaged wiring harness
The last check is continuity of the harness between the sensor and the boiler's electronics. A broken or chafed wire gives the same symptom as a failed sensor, so it is tested before parts are swapped.
Checks you can make yourself
- Note the code 1021 and what the hot taps are doing. Check whether heating still works too.
- Check the fused spur feeding the boiler is on, and whether the fault followed a power cut.
- Look at your hot water settings on the boiler display or app, in case something has been changed.
- Look underneath the boiler, without opening it, for drips or water. Mention anything you see.
- Reset as the 8000 Life user instructions describe if the code does not clear by itself, no more than two or three times.
Our Worcester boiler no hot water guide covers other reasons a Worcester combi stops heating your taps, and our Worcester boiler reset guide explains when a reset is worth trying.
The sensor and its wiring are inside the casing, next to hot pipework and mains-powered electronics. Do not remove the casing or unplug anything. Testing and replacing the sensor is work for a Gas Safe registered engineer.
What to tell the engineer
Give the engineer the code 1021, the exact model (for example GR8300iW 40 C), whether the code is constant or comes and goes, and what the hot water does when it appears. Mention any recent work on the boiler, as that points towards a disturbed plug. An engineer who knows it is the hot water sensor can usually check the plug, resistance and harness in one visit.
Expect the engineer to isolate the power before opening the case, check the plug on the sensor first, then measure the sensor's resistance and compare it with Worcester's figures. Only if the sensor reads correctly will they move on to testing the harness. Working in that order avoids paying for a new sensor when the real fault is a loose connector or a damaged wire.
Typical repair costs in the UK
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Refitting a loose plug or repairing the sensor wiring | Typically £100 to £180 |
| Hot water temperature sensor (NTC thermistor) replacement | Typically £100 to £200 |
Sensor faults are among the cheaper boiler repairs. If the harness or plug is the cause, the cost may be at the lower end.